AP3456 - 6-1 - Human Performance
CHAPTER 1 - HUMAN PERFORMANCE
Introduction
1.
The investigation and study of error has received more attention in aviation than in many other
domains, and part of the explanation probably lies in the fact that aviation accidents tend to be
expensive and unavoidably public. About 40% of serious accidents in the Royal Air Force are ascribed
to aircrew error. Other military operators report similar proportions. In the commercial aviation sector
a variety of figures have been quoted, some as high as 80%. In aviation, a post-accident inquiry often
faces a difficult question: why should an experienced, motivated professional, with undoubted skill,
make such a mistake? In individual cases, the question can seem very perplexing. Viewing a large
number of such events adds to the confusion. The first step necessary in addressing this problem is to
recognize the wide variety of ways in which mistakes come about, and to consider classifying errors
according to the characteristic mechanisms involved or the contributory factors. There are several
ways of approaching this problem, depending on the degree to which theory or data determine the
classification scheme and, of course, the theoretical bias of the classifier.
2.
A long term (20 year) study, which still continues in the RAF, involves independent investigation of
aircrew error accidents. The results of these investigations are classified according to a scheme which is,
as far as possible, data-driven. No particular theoretical view is adopted. The broad findings of the study
are presented in Table 1. This shows all those factors found to be at least possible contributory causes in
more than 10% of the investigations. They are fairly arbitrarily assigned to three broad, common-sense
categories: predispositions contributed by the aircrew themselves; enabling factors contributed by the
organization, tasks and equipment imposed on aircrew; and what can only be described as immediate
causes - the actions, conditions or events that lead directly to an error.
Table 1 Factors Implicated In Aircrew Error
Predisposition
Enabling Factors
Personality
22%
Ergonomics
22%
Inexperience
20%
Training/briefing
18%
Life stress
14%
Administration
17%
Immediate Causes
Acute stress
25%
False hypothesis
13%
Cognitive failure
18%
Disorientation
12%
Distraction
16%
Visual illusion
12%
Notice that there is no clearly dominant factor. In addition, accidents are usually complex events, so
often several factors, out of a repertoire of about 40, are cited as contributors. These facts suggest
that simple, global remedies will not be found. Reducing the toll of accidents is likely to require a
combination of specific remedies targeted on relatively small sub-groups of accidents (improving the
conspicuousness of aircraft to reduce mid-air collisions, for example, or modifying a regulation or
instruction to avoid ambiguity) and a broad assault designed to improve the routine identification and
elimination of risks.
3.
Several of the terms in Table 1 (for example disorientation, visual illusion) are clearly peculiar to
aviation though several of the larger categories do, however, appear to be potentially more generally
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relevant. They are personality, life stress, acute (reactive) stress, cognitive failure (a mismatch
between intentions and actions), and a clutch of enabling factors (ergonomics, training and
administration) which together account for about 40% of aircrew error accidents. The topics to be
considered are, therefore, cognitive function and its limitations, long term predispositions to error, short
term factors degrading performance, and enabling factors that make errors more likely.
COGNITIVE FUNCTION AND ITS LIMITATIONS
Experience
4.
The gulf between an expert’s and a novice’s performance can seem immense. The novice’s
mistakes are easy to understand, but inexperience seems (from Table 1) not to make a dominating
contribution to aircrew error - and this in military aviation, a profession which by its very nature puts
large demands on young shoulders. To understand why skilled operators sometimes make mistakes it
is first necessary to understand how normal skilled behaviour is achieved, because it appears that
errors are a consequence of the normal characteristics of skill and expertise. The cognitive strategies
that enable, say, the expert pianist to achieve a polished performance are the same ones that
distinguish the captain of an airliner from a trainee pilot. They also form the basis of more common
skills that everyone takes for granted, and entail particular risks.
Mental Activity
5.
An important feature of human behaviour is that some activities seem to require little or no mental
effort, and so can be performed in parallel with other activities. Others seem to absorb all our mental
capacity. It is not just a matter of conflicting motor actions; it is a central, mental resource that is
implicated. Most adults can ride a bicycle and talk or do mental arithmetic at the same time, but try
multiplying two two-digit numbers during a hard fought game of, say, squash or badminton. The
unpredictability of the overtly physical game precludes unrelated mental effort. It is also significant that
activities can be moved from the mentally demanding group to the low-mental-effort group simply by
practice; there is no hard and fast categorization. Indeed a puncture or unexpected obstacle at high
speed can suddenly, temporarily reverse the process and change riding a bicycle into the sort of task
that excludes all other thoughts.
6.
Introspection offers an indication of the nature of this limited mental resource. When we perform
a difficult or novel task, what it demands is our attention; the task becomes the principal thing of which
we are conscious, and attention seems to be flexible, both in terms of the sensory modalities to which it
refers, and temporally. The contents of consciousness need not be the results of current stimulation;
they can be formed from memories of past events, or imaginative constructions. Attention has the
character of memory for the present. It enables information of present interest, from a variety of
sources, to be held in consciousness while it is evaluated or used in decision making or computation.
Working Memory
7.
Working memory appears to have three components. The best understood, known as the
articulatory loop, handles speech based information. Its capacity is limited to only a few items - about
enough for a telephone number - and decay takes only a few seconds. The memory can, however, be
maintained indefinitely by rehearsal using articulatory processes connected with speech. There
appears to be an independent but structurally similar component used for storing spatial information.
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Like the articulatory loop it involves a passive information store and an active rehearsal mechanism, in
this case possibly based on the system that controls eye movements.
8.
The least well understood component of working memory is known as the central executive. It is
believed to be capable of handling any type of information, and to be responsible for the integration of
information from disparate sources as well as scheduling the allocation of mental resources. Its
capacity is believed to be limited, but has so far defied measurement. The central executive’s rather
grand title reflects the importance attached to its functions. It has also been aptly described as an area
of residual ignorance. It is, perhaps, inevitable that all the (so far) experimentally inaccessible
functions of working memory should seem to reside in one enigmatic block. If any progress can be
made in this area, a more complex, but also more satisfying picture should emerge. For the moment
some important features of working memory are clear: although both spatial and speech based
information can be stored, the overall capacity is limited, and maintaining the memory for more than a
few seconds demands effort and consumes precious resources. Information in working memory is
also vulnerable to interference from new inputs. These limitations demand effort-saving strategies in
gathering information from the world and controlling our actions.
Long Term Memory
9.
In contrast to working memory, long term memory appears to have an enormous capacity, and to
store information indefinitely. We are only aware of this information, however, when it is transferred to
working memory. Long term memory also seems to involve several sub-systems, but the distinctions
involved are not always clearly drawn. For example, it is clear that some of the information in long
term memory can be described as semantic. It involves knowledge about the world. Other information
is best described as episodic. It relates to one’s own personal experience. It is not clear, however,
that different processes are involved in forming these two types of memory, and, sometimes, the
distinction between them is difficult to make. It is interesting that interviewing accident survivors often
reveals an apparent change in the way the story is told after several repetitions from a detailed,
possibly confused pattern, which seems to invoke actual impressions of the event, to a more coherent,
sparser account that is often less informative (and sometimes at variance with other evidence). This
may reflect a change in the way the information is stored, semantic coding being more economical.
The change that takes place in fishermen’s tales with retelling probably involves a similar shift in
balance between the more truthful episodic encoding and the more "meaningful", semantic encoding.
10. Another useful distinction is between declarative knowledge (which embraces both semantic and
episodic memory) and procedural knowledge. Procedural knowledge is of particular interest in the
context of error because it involves the mechanisms that control or guide performance of a task
without reference to underlying factual knowledge. Knowing how to ride a bicycle is a good example of
procedural knowledge. Once attained, the knowledge persists indefinitely, and is instantly available
should the opportunity to exercise it arise. It is also peculiarly difficult to communicate the
fundamentals of the skill verbally. This last point is not true of all procedural knowledge, however, and
the distinction with declarative knowledge is not entirely clear cut. In the context of error analysis some
have found it useful to classify tasks according to the type of knowledge involved in their execution.
The most automatic activities are described as skill-based; they demand little conscious attention, and
explanation of the processes involved may be difficult. Rule-based behaviour involves more easily
described procedures, for example "i" before "e" except after "c". It demands a little more conscious
monitoring if actions are to be performed in the right order, without omissions. The most demanding
activities are described as knowledge-based. Here the activity is largely unautomatic, the mental load
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is considerable, and artificial memory aids may be required, for example; check lists, computational
notes, diagrams and maps.
11. Again it is often difficult, if not impossible, to make precise distinctions in practical cases. Most
complex tasks involve more or less unconscious procedural elements and overall strategies based on
explicitly definable knowledge. With increasing experience, the trainee’s behaviour on some aspects
of his tasks might be said to progress from one level to the next as less and less conscious attention is
required. This is surely not a general description of skill or expertise acquisition, however. It is, for
example, possible to describe the sequence of actions involved in changing gear in a car. Some
people are even capable of explaining in detail the mechanical consequences of these control
movements. For most people, however, gear changing is simply a 'knack'; attempting to convey it in
words to a learner driver can be almost pointless. With a little practice, however, the knack is acquired
- whether or not the learner understands the mechanical details. Nevertheless, the distinction between
skill, rule, and knowledge-based behaviours does not have the merit of reflecting an important
descriptor of a task in the degree of conscious attention it demands of the operator. It is interesting
that errors in knowledge-based behaviour do not appear under immediate causes in Table 1, but
cognitive failures are well represented. Cognitive failures involve a mismatch between intentions and
actions. The correct drill is selected, but items are omitted, confused with others from a similar drill, or
operated in reverse (raising rather than lowering a lever). Cognitive failures are often associated with
distractions or preoccupation in otherwise normal and undemanding circumstances. Where
knowledge-based errors are identified in an investigation, they are very often associated with failures in
training or briefing, or the administrative background (which includes the framing of orders, manuals,
and instructions).
12. Aircrew-error data point to cognitive failure as an important area of vulnerability in skilled
performance, though it is certainly not confined to aviation. It is true, however, that aviation, military
aviation in particular, involves highly standardized routines and rituals, which are designed to minimize
the workload entailed in knowledge-based performance. Such a strategy promises a high probability of
success in extremely demanding missions, without raising personnel selection criteria to unrealistic
levels. It also necessarily biases the type of error likely to predominate in any collection of accident
investigations.
Sensory Stores
13. Very little of the information available at any moment in the sensory domain is allowed into the
focus of attention. But that focus can shift very quickly - from reading this text to sounds coming from
another room, say, or the sensations produced as your hands support the book. When the impetus for
a shift of attention is produced by an unexpected external event, such as your own name cropping up
in the conversation in the next room, some antecedents of that stimulus (the beginning of the
sentence, for example) may also be noticed. Experimental evidence suggests that this remarkable,
and useful, feat is not achieved through prescience, but by routine, very short term storage of sensory
information. Sensory stores seem to have unlimited capacity, but retain information for, at most, a
second or so. This allows not only selection of the information to be processed, but also some
interpretation on the basis of past experience and current context.
14. Perception is, therefore, in part driven by expectation. This is a labour-saving ruse that takes
advantage of redundancy and predictability in the real world in constructing a representation of it. The
written word is particularly redundant, as this sentence about a tailoring deficiency shows: "Th# sl##v#s
#f th# sh#rt w#r# a l#ttl# t## sh#rt". Despite the lack of vowels, it is unlikely to take much longer to
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decipher this sentence than it would normally, and the interpretation of "sh#rt" should change naturally
with context. The advantages of this system are considerable: it reduces the resources required to
interpret the world, and allows some flexibility in selection and interpretation based on succeeding as
well as preceding information.
15. The disadvantage is, of course, a risk of misinterpretation by too great a reliance on previous
experience and present expectations. In Table 1 these errors appear in the false hypothesis category. Often
they are associated with a preceding cognitive failure. The pilot is distracted or preoccupied during his pre-
landing checks. The checks, which he has done so often that he hardly need think about them, are
completed, but with an omission. He "knows" he has lowered the undercarriage, so a routine glance at the
undercarriage indicator gives the expected result, not the true state of affairs, and the landing continues
without wheels. Even after landing, the pilot may not correctly diagnose the cause of the strange noise and
bumps as the aircraft slides down the runway, so strong is his expectation.
Overview
16. The system described above is flexible and efficient. In familiar situations the effort required is
minimized; well practised routines and rules of thumb operate almost automatically, and the signals
required to direct actions or initiate new responses are selected without much deliberation. In taxing or
problematic situations, a more effort-intensive approach can be adopted. The environment is scanned for
the signs that identify the problem or situation; previously effective solutions are recalled from past
experience and implemented. When the situation is novel, a deliberate, more or less systematic exercise
in information gathering and conceptual reasoning may be required. The expert approaches his task with
all three strategies at his disposal. Long-term goals may be consciously set, and these define the skills
required and the experience he will have to draw on in executing his shorter-term plans.
17. The weaknesses of the system are characteristic of the resources deployed in each type of
approach. The capacity of working memory is an all too evident limit on efficiency in conceptual
reasoning. When diagnosis and response are required in a fairly short time, and aides memoire are not
available, then it is common that some relevant information is overlooked or given insufficient weight,
particularly if it does not fit the first tenable hypothesis that comes to mind. Solutions may be proposed
that are focused on the observable symptoms, but without thought for possible side-effects of the
solutions (a trivial example is replacing a blown fuse with one of higher rating). When there is just too
much to think about, there is a strong temptation to test hypotheses in a concrete manner without
considering the possible consequences of the intervention. In more routine circumstances, minor slips
and lapses are more likely. Monitoring may fail to detect the signs; the situation is seen as normal - as
expected. About two thirds of the cognitive failures reported in Table 1 happened in routine,
undemanding circumstances. Often all that was required was a minor distraction. The consequences
seem out of proportion to the precipitating event. It is an important finding for any safety oriented
occupation that normal behaviour, in normal circumstances, carries a significant risk of serious error.
FACTORS AFFECTING PERFORMANCE
The Environment and Arousal
18. Military aircrew have to contend with a variety of environmental stress creators not commonly
encountered in other occupations; heat, vibration, noise, and acceleration are all catered for with
special equipment. The acute, reactive stress associated with life-threatening emergencies is not so
easily countered.
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19. The effects of stress creators on performance are complex and varied. To some extent the
concept of arousal simplifies (perhaps oversimplifies) discussion of these effects. It implies a
continuum of activation from extreme drowsiness to extreme excitement. Psychological indicators of
arousal level include alertness, sensitivity to simulation and performance on tests. Physiological
indicators, such as heart rate, skin resistance, etc, sometimes, but only sometimes, show useful
correlations with psychological variables. Fig 1 embodies two ideas which have proved a useful, if
incomplete, description of the relationship between arousal level and performance for many years.
The first idea is that there is an optimum arousal level for any task. This implies an inverted "U"
relationship with performance. This is a difficult hypothesis to test experimentally, of course. The
second idea is that easy tasks are more tolerant of high arousal levels than difficult ones. Difficulty
level in this context obviously depends on the training and experience of the operator. Further
individual differences (see the section on personality) also complicate the picture. Variations in arousal
level seem to affect performance largely by changing attentional capacity and processing speed. To
some extent these changes are moderated by learned strategies in the control of attention.
20. At low levels of arousal, such as might occur after a long period of work, at night, particularly if the
work is unstimulating or monotonous, responses take longer and lapses of attention and omissions are
more likely to occur. Given noise, stimulants (such as caffeine or interesting conversation), or
sufficient motivation, apparently normal levels of efficiency can be achieved - though the less important
tasks may be neglected.
21. Fatigue and sleep deprivation commonly produce stress, but they do not figure in Table 1. This is
not to suggest that they never contribute to aircrew error, but the evidence is that they make only a
minor contribution. In civil aviation, which routinely involves long periods on duty, time zone shifts and
disruption of circadian rhythms, there may be more scope for fatigue and sleep deprivation to affect
performance. In both the military and civilian sectors, however, duty cycles and rest periods are
regulated and closely monitored.
6-1 Fig 1 Arousal Levels
22. At high levels of arousal, such as might be provoked by an emergency, information may be
processed more quickly, but at the expense of a reduction in the capacity of working memory. Control
of attention becomes more of a problem. The reduction in capacity of working memory can be
compensated for by increased attentional selectivity - focusing intently on the important information -
but impairment of perceptual discrimination may allow superficially relevant stimuli to become
distracting, so disrupting performance.
Acute Reactive Stress
23. The errors coded under "Acute stress" in Table 1 were mainly caused by mechanical problems -
engine fires, bird strikes, etc. A few were due to prior mishandling by the pilot, or disorientation. The
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major category of problems generated (about 30%) are best described as a disorganization of
responses: the wrong drills were selected, or the pilot’s analysis of the emergency was haphazard and
ineffective. Slow responses and precipitate action were about equally likely (about 10% of the total
each). Narrowing of attention and cognitive failure together accounted for another 30% of the cases.
24. There is no reason to suppose that this pattern is in any way unusual nor likely to be
representative of that found in non-aviation emergencies. Extremely detailed knowledge of the job, the
emergency, and the operator would be required to predict the type of failure to be expected. Some
general guidance on the operator’s contribution is given in the section on predisposition.
PREDISPOSITION
Trait : Life Stress
25. A popular lay explanation for aircrew error involves domestic and other pressures. The
association between stressful life events (both positive and negative) and heart disease and other
illnesses is well known. A similar statistical association between the incidence of life events and
involvement in flying accidents has been reported at least once, but this is clearly a difficult area for
research, and further analysis can suggest other interpretations. Close examination of the accidents
coded in Table 1 under "Life stress" reveals at most only two cases in which a link with life events can
confidently be averred. Both were arguably rather special cases, and did not involve a general
depletion of ability to cope with the stresses of work.
26. It is possible that military aviation allows greater compartmentalization than some other activities.
The crew are isolated from other distractions and pressures while performing the task and, in many
cases, the critical parts of the task (ie whilst airborne) last for relatively short periods. Many individuals
can cope under these circumstances, unless the stress is causing noticeable sleep disruption. It is also
likely that individual differences play a large part in determining the impact of life events.
Trait: Personality
27. The scientific description of personality can be approached in a variety of ways. Two dimensions
that have proved useful in many fields of investigation are extraversion and neuroticism. Questionnaire
tests of extraversion and neuroticism distinguish different types of deviant personality and psychiatric
disorder, and also show reliable differences between professional groups. In addition, scores on such
tests account for some of the variation in the way people approach tasks, cope with a range of
stressful situations, and behave generally. Extraverts are assumed to require more stimulation than
introverts to excite the central nervous system. As a result, while extraverts are active, sociable and
impulsive, introverts are passive, reserved and thoughtful. A high neuroticism score indicates an
unstable autonomic nervous system; it would be associated with an emotional or moody disposition. A
low score would indicate stability.
28. Introverts tend to work in a methodical manner, and hence to be slower than extraverts, who may
make more mistakes in the interests of speed. Stimulants and threatening circumstances, by raising
arousal level, would tend to be detrimental for introverts (by over-arousal), but may improve extraverts’
performance, since they tend to be chronically under-aroused. The introvert performs better, however,
when sustained vigilance is required.
29. A high neuroticism score has implications for performance in threatening circumstances. Anxiety
may divert mental resources into unproductive worry and degrade performance. Psychosomatic illness
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can result from prolonged exposure to such stress. A high score may also accentuate the differences
between introverts and extraverts in terms of liability to accidents.
30. Several studies in aviation or road safety have implicated neuroticism or some form of
maladjustment. High extraversion scores have also been found to be associated with accident
involvement. Contradictory findings and failures to find any association are by no means unknown,
however, and it is not possible to claim that a clear picture has emerged. Bearing in mind that not all
accidents are likely to involve an important contribution from personality variables, it is obvious that
large numbers would be needed to establish any correlations. It is also likely that some attention
should be given to classifying types of accident (further increasing the numbers required).
31. Table 1 shows that about 22% of aircrew-error accidents have a possible association with
personality. It has been possible to classify about two thirds of these on the basis of descriptors used
in personal records. Two groups have emerged. One is described as under-confident, nervous or
prone to over-react; the other as over-confident, reckless, heedless of rules. It is tempting to apply the
labels 'unstable introvert' and 'unstable extravert' respectively, but more evidence is required. It is
clear, however, that one group (the first) tends to be associated with accidents involving mishandled
emergencies, and the other with accidents involving unauthorized or risky manoeuvres, or failure to
appreciate risk.
32. Parallels probably exist in many other professions. Even if "joy riding" is not possible, there is
always some scope for ill-considered experimentation, corner-cutting and, of course, mishandling of
emergencies. It is also clear that future research should not be expected to produce simple
correlations between personality measures and accident involvement. It would be wise to expect a
bipolar relationship with extraversion mediated by neuroticism, and to classify accidents according to
the types of error involved. Personality tests can provide some guidance in selecting aircrew, and are
used by many airlines. They are, however, relatively imprecise instruments and their utility in selection
obviously depends on the ratio of suitable candidates to vacant posts. In most contexts, differences in
personality remain a management issue.
Trait: Cognitive Function
33. Individual differences in cognitive functioning may play a part in liability to accidents. The ability to
cope with chronic, mild stress, and liability to cognitive error may both be related to stable biases in
cognitive style, those with a more obsessional style being less vulnerable to stress and less prone to
cognitive failure. There is also some evidence that under stress cognitive styles may become more
extreme. Thus cognitive style may identify those who are vulnerable to life stress and even, possibly,
mediate a relationship between life stress and accident involvement.
ENABLING FACTORS
System-induced Errors
34. The first three enabling factors listed in Table 1 together account for about 40% of the accidents
conventionally described as due to "aircrew error". The potential for such system-induced errors
increases with the sophistication and power of the systems employed. Aviators increasingly rely on
indirect apprehension of important data and indirect control of the system. There are obvious benefits
in the use of technology to supplement human capabilities, but the designer of equipment faces real
challenges in devising suitable interfaces. Conflicting requirements have to be met. Both the novice
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and the expert require easily interpretable displays and accessible, simple controls. The expert,
however, may require more detailed information, or a more flexible operating style than the novice.
Ease of operation in controls is obviously desirable, but may facilitate mis-selections. In addition, the
training and administrative background set the context in which the operator works, and both can easily
provide opportunities for mis-information and confusion.
Professionalism
35. The variety of enabling factors is enormous. One unifying aspect is the fact that such problems
are identifiable before they cause an accident, and, in contrast with most of the psychological factors
discussed above, are in principle amenable to relatively simple remedies. The inquiries into many
major disasters (Chernobyl, Challenger, the Herald of Free Enterprise) have provided examples. The
failure to remedy this type of problem may be due to lack of imagination, error of judgement or an
unfortunate ordering of priorities. Professionals expect to be able to cope. Performing under less than
optimum conditions does afford some satisfaction. Complaining about inadequate equipment, or
questioning common practice may seem "unprofessional", particularly if it involves an admission that
something is not understood. In both military and civilian aviation steps have been taken to circumvent
this problem, and these are dealt with under "Remedies" below.
REMEDIES
Analysis
36. The picture of the human presented above appears somewhat discouraging. Although capable of
acquiring complex skills and of retaining large amounts of knowledge, he is yet likely to fail in a number of
ways. If the task is unusually demanding or the context too stressful, his behaviour is likely to become
disorganized, skills may break down, information may well be overlooked, disregarded, or improperly
interpreted; if things are going well, and the task demands are slight, the experienced operator may yet
fail to execute 'simple' procedures properly, or become inattentive. Depending on his personality, he may
tolerate stress only poorly, or impulsively deviate from standard practice. Some of these problems are
relatively intractable features of human nature. Long term research may eventually provide palliatives.
But our perspective so far has been exclusively error-orientated. It is as well to bear in mind that, in
contrast with the situation in experimental studies, it is not unusual for many hours of successful
performance to separate errors in real tasks, and humans do routinely identify and correct slips, lapses
and misunderstandings. The very rarity of error makes applied research and planning for practical
remedial action difficult. Nevertheless, pro-active remedial action is possible. Three major features of
pro-active effort in aviation are standardization, simulation and competency checks.
Standards
37. Standardization applies to all aspects of aviation. As far as possible all the equipment fitted to a
fleet of aircraft will conform to a common standard. To some extent common standards are applied
across fleets. There are economic reasons for this approach, but safety advantages accrue as well.
More importantly, standard operating procedures are defined according to the best available advice
and experience. This ensures that crews who have never met before can work together efficiently and
safely, and that the best practice is applied universally. When flaws in equipment design or procedures
do come to light, again remedies can be applied universally; the number and variety of risks latent in
the system is minimized.
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38. An advantage of standardization is the criteria it provides against which to judge performance.
Aircrew are regularly assessed for basic skills and for their ability to cope with emergencies. A
standard set of emergencies is defined (for example engine failure during take-off), and the crew-
members have to demonstrate their ability to handle these emergencies at regular intervals. The
procedures under which these skills are assessed are themselves monitored and standardized by
independent authorities.
Simulators
39. Simulation has provided the means of testing skills in emergency situations safely, and more
effectively. The technology used in flight simulators can support effective assessment and instruction
through the use of replays, graphical records etc. Regular simulator training in emergencies, besides
ensuring competence in handling the most likely and most threatening mishaps, also increases the
crew’s general confidence and may make them more resistant to the stress of unexpected problems.
In addition, simulators are increasingly being used in support of special courses designed to make
aircrew aware of human factors issues and, in particular, the effective use of resources, both material
and human. A prominent element of many such courses is guidance in interpersonal relations and
successful crew co-operation.
Feedback
40. The management of any modern flying enterprise necessarily involves consideration of safety
issues. Not only accidents, but also minor incidents are investigated thoroughly. The lessons learned
are fed back into operational practice through flight safety organizations that permeate almost every
level of the management structure. These organizations also seek out hazards through flight safety
inspections or routine monitoring of the records made by airborne data recorders. Such measures
obviously have to be applied with a degree of tact and sympathy, if the co-operation of the operators is
to be maintained.
Reporting
41. In addition to the mandatory incident reporting schemes, both military and civilian operators also
have confidential reporting systems. Again there is potential for unnecessary embarrassment or
mistrust but, with care, such schemes do provide valuable information and, obviously, have the
potential to prevent accidents.
CONCLUSIONS
42. Many of the most important errors derive directly from normal characteristics of human skilled
behaviour. General principles on how to cater for this vulnerability are by no means established, but
recognizing the fact has, at least, moved the debate on in aviation from issues of blame to research
issues and possible remedy. In general, by relying on standardized procedures, aviation seems to
have reduced the potential for errors in knowledge-based activity; as a result the predominant, primary,
immediate cause of aircrew error appears to be cognitive failure.
43. There is some evidence suggesting that personality may predispose some individuals to a
particular type of risk. Many airlines use personality tests in their selection procedures. Such tools
obviously provide only guidance rather than identification, and their utility in a selection process
inevitably depends on the ratio of high quality candidates to vacant posts.
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44. The stress due to emergencies does contribute to aircrew error, but regular simulator training, and
competency checks, serve to reduce its impact. The role of domestic stress is best described as
undecided. The nature of the military aviation task may provide a measure of protection. Many other
stresses associated with flying are routinely contained. Fatigue may, in general, be counted among
these because of regulation and monitoring.
45. Aviation has, in large measure, embraced a safety orientated culture. Standardization, simulation
and competency checks are entrenched in the system, and serve to limit the potential for risk and its
impact. In addition, there is an active interest in identifying risks through inspection, investigation and
incident reporting schemes.
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AP3456 - 6-2 - Leadership and Captaincy
CHAPTER 2 - LEADERSHIP AND CAPTAINCY
Introduction
1.
The Aircraft Commander is the aircrew member designated by a competent authority as being in
command of an aircraft, and responsible for its safe operation and accomplishment of the assigned
mission (see Military Aviation Authority (MAA) Regulatory Article (RA) 2115).
2.
Captaincy is the generic term used for the judgement and asset management skills of aircrew when
performing their primary duties as an aircraft commander. Although the Aircraft Commander is usually a
pilot, in some aircraft roles the Aircraft Commander may be of other aircrew category. MAA RA 2115
establishes the authority and defines the responsibilities of an Aircraft Commander. It does not, however,
cover the qualities required by such a team leader. Those qualities are described within this chapter.
Leadership
3.
Leadership is a quality difficult to define, although it has long been recognized and appreciated by
the human race. Leadership is required to carry through any enterprise of importance and often
seemingly hopeless causes have been brought to a successful conclusion due to the determination
and personal influence of some individual. A leader has an aim and plans out the activities required to
achieve it. Tasks are allocated to the most suitable subordinate and, by encouragement, drive and
example, the leader inspires them to success. The leader considers the problems of others and
exercises sound judgement in whether to allow these to influence progress towards the aim.
4.
Leadership is already detected in all aircrew because many of those personal attributes are
required to become aircrew. Many of these attributes are inherent, and the great leaders throughout
history were probably born such. However, much can be done to acquire leadership qualities and
those qualities already present can be honed by careful study, thought and training.
The Aircraft Commander
5.
An aircraft’s crew is composed of professionally competent people all possessing leadership
qualities to a greater or lesser extent. Nevertheless, it is essential that one member should be
appointed as the Aircraft Commander, and should be recognized as such, in order to direct the crew’s
efforts and take overall responsibility for achieving the task. The Aircraft Commander should possess
and demonstrate suitable qualities, including:
a.
The ability to influence by personal example, in terms of character and ideals.
b.
Sufficient professional ability to command the respect of others.
c.
Attentiveness to the administration and welfare of crew members, thereby fostering an
opportunity for mutual loyalties to develop.
The Qualities of a Captain
6.
The qualities of a captain are broadly those of a good officer and include:
a.
Skill and experience.
b.
Moral character, which includes:
(1) Personality.
(2) Tenacity.
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(3) Loyalty.
(4) Sense of responsibility.
(5) Personal influence.
(6) Courage.
(7) Initiative.
c.
Physical and mental fitness.
The ways in which an Aircraft Commander employs these personal qualities, and develops the skills of
captaincy, will influence the level of achievement by the crew.
7.
Skill and Experience. A very high degree of skill is needed to ensure that an aircraft is operated
to its maximum capability. Flying is a professional business and a good captain is one whose
professional standards are such as to be beyond the criticism of the crew. The captain must
endeavour to extract the maximum value from every sortie and should consult other aircrew of known
ability and experience. Experience comes only with time and exposure to the problems of aviation.
Not all captains can be well experienced at the outset but the ability to learn quickly, and thus gain
experience, will rapidly improve captaincy skills. The more experience gained, coupled with foresight
and careful planning, the more successfully a captain will be able to anticipate difficult situations and
lead the crew to deal with them.
8.
Personality. Personality is generally understood to be the distinction of personal character, the means
whereby one individual is distinguished from another. Personal integrity is essential to a good personality
and is a quality which promotes trust. A captain’s integrity must be unquestionable and beyond reproach; a
good example must be set to the crew members in all things, both professional and social. A captain should
be patient, cheerful, understanding and flexible. However, the captain’s personality must be strong enough
to leave the crew in no doubt that the captain’s role is primarily as a commander, with authority over them
and with responsibility for crew discipline. If necessary, individual crew members must be prevailed upon to
adjust their own personalities in the interests of crew harmony.
9.
Tenacity. Tenacity is resoluteness combined with persistence. It is closely allied to determination
and encompasses the desire and ability to see a difficult matter through to a successful conclusion in
spite of disheartening or apparently overwhelming odds.
10.
Loyalty. An Aircraft Commander must be loyal to superiors and subordinates alike and this
loyalty must be manifestly sincere. A captain should feel a moral obligation to justify and respond to
the faith and trust proffered by others.
11.
Sense of Responsibility.
Aircrew are expected to have a highly developed sense of
responsibility. It is the Aircraft Commander’s task to foster this and give it purpose. Members of the
crew must be made aware of the importance of their tasks. The captain should take a detailed interest
in an individual’s activities and offer good advice or make valid criticism in order to encourage
excellence. The Aircraft Commander is responsible for crew coordination. This implies obtaining the
wholehearted and active cooperation of the crew in ensuring that there is no unnecessary duplication
of effort and that all the aircraft’s systems and facilities are utilized to the maximum efficiency. The
Aircraft Commander should take an interest in crew welfare in a tactful and unobtrusive way to alleviate
problems which are likely to affect the efficiency of the crew.
12.
Personal Influence. Personal influence is the ability to inspire a crew to further efforts when their
inclination is to give up or turn back. The personal influence of a good Aircraft Commander should
ensure that the crew members invariably give of their best.
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AP3456 - 6-2 - Leadership and Captaincy
13.
Courage. Courage is of two kinds - mental or moral, and physical. Courage is not synonymous
with fearlessness. Indeed, if one is not afraid, one cannot show courage since courage is an effort of
will to overcome fear. Physical courage is the ability to stick to the job to the end despite injury,
privation and approaching death. Moral courage is reflected in the attitude of mind required to make
just but unpopular decisions.
14.
Initiative. Initiative may be said to be the ability to combine and utilize common sense, foresight
and imagination under difficult conditions. More specifically, it is the ability of an individual to originate
a course of action without prior reference to his superiors, in order to cope with unexpected
circumstances. It consists of refusing to be defeated by circumstances or events for which no specific
instructions have been given. It is affected by personal integrity, professional knowledge, courage and
confidence and should be in the make-up of all aircrew.
15.
Physical and Mental Fitness. The ever-increasing performance of modern aircraft demands the
highest levels of physical and mental fitness. One of the essential requirements of leadership, mental
stamina (the ability to think clearly and act decisively and quickly in an emergency) will suffer if fitness
is not maintained. Cockpit complexities in a fast-moving scenario require the captain to have high
levels of mental and physical stamina although some of the burden can be relieved, but not eliminated,
in multi-crew aircraft. The possibility of having to react to emergencies often calls on hidden reserves
of physical strength. Furthermore, the fortitude derived from physical and mental fitness may well be
required in any ensuing survival situation, particularly if captured or hurt or if injured crew members are
involved. It is important for crews to understand the factors which go to make up the particular state of
health necessary for maximum efficiency, and Aircraft Commanders should set a good example to
their crews.
Training
16. Training instils confidence and the ability to react instinctively to both routine and unexpected
incidents. It is of particular value in an emergency where instinctive execution of emergency drills is
vital. A good captain should appreciate the value and importance of training in the broader concept.
Every flight should be analysed for its training value and post-flight discussions should be held to
augment knowledge and take the benefit of experience. Regular training opportunities must be taken,
particularly to practise emergency procedures.
Conclusion
17. The qualities of a good captain are present, to a greater or lesser extent, in the make-up of all
aircrew. The responsibilities of the Aircraft Commander are laid down in MAA RA2115 and are reinforced
by other local orders and instructions. The Aircraft Commander has a most responsible job which calls
for mature judgement and sound leadership, often under the most difficult of circumstances. The Aircraft
Commander is ultimately responsible for the safety of the aircraft, its crew, its passengers and its cargo
whilst carrying out a specified mission. An intelligent study of the requirements of captaincy, the fostering
of natural talent and the acquisition of the appropriate skills and techniques, together with the genuine
desire to be a good leader, go a long way towards achieving such an aim.
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AP3456 - 6-3 - Airmanship
CHAPTER 3 - AIRMANSHIP
Introduction
1.
The term 'airmanship' is akin to 'seamanship' as used for mariners. Both terms represent a level
of knowledge and skill that is desired, and essential, to one who aspires to operate safely within such
environments.
2.
A demonstration of poor airmanship will produce poor professional results, failure on courses of
training, and possibly result in an aircraft accident. Some aspects of airmanship are not as
straightforward as "see something, do something". These more complex aspects involve the thought
processes concerned with decision-making. Breaking airmanship down into its constituent parts would
provide a basis for improved, specifically targeted, training.
3.
The Royal Air Force has developed training objectives and a diagnostic tool for the assessment of
airmanship performance. This assessment method has been mandated at all RAF flying training units,
and is known as the 'RAF Airmanship Model'. However, the airmanship performance principles upon
which the RAF Airmanship Model is based can be applied to all aircrew. Within this chapter, therefore,
the terms 'aviator', 'aircrew' and 'student' are used synonymously.
MEMORY FUNCTIONS
4.
To understand the requirements of the RAF Airmanship Model, some knowledge of basic
psychology and memory function is beneficial. Volume 6, Chapter 1 explained human performance in
such respect. The following paragraphs summarize the relevant background points and explain
terminology.
Memory
5.
For ease of illustration, a computer analogy is used to describe the functional areas of the brain
that are relevant to airmanship. Although these areas are complex and not fully understood, there is
agreement amongst scientists and psychologists at the following level of detail. Memory is itself
divided into long and short term:
a.
Long-term Memory. Long-term memory can be compared to a computer’s hard drive. It is
where all permanent data is stored and can be further divided into several areas. For simplicity,
only the following areas need to be considered:
(1)
Semantic Memory. Semantic memory is where data such as facts, rules and
information are stored. Examples would include cockpit checks and procedures, emergency
drills, and technical data.
(2)
Motor Memory. Motor (or mechanical) skills, such as aircraft handling, are stored in
motor memory after practice or consolidation. They can be accessed and used sub-
consciously and, as such, do not always use up short-term memory either for processing or
for monitoring.
(3)
Episodic Memory. Specific episodes or events, such as a first solo flight, complex in-
flight emergencies, or memory of the last sortie or combat are stored in episodic memory.
These stored events are the individual’s perception of what happened, rather than what
actually happened, and may be altered by more recent events, new perceptions or a debrief.
Episodic memory can be likened to a stored low definition photo or video clip.
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AP3456 - 6-3 - Airmanship
b.
Short-term Memory. Short-term memory (or working memory) equates to a computer’s RAM
(Random Access Memory). The capacity of this memory depends on the individual, but is limited to
between 5 and 9 'slots' available for holding data. Some data may be compressed to maximize storage
space in short-term memory. Such examples are regularly-used radio frequencies, or telephone
numbers, that tend to be stored and recalled as one 'chunk' of data rather than individual digits. The
data will only be held for between 10 and 30 seconds, unless it is refreshed or updated, and it may be
corrupted by other data inputs. This is effectively where data from the senses (seeing, hearing,
touching etc), and that retrieved for use from long-term memory, is held whilst it is processed. The
number of available 'slots' dictates how many tasks (or what types of tasks) an individual can perform
simultaneously; this is often labelled 'mental capacity' by aircrew.
Mental Capacity
6.
Figure 1 illustrates how short-term memory deals with primary and secondary tasks. The
rectangle represents a person’s full information-processing capacity. The shaded area represents a
hypothetical primary task with the clear area showing the amount of resource available for completing
a secondary task, ie spare mental capacity.
6-3 Fig 1 Short-term Memory - Processing Capacity
Secondary
Task
Secondary
Task
Primary
Primary
Task
Task
a Dealing with a Familiar Primary Task
b Dealing with a Less Familiar Primary Task
7.
There is a limit to the amount of information that the human mind can process at any one time.
The human mind can allocate the mental resources required by the primary task, and, by default, the
available reserve capacity will be determined. Therefore, the higher the demand of the primary task,
the lower the spare capacity that can be allocated to the secondary task and, consequently, the worse
the performance of the secondary task. During flying training, pilots usually manage to successfully
defend the performance of the primary task (flying the aircraft) from secondary task intrusion.
Consequently, flight control error does not normally increase as a result of secondary tasks being
present. So, within the broad concept of the human mind having limited information processing
capability, and that the performance of a primary task uses some of those resources, the ability to carry
out a secondary task is a useful tool in determining the spare 'mental capacity' of a pilot. It has also
been identified that, in flying training, it is important to ensure that the primary skill is automated as
much as possible (by practice and experience), thereby freeing up more capacity for the secondary
task, which includes further learning.
Perception
8.
Perception is an important factor when studying both cognitive and psychomotor performance.
This is because sensory data, episodic memory and the way the analysis is performed are all subject
to modification based on stored perceptions. An example of this is that, sometimes, people hear what
they expect or want to hear, rather than what is actually said. This can also manifest itself in a sortie
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AP3456 - 6-3 - Airmanship
debrief, which may reveal different versions of the same sortie from different aircrew. In essence,
perception plays a major part in the construction of the mental picture of the surrounding environment
that is essential to situational awareness, airmanship and the decision-making process.
9.
Investigation has demonstrated that perception emerges repeatedly as one of the most important
factors in the whole learning and thinking process. Fig 2 illustrates the stages of the decision-making
process. The required end product is an action, but to perform an action a prior decision has to be
made. To make the decision, a number of factors must be analysed, but first, those factors must be
correctly recognized or perceived. If they are not correctly recognized or perceived, the required
decision-making cycle will not be followed. Thus, perception of an event or factor is critical to the
whole airmanship process.
6-3 Fig 2 Perception in the Decision-making Process
Perception
Recognition
Analysis
Decision
Action
10. Because of the importance of perception in any decision-making process, one of the core aims
during any training course should be to assist in the formation of perceptions. In order for a factor to
be perceived correctly, knowledge must be placed in context. Perception can be considered to consist
of both knowledge and contextual experience, as shown at Fig 3. Therefore, training may need to
focus on these discrete, but closely related, areas.
6-3 Fig 3 Perception Formation
Contextual
Knowledge
Experience
Perception
11. Effective decision-making loops are essential to good airmanship. Aircrew are required to
observe, and react to, events that occur both within the cockpit and in the environment outside the
aircraft. They must then use the information sensed in order to make decisions and take actions,
which will ensure the safety of the aircraft.
THE DECISION-MAKING LOOP
The Need for Effective Decision-making
12. The importance of decision-making loops has been recognized for a long time. An early concept,
developed by Col John Boyd USAF, was the 'OODA Loop' - Observe, Orient, Decide, Act. This loop
was used by members of the USAF’s fighter community of the late 1960’s, as the basis to redefine
fighter tactics. The derivation of this loop was based on air-to-air combat, where time is a critical
factor. The participant who was able to complete the loop in the least time was likely to gain the
advantage, as he would be able to actively dictate the fight whilst his opponent would be forced to
become entirely reactive. Because of the specific and time-related nature of this decision-making loop,
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AP3456 - 6-3 - Airmanship
its use in routine airmanship assessment is limited. However, the OODA Loop served as a useful
basis for developing a decision-making cycle for the RAF.
The RAPDA(R) Decision-making Loop
13. As part of the development of the RAF Airmanship Model, the need was identified for a
comprehensive decision-making loop, which could be applied to general situations. The resultant
decision-making loop, which encompasses the principles of applied airmanship, is the
Recognize
Analyse
Prioritize
Decide
Act (
Review) loop (Fig 4). This loop requires the student to:
a.
Recognize. Everyone observes their environment, but the key skill is to recognize those
significant events or factors, within that environment, that are important or likely to impact on the
performance of the task. Therefore, awareness or perception of what is, and what is not,
important is required. If an event or factor is not recognized as important, it may be ignored and
not receive attention and resources for analysis.
b.
Analyse. Once an event or factor has been recognized as significant, it must be analysed for
its likely effects.
c.
Prioritize. When faced with multiple or newly recognized events or factors, an appropriate
priority must be allocated during the analysis phase so that the task can be accomplished
efficiently. This requires an awareness of the relative importance of each individual recognized
event or factor. It could be argued that prioritization is required before the sequential analysis of
multiple factors; however, this order was chosen as some analysis is likely to be required before
accurate priorities can be allocated, and there may not always be multiple factors to prioritize.
d.
Decide. The student must then decide on the most appropriate course of action.
e.
Act. Having made the decision, the student must initiate the most appropriate course of action.
f.
Review. An important part of any decision-making process is to ensure that the correct
actions have been taken. The environment will also continue to change, possibly as a result of
the action. Any new significant events or factors must be recognized to prompt the loop to be re-
started. Since the action originates from the recognition and analysis phases of the decision
cycle, it is essential that the whole process is reviewed, not just the action taken.
6-3 Fig 4 The RAPDA(R) Decision-making Loop
Recognize
Analyse
Prioritize
Decide
Act
REVIEW
Environmental change as a result of actions
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AP3456 - 6-3 - Airmanship
THE RAF AIRMANSHIP MODEL
14. The RAF Airmanship Model comprises the following elements:
a.
Situational Awareness.
b.
Decisiveness.
c.
Communication.
d.
Resource Management.
e.
Mental Performance.
f.
Spare Mental Capacity.
Situational Awareness
15. Situational Awareness (SA) is widely regarded as the dominant factor in aircrew cognitive ability -
those aircrew with good SA tend to be successful whilst those with poor SA are not. SA requires
awareness of current position within the environment and recognition of factors significant to the sortie
aim within that environment. There are two types of SA demanded of aircrew - positional and tactical.
16.
Positional SA. Aircrew must be aware of all the information required for the normal, safe
positioning of the aircraft. This requirement is known as 'Positional SA'. To possess positional SA,
aircrew must be aware of current and projected aircraft position in terms of:
a.
Height, attitude, and speed.
b.
Geographic location and proximity to geographical features such as coastlines, mountains,
cities and airfields.
c.
Aeronautical features including airways and other controlled airspace, navigation aids and
procedural flight paths.
d.
Meteorological features including clouds, precipitation, turbulence, tropopause, reduced
visibility and jetstreams.
e.
Other aircraft.
17.
Tactical SA. Aircrew must also recognize events or factors that may affect the current, future or
possible operation of the aircraft or formation. This task is complex as it both complements positional
SA and is separate from it. This task is known as 'Tactical SA'. For example, once the aviator
recognizes that he is about to penetrate an airway (positional SA), tactical SA would involve recognition
that permission was required and that, if it had not been gained, to continue would be highly likely to
prejudice the timely achievement of the sortie aims. As another example, tactical SA would include the
realization, from a background transmission to another aircraft, that the weather had deteriorated and a
change to diversion fuel allowance may be required. Emergencies also fall into tactical SA, in that a
malfunction is an event that is likely to affect the future operation of the aircraft or formation.
18.
Summary. Many events and factors are included in SA. Positional SA is awareness of position within
the environment, while tactical SA is recognition of other significant factors within that environment, and the
appreciation of the unexpected. A poor performance in SA may show that the student has not developed a
perception of the importance or relative importance of environmental factors. It does not show that the
student is incapable of analysis or that there is insufficient spare mental capacity.
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AP3456 - 6-3 - Airmanship
Decisiveness
19. Making decisions is a critical part of airmanship. In addition to recognizing and analysing factors,
aircrew must be capable of making a decision as to the required action. Within decision-making itself,
there is a clear distinction between merely making a decision and making the correct or most
appropriate decision. Furthermore, once a decision has been made, on many occasions it must be
implemented by positive action. Initiating positive action itself requires a further decision.
a.
Decision-making. A student may be able to recognize and analyse factors, but if there is no
resultant decision there will be no subsequent action. A poor performance in this area indicates
an indecisive student.
b.
Quality of Decisions. Ideally, the decision should be correct for the circumstances.
However, in some situations there may be more than one acceptable solution, in which case the
decision should be 'reasonable'. An incorrect decision may be derived from poor analysis,
incorrect perceptions, or failure to recognize significant environmental factors.
c.
Translation of Decision into Action. Having made a decision, it must be implemented.
This will require the correct action(s) to be taken. Some decisions result in positive action but, in
some cases, the best course of action may be to do nothing. However, that must also be a
positive decision, and not occur through failure to act or a lack of decision. Poor ability in this area
may indicate a lack of self-confidence on the part of a student.
Communication
20. Communication is the passing of information and is required to support SA and elements of
analysis and decision-making. It is also likely to be required to enable a crew or formation to act once
a decision has been made. In all circumstances, communication must be effective. It is also
necessary to pass or acquire information using standard R/T phraseology where it exists.
Communication may be:
a.
Within the aircraft.
b.
Within a formation.
c.
With external agencies.
To meet these demands, aircrew must have fluent ability in R/T phraseology and be knowledgeable in
terms of standard terminology and phrases used at unit level.
Resource Management
21. Management of available resources is a key component skill within airmanship. Use of the
available resources may be categorized as:
a.
Management of Aircraft Systems. In addition to having sufficient knowledge of the aircraft
systems, the aviator must be able to apply that knowledge to ensure the efficient and safe
operation of the aircraft in flight, particularly within an operational environment.
b.
Management of Cockpit Resources. The organization of cockpit resources includes flight
instrument displays, maps and documents to allow timely and efficient access to relevant
information for the operation of the aircraft or formation. One example might be the positioning of
maps, to allow smooth transition from the navigational chart to the target map during the approach
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AP3456 - 6-3 - Airmanship
to the target area. Another example would be the appropriate layering of display pages on multi-
function instruments.
c.
Management of Crew/Formation Resources. Control of other crew members, and other
aircraft within the formation must be effective. It must also make a timely contribution towards
sortie aims. Management and/or direction of individual contributions should enhance team
awareness and analysis in pursuance of the task.
d.
Management of External Resources. Resources external to the aircraft must also be managed
to ensure safe operation, and efficient pursuance of the task. Those resources external to the aircraft
include other formation members, air traffic control (ATC), ground control intercept (GCI) facilities, or
other airborne or surface forces. Aircrew need to be knowledgeable in terms of the services on offer
from, or requirements specific to, the external resource concerned.
Mental Performance
22. There are a number of cognitive skills that must be mastered by aircrew for successful operation
of an aircraft:
a.
Situational Analysis. It is important to analyse recognized events or factors, including
associated risks, and make appropriate plans and projections whilst continuing to operate the
aircraft safely. This requires the holding of a sufficient quantity of information in short-term
memory and processing it whilst simultaneously performing routine psychomotor skills.
b.
Priority Allocation. Where multiple events are presented, and recognized, it is essential for
the aviator to prioritize, make plans, and implement actions, in an appropriate order, and in
pursuance of the task.
c.
Mental Flexibility. The aviator may have to make new plans (or change existing ones), as a
result of changed circumstances which occur during flight. As always, the aim is to continue the
pursuance of the task.
Spare Mental Capacity
23. Flying an aircraft requires the aviator to perform the required tasks to a high standard. In addition,
the aviator must be able to recognize and deal with unexpected tasks and events, such as
emergencies. It is the ability to cope with this additional workload that is termed 'Spare Mental
Capacity'. For example, a student pilot must be capable of flying a loop to a prescribed standard.
However, whilst doing so, should an emergency occur, it must be recognized, and the correct actions
taken - whilst still maintaining control of the aircraft.
24. In the training environment, experience will have an important effect on a student’s level of spare
mental capacity.
a.
Relatively new pilots are using cognitive skills all the time, in order to learn and practise new
flying skills. With time, many of these tasks will become automated and move into motor memory.
b.
As a student advances through a course, the level of spare capacity exhibited will vary
markedly depending on the exercise that is being undertaken. For example, it could be
anticipated that a student on an initial formation sortie may have very little, or even zero, spare
capacity due to the high workload. However, the same student, on the final sortie of formation
training, would be expected to demonstrate a fair degree of spare capacity.
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AP3456 -.6-4 - Physiological Effects of Altitude
CHAPTER 4 - PHYSIOLOGICAL EFFECTS OF ALTITUDE
Introduction
1.
Flight at high altitude exposes flying personnel to environmental conditions in which the
unprotected human body may not be able to function. It is important, therefore, that the physical
limitations of the body, and method of extending these limitations, are thoroughly understood by all
aircrew, and particularly by captains of aircraft who may be responsible for the safety and well-being of
untrained passengers.
2.
In order to understand the effects of altitude on humans, it is essential to know something about
the characteristics of the atmosphere, and also to have a basic understanding of the requirements of
the human respiratory system.
Physics of the Atmosphere
3.
Physics of the atmosphere is dealt with fully in Volume 1, Chapter 1; it is only necessary here to
emphasize a few factors which are of particular significance in a study of the effects of altitude on the
aviator.
a.
Composition of the Atmosphere. For all practical purposes, the composition of the
atmosphere can be considered constant from ground level to 300,000 ft. The composition, by
volume, of dry air is:
(1) 21 % oxygen.
(2) 79 % nitrogen (including the rare gases, of which argon is the main one).
(3) A trace of carbon dioxide.
Ozone, which is formed by the action of ultraviolet radiation on oxygen, is also present at trace
concentrations. The concentration of water vapour in the atmosphere varies with the degree of
saturation (relative humidity) and the temperature. Typically, water vapour forms 1 to 2% by
volume of atmospheric air.
b.
Atmospheric Pressure and Altitude. With ascent from the surface of the earth, the
atmosphere becomes progressively less dense. Thus, the pressure exerted by the atmosphere
falls in an approximately exponential manner with vertical distance from the ground, the pressure
at an altitude of 18,000 ft being half that at sea-level. The relationship between the pressure
exerted by the atmosphere and altitude (ICAO international standard atmosphere) is given, in an
abbreviated form, in Table 1. Since the relationship between atmospheric pressure and altitude is
exponential, the change of pressure for a given change of altitude falls with ascent to altitude.
The change of pressure per 1,000 ft change of altitude is illustrated in Table 2.
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AP3456 -.6-4 - Physiological Effects of Altitude
Table 1 Relationship Between Atmospheric Pressure and Altitude
Pressure
Altitude (ft)
mm Hg
psi
KPa
mb
0
760
14.70
101.38
1013.25
8,000
565
10.92
75.37
752.91
18,000
380
7.34
50.69
506.08
25,000
282
5.45
37.62
376.00
34,000
188
3.63
25.08
250.28
40,000
141
2.72
18.81
187.60
50,000
87
1.68
11.61
116.00
60,000
54
1.04
7.20
71.71
100,000
8
0.16
1.07
11.03
Table 2 Change of Pressure with Altitude
Change of Pressure per
Altitude (ft)
1,000 ft Altitude
mm Hg
KPa
500
27.10
3.62
10,000
20.20
2.70
20,000
14.60
1.95
30,000
10.30
1.37
40,000
6.80
0.91
c.
Partial Pressure of Gases. The physiological effects of a given gas are related to its
molecular concentration, which is expressed as the 'partial pressure' of the gas. The partial
pressure exerted by a gas, in a mixture of gases, is the pressure which it would exert if it alone
occupied the same volume as the whole mixture. Thus, the partial pressure of a gas x, (Px),
which constitutes y% by volume of a gas mixture having a total pressure of PT is given by:
y
P =
× P
x
T
100
For example, the partial pressure of oxygen (PO ) in dry air at a pressure of 760 mm Hg is:
2
21× 760
P
=
= 160 mm Hg
O2
100
Similarly, the partial pressure of nitrogen (PN ) in dry air at a pressure of 760 mm Hg is:
2
79 × 760
P
=
= 600 mm Hg
N 2
100
The sum of the partial pressures of the constituents of a gas mixture equals the total pressure
(PT) exerted by the mixture. Thus, for dry air:
+
=
O
P
N
P
T
P
2
2
Since the total pressure exerted by the atmosphere falls exponentially with altitude (see sub-para 3b), it
follows that the partial pressure of oxygen in dry air falls with altitude in a similar manner, as illustrated in
Table 3.
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AP3456 -.6-4 - Physiological Effects of Altitude
Table 3 Partial Pressure of Oxygen in the Atmosphere at Altitude
Partial Pressure of Oxygen
Altitude (ft)
mm Hg
KPa
0
160.00
21.34
8,000
118.70
15.83
18,000
79.80
10.65
25,000
59.20
7.90
40,000
29.60
3.95
d.
Temperature and Altitude. Solar radiation heats the surface of the Earth, and this warms
the lowest layer of the atmosphere. Above the surface of the Earth, the temperature falls steadily
with altitude throughout the troposphere at the adiabatic rate of approximately 2 ºC per 1,000 ft.
This fall in temperature ceases at the tropopause, which varies in height around 35,000 ft (the
tropopause is higher over the equator, and lower over the poles). In the stratosphere, the
temperature is fairly constant at about –55 ºC.
e.
International Standard Atmosphere. The International Standard Atmosphere is derived
from average conditions, and is defined fully in Volume 1, Chapter 1, Paragraph 10.
Anatomy and Physiology of Respiration
4.
The energy essential for living processes is obtained by the oxidation of complex foodstuffs. Thus,
oxygen is one of the most important materials required for the maintenance of normal function by living cells.
The cells of the brain are particularly sensitive to a lack of oxygen. The human body is only able to store very
small quantities of oxygen. Thus, cessation of the oxygen supply to the brain results in unconsciousness in
six to eight seconds, and irreversible damage ensues if the oxygen supply is cut off completely for longer
than about four minutes. The maintenance of normal function requires that oxygen be delivered to the cells
of all tissues of the body, and that the supply is matched to the rate of consumption of oxygen, so that the
partial pressure of oxygen (PO ) is maintained above a certain critical value. Oxidation of complex foodstuffs
2
produces, amongst other substances, carbon dioxide. The carbon dioxide so formed must be removed from
the tissues and vented to the atmosphere, since accumulation in the tissues interferes with normal function.
The process whereby the oxygen in the atmosphere is transported to the tissues, and the carbon dioxide in
the tissues is transported to the atmosphere, is termed 'respiration'. Several steps are involved in these
transport systems:
a.
Exchange between the atmosphere and the gas within the lungs - by ventilation of the lungs
(breathing).
b.
Carriage of oxygen and carbon dioxide between the lungs and the tissues by the circulating
blood.
c.
Exchange between the circulating blood and the tissues, where oxygen is consumed and
carbon dioxide is produced.
5.
Gas exchange between the external atmosphere and the blood, which transports oxygen and
carbon dioxide around the body, takes place within the lungs. The structure of the lungs is well suited
to promoting the rapid transfer of oxygen and carbon dioxide between the lung gas and the blood.
Within the lungs, the air passages divide repeatedly, ending eventually in very small air sacs (alveoli),
of which the adult lung contains some 300 million, giving an effective area for gas exchange of
between 50 and 100 square metres. The walls of the alveoli are very thin, and the blood flowing
Reviewed Nov 15
Page 3 of 22
AP3456 -.6-4 - Physiological Effects of Altitude
through the lungs is, therefore, brought into very close proximity to the gas in the air sacs (alveolar
gas). The passage of a gas across the walls of the alveoli is controlled by the difference of the partial
pressures of the gas in the blood and alveolar gas. Thus, oxygen is taken up by the blood flowing
through the lungs, as long as the partial pressure of oxygen (PO ) in the alveolar gas is greater than the
2
PO in the blood flowing into the lungs. As oxygen enters the blood, increasing the concentration of
2
oxygen in it, the PO of the blood also rises. The area of the alveolar wall is so great, and the wall
2
separating the alveolar gas and the blood is so thin, that the PO of the blood leaving the lungs is nearly
2
always equals to the PO in the alveolar gas. Similarly, the exchange of carbon dioxide is driven by the
2
difference between the partial pressure of carbon dioxide (PCO2) in the blood flowing into the lungs and
the lower PCO2 in the alveolar gas. In addition, the PCO2 of the blood leaving the lungs is equal to the
PCO2 in the alveolar gas. Thus, the PO and P
2
CO2 in the alveolar gas closely reflect the partial pressures
of these gases in the blood flowing from the lungs to the tissues of the body. The oxygen removed
from the alveolar gas by the blood is replenished by the ventilation of the lungs with air. This process
(external respiration) also removes the carbon dioxide added to the alveolar gas by the blood flowing
through the lungs.
6.
Air enters the nose and mouth during inspiration and is carried down, through the larynx (voice
box) and the trachea (windpipe), to the lungs. During its passage, the air is:
a.
Warmed to body temperature (37 ºC).
b.
Humidified, so that it becomes saturated with water vapour at body temperature (partial
pressure of water at 37 ºC is 47 mm Hg).
c.
Filtered.
Within the lungs, the inspired air mixes with the alveolar gas, thereby adding oxygen to it. Carbon
dioxide is carried to the atmosphere by the portion of the alveolar gas expelled from the lungs during
expiration. The ventilation of the lungs with air is normally regulated so that the PCO2 of the alveolar
gas is held constant over a wide range of rates of production of carbon dioxide by the tissues of the
body. Thus, at rest, the average volume of each breath is approximately 0.5 litres, and the average
rate of breathing is approximately 16 breaths per minute, so that the lung ventilation is 0.5 × 16 = 8
litres per minute. When the rate of production of carbon dioxide is increased, as in physical exercise,
both the depth and rate of breathing are increased.
7.
The composition of the alveolar gas depends on the composition of the inspired gas, and the
balance between ventilation of the lungs on the one hand and the rates of consumption of oxygen and
production of carbon dioxide on the other. It has already been stated (para 6) that the ventilation of the
lungs is normally regulated in relation to the latter, so that the PCO2 of the alveolar gas is held constant.
The 'normal' average value of the alveolar PCO2 is 40 mm Hg (range 38 to 42 mm Hg). The
composition of the alveolar gas when breathing air at sea level is given in Table 4. The table also
shows the concentration of each gas by volume of the dry gas.
Reviewed Nov 15
Page 4 of 22
AP3456 -.6-4 - Physiological Effects of Altitude
Table 4 Composition of Alveolar Gas - Breathing Air at Ground Level
Partial Pressure
Concentration
Gas
of Dry Gas by
mm Hg
KPa
Volume %
Oxygen
100
13.34
14.00
Carbon Dioxide
40
5.33
5.60
Nitrogen
573
76.44
80.40
Water Vapour
47
6.27
-
Total
760
101.38
100.00
8.
When breathing air at higher altitude, the fall of the PO in the atmosphere (see sub-para 3c)
2
produces a fall in the PO in the alveolar gas. Reduction of the alveolar oxygen tension to below 55 to 60
2
mm Hg produces a reflex increase in the ventilation of the lungs, so that the ventilation increases relative
to the rate of production of carbon dioxide by the body, and the alveolar PCO2 is reduced below normal.
The lower the alveolar PO is below 55 to 60 mm Hg, the greater is the increase in ventilation, and the
2
larger is the reduction of alveolar PCO2. The partial pressure exerted by the water vapour in the alveolar
gas is unaffected by ascent to altitude, as it depends solely on the temperature of the gas in the lungs,
which remains constant at 37 ºC. Typical values of the partial pressures of the constituents of the alveolar
gas, when breathing air at various altitudes, are illustrated in Table 5 and Fig 1.
6-4 Fig 1 Composition of Alveolar Gas - Breathing Air at Altitude
800
)
700
g
H
m
(m
600
re
u
s
s
re
500
P
l
rtia
a
400
P
300
Nitrogen
200
Oxygen
100
Carbon
Dioxide
Water
Vapour
0
10,000
20,000
30,000
Altitude (ft)
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Page 5 of 22
AP3456 -.6-4 - Physiological Effects of Altitude
Table 5 Typical Partial Pressures of Alveolar Gases when Breathing Air at Various Altitudes
Partial Pressure in Alveolar Gas of:
Altitude (ft)
Water Vapour
Oxygen
Carbon Dioxide
Nitrogen
mm Hg
KPa
mm Hg
KPa
mm Hg
KPa
mm Hg
KPa
0
47
6.27
100
13.34
40
5.33
573
76.44
8,000
47
6.27
65
8.67
40
5.33
413
56.43
18,000
47
6.27
40
5.33
28
3.74
265
35.35
25,000
47
6.27
30
4.00
22
2.93
183
24.41
35,000 (*)
47
6.27
18
2.40
12
1.60
102
13.74
(*) Immediately after rapid decompression to 35,000 ft
Carriage of Oxygen in the Body
9.
Oxygen is transported from the lungs to the tissues, and the carbon dioxide produced by the
tissues is transported to the lungs, by the circulating blood. Although both oxygen and carbon dioxide
are soluble in water, the amount of oxygen that is carried in solution in the blood is much too small to
meet the demands of the tissues. The blood red cells contain a red pigment (haemoglobin), with which
oxygen forms a loose compound (oxyhaemoglobin). The amount of oxygen held in the blood as
oxyhaemoglobin is a function of the partial pressure of oxygen in the blood (PO ). Oxygen is taken up
2
where the PO is higher, as in the lungs, and released where the P is lower, as in the tissues. A
2
O2
special mechanism also exists in the blood, whereby its capacity to dissolve carbon dioxide is greatly
increased compared to water. Carbon dioxide is taken up where the PCO2 is higher, as in the tissues,
and released where the PCO2 is lower, as in the lungs. As has been described earlier (para 5), the
partial pressures of oxygen and carbon dioxide of the blood leaving the lungs are equal to the partial
pressures of these gases in the alveolar gas. The blood pumped to the tissues, by the heart through
the systemic arteries, also has the same PO and P
2
CO2 as the alveolar gas. As the blood flows through
the extensive network of thin walled, small vessels (capillaries) which permeate all the tissues of the
body, oxygen is released and carbon dioxide is taken up. The blood flow to an organ is normally
regulated so that it matches the demands for oxygen delivery and carbon dioxide removal of its
tissues. When these increase, as in muscle tissue during physical exercise, the muscle blood flow,
and indeed the amount of blood pumped by the heart, are greatly increased. Thus, heavy physical
exercise, such as running, increases the output of the heart by about five times the resting value. The
matching of blood flow to tissue demands for oxygen is normally such that between 25% and 75% of
the oxygen contained in the arterial blood is given up by the blood as it flows through the tissues. The
blood flowing from the tissues to the lungs has therefore a lower PO , and a higher P
2
CO2, than the
arterial blood and the alveolar gas. These differences of partial pressure result in oxygen being taken
up, and carbon dioxide unloaded, as the blood flows through the lungs and comes into intimate contact
with the alveolar gas.
10. Because of the fall of the PO in the alveolar gas which occurs with ascent to altitude whilst
2
breathing air (para 8), the blood leaving the lungs and arriving at the tissue capillaries has both a
reduced PO and a lower oxygen content (see Fig 2). This reduction, if moderate, will not decrease the
2
rate at which oxygen is delivered to the tissues, but will reduce the partial pressure of oxygen in the
tissue. Several mechanisms, including an increase in blood flow, come into operation to minimize the
fall of PO in the tissues. If the reduction is more severe, then the P in parts of the tissues may fall to
2
O2
zero, in spite of the compensatory mechanisms coming into play. The critical level of alveolar PO at
2
which this situation arises in the brain, causing unconsciousness, is of the order of 30 to 35 mm Hg.
Reviewed Nov 15
Page 6 of 22
AP3456 -.6-4 - Physiological Effects of Altitude
6-4 Fig 2 Relationship between Blood and Alveolar Oxygen Pressure at Various Altitudes
Altitude (X 1,000)
Sea Level
22 20
15
10
5
100
n
e
g
80
y
x
O
ith
w
60
in
b
lo
g
o
m
e
40
a
H
f
o
n
tio
20
ra
tu
a
S
%
0
20
40
60
80
100
Partial Pressure of Alveolar Oxygen (mm Hg)
Hypoxia
11. Oxygen is one of the most important elements required for the maintenance of normal function by
living matter. The human body is extremely sensitive and vulnerable to the effects of deprivation of
oxygen. The absence of an adequate supply of oxygen (either in terms of quantity or partial pressure),
is called 'hypoxia', and almost always results in a rapid deterioration of most body functions, and may
cause death. A 25% reduction of the partial pressure of oxygen (PO ) in the atmosphere, associated
2
with ascent to an altitude of 8,000 ft, produces a detectable impairment of mental performance; whilst
sudden decompression to 50,000 ft, which reduces the alveolar PO to 10 mm Hg, causes
2
unconsciousness in ten seconds, and death in four to six minutes.
12. It is generally recognized that the most serious single hazard to humans during flight is the
reduction of the PO as a result of ascent to altitude. Failure of oxygen equipment and/or cabin
2
pressurization, so that the individual has to breathe air at high altitude, quickly leads to incapacitation,
and even death. The risks are greater in aviation, in that a degree of hypoxia which, from the
physiological viewpoint might not be fatal in itself, may have fatal results because of deterioration of
performance in an individual, leading to loss of control of an aircraft. Although improvements in the
performance and reliability of cabin pressurization and oxygen delivery systems have greatly reduced
incidents and accidents due to hypoxia, constant vigilance remains essential.
13. The causes of hypoxia in flight are:
a.
Ascent to altitude without supplemental oxygen.
b.
Failure of personal breathing equipment to supply oxygen at an adequate concentration
and/or pressure.
c.
Decompression of pressure cabins at high altitude.
d.
The presence of toxic fumes in the cabin.
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Page 7 of 22
AP3456 -.6-4 - Physiological Effects of Altitude
The rate at which the changes produced by breathing air at altitude take place, is a function of the
manner in which the condition is induced. Typically, the changes occur slowly as a result of ascent at
the usual rate for an aircraft (2,000 to 3,000 ft per minute); more rapidly by the reversion to breathing
air after failure of oxygen delivery equipment; and fastest by a rapid decompression. Although
breathing air during a steady ascent at 2,000 to 3,000 ft per minute is now an uncommon cause of
hypoxia, it is convenient to describe the changes induced in this way, since the relatively slow rate of
climb allows a semi-steady state to be maintained. The manner in which these changes are modified
by other causes of hypoxia will then be described.
Symptoms and Signs of Hypoxia
14. The speed and order of appearance of signs, and the severity of symptoms, produced by
breathing air at altitude depend on the final altitude, rate of ascent (or the rate of failure of the oxygen
supply at altitude) and duration of the exposure to altitude. Generally, the higher the altitude, the more
marked the symptoms. Rapid rates of ascent, however, allow higher altitudes to be reached before
severe symptoms occur. In these circumstances, unconsciousness may occur before any, or many, of
the symptoms of hypoxia appear. Even when these factors are kept constant, there is considerable
variation between individuals in the effects of hypoxia, although for the same individual the pattern of
effects does tend to follow the same trend from one occasion to another. Other factors affecting the
intensity of hypoxia at altitude include:
a.
Physical activity: exercise exacerbates the features of hypoxia.
b.
Ambient temperature: a cold environment will reduce tolerance to hypoxia, in part at least, by
increasing metabolic workload.
c.
Illness: the additional metabolic load imposed by ill health will increase susceptibility to
hypoxia.
d.
Use of certain drugs, including alcohol.
15. The effects of slow ascent (less than 4,000 ft per min) to altitude whilst breathing air are as
follows:
a.
Altitudes up to 10,000 ft. At altitudes up to 10,000 ft, the seated individual has no symptoms
(except during heavy exercise). The ability to perform most complex tasks is unimpaired. The speed of
reaction to novel conditions is, however, significantly impaired at altitudes above about 8,000 ft. It is
possible to show in the laboratory, that the ability to detect targets at low levels of illumination is impaired
at altitudes above 6,000 ft.
b.
Altitudes between 10,000 and 15,000 ft. At altitudes between 10,000 and 15,000 ft, the
resting individual has little or nothing in the way of symptoms, but the ability to perform skilled
tasks, such as aircraft control and navigation, becomes progressively impaired; the impairment
increasing with altitude above 10,000 ft. The individual is frequently unaware of the hypoxia, or of
the impairment of performance it produces. Indeed, a common misconception is that performance
is better than usual! Physical exercise, particularly at altitudes above 12,000 ft, frequently
produces mild symptoms, especially breathlessness. Exposure to these altitudes for longer than
10 to 20 minutes often induces a severe headache.
c.
Altitudes between 15,000 and 20,000 ft. Above about 15,000 ft, symptoms of hypoxia
occur, even in individuals at rest. There is marked impairment of performance, even of simple
Reviewed Nov 15
Page 8 of 22
AP3456 -.6-4 - Physiological Effects of Altitude
tasks, together with a loss of critical judgement and will power. Because of the loss of self-
criticism, there is usually a lack of awareness of any deterioration in performance or indeed the
presence of hypoxia. Thinking is slowed, there is muscular inco-ordination, with trembling and
clumsiness, and marked changes in emotional state. Thus, the individual may become euphoric,
garrulous, pugnacious, or morose, and perhaps physically violent. Again, the individual usually
has no awareness of the condition; an effect which makes hypoxia such a potentially dangerous
hazard in aviation. The individual frequently feels light-headed and experiences tingling in the lips
and limbs. Darkening of vision is a common symptom, although, generally, the subject is unaware
of the change until oxygen is restored, when there is a marked apparent brightening of the level of
illumination. Hearing is not usually markedly impaired, until the hypoxia becomes severe.
Physical exertion greatly increases the severity of all of the effects. It often causes
unconsciousness.
d.
Altitudes above 20,000 ft. Breathing air at altitudes above 20,000 ft results in severe
symptoms, even in individuals at rest. Mental performance and comprehension decline rapidly,
and unconsciousness supervenes with little warning. Jerking of the upper limbs occurs quite often
before consciousness is lost, and convulsions may occur after unconsciousness has occurred.
Physical exertion at altitudes above about 20,000 ft rapidly leads to unconsciousness.
16. In moderate and severe hypoxia, the depth and rate of breathing are increased. This effect can usually
be seen on exposure to breathing air at altitudes above 15,000 to 18,000 ft. Above 18,000 ft, the presecence
of high concentration of haemoglobin that has given up its oxygen in the capillaries of the skin, gives rise to
blueness of the lips, tongue and face, as well as the skin of the limbs (most noticeable in the finger nails).
17. Interruption of the supply of supplemental oxygen at altitudes above 10,000 ft, with reversion to
breathing air, is a more frequent cause of hypoxia in flight than ascent without added oxygen. As the altitude
is increased, the time between the reversion to breathing air and the consequent impairment of performance
rapidly decreases (as does the time to loss of consciousness at higher altitudes). The time which elapses
between sudden reversion to breathing air and loss of useful consciousness, i.e. the point at which an
individual is no longer able to carry out a purposeful action, is very variable, especially at altitudes below
28,000 to 30,000 ft. This, so called, Time of Useful Consciousness (or Effective Performance Time) at
various altitudes, is presented in Table 6 and Fig 3.
Table 6 Time of Useful Consciousness Following Sudden Reversion to Breathing Air
Time of Useful Consciousness
Altitude (ft)
(range - seconds)
25,000
150 to 360
27,000
130 to 250
30,000
100 to 180
34,000
60 to 100
36,000
55 to 85
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Page 9 of 22
AP3456 -.6-4 - Physiological Effects of Altitude
6-4 Fig 3 Time of Useful Consciousness at Given Altitudes
6
5
4
3
Reversion from Breathing
Oxygen to Breathing Air
2
Mean
1
Range
Rapid Decompression
(When Breathing Air)
from 8000 Ft to Altitude
Shown
0
25,000
30,000
35,000
40,000
Cabin Altitude (ft)
18. When hypoxia is induced by a sudden failure of the pressure cabin of an aircraft (i.e. the time for
decompression to an altitude in excess of 20,000 ft is less than 1½ minutes), the severity and rate of onset
are considerably greater than when the hypoxia is induced by cessation of supplemental oxygen at the same
altitude. Thus, serious impairment of performance will occur within 1½ minutes on rapid decompression to
25,000 ft, whilst breathing air. It may be seen (Fig 3) that the higher the final altitude, the shorter the time
between the decompression and the consequent impairment of performance. Oxygen breathing must be
commenced within a few seconds of the beginning of a rapid decompression to altitudes between 15,000
and 30,000 ft, if no impairment of performance due to hypoxia is to occur. Rapid decompression to altitudes
above 30,000 ft, will result in transient impairment of performance even if 100% oxygen is breathed as the
decompression commences. These facts emphasize the importance of the correct use of oxygen
equipment in the event of the decompression of an aircraft which is pressurized to provide a cabin altitude
below 8,000 ft (i.e. one in which the occupants will probably be breathing air whilst the aircraft is at high
altitude). This is even more important in aircraft with small, highly-pressurized cabins when loss of a
windscreen or door will result in an explosive decompression of the cabin, and hence the very rapid
development of hypoxia.
Prevention of Hypoxia at Altitude
19. It has been explained (para 15) that the hypoxia associated with breathing air at altitudes greater
than 8,000 ft (an alveolar partial pressure of oxygen of 65 mm Hg), produces a significant impairment
of the skills required for flying. The maximum cabin altitude at which aircrew may operate without
supplemental oxygen is, therefore, 8,000 ft. In a low differential pressure cabin aircraft (in which the
cabin altitude reaches 16,000 to 25,000 ft at the ceiling of the aircraft), it is normal practice to use
supplemental oxygen from ground level since, with high rates of ascent, it is possible to exceed a cabin
altitude of 8,000 ft rapidly. The reduction of the partial pressure of oxygen (PO ) in the air, which occurs
2
with ascent to altitude, and which gives rise to hypoxia, can be prevented by increasing the
concentration of oxygen in the inspired gas. In all RAF oxygen delivery systems designed for use by
aircrew, the concentration of oxygen is increased with ascent to altitude so that the PO2 of the alveolar
gas does not fall below that associated with breathing air at ground level (i.e. an alveolar PO of 100
2
mm Hg (Table 4)). The oxygen concentration required at an altitude of 34,000 ft in order to maintain
an alveolar PO of 100 mm Hg is 100% (Table 7).
2
Reviewed Nov 15
Page 10 of 22
AP3456 -.6-4 - Physiological Effects of Altitude
Table 7 Partial Pressure of Alveolar Gases, Breathing 100% Oxygen at Altitude
Altitude (ft)
Partial Pressure
34,000
40,000
45,000
of:
mm Hg
KPa
mm Hg
KPa
mm Hg
KPa
Oxygen
100
13.34
60
8.00
36
4.80
Carbon Dioxide
40
5.33
34
4.54
28
3.74
Water Vapour
47
6.27
47
6.27
47
6.27
Total Pressure
187
24.94
141
18.81
111
14.81
20. Ascent to altitudes above 34,000 ft, even whilst breathing 100% oxygen, results in the alveolar PO2
falling below that resulting from breathing air at ground level (PO of 100 mm Hg). Breathing 100%
2
oxygen at an altitude of 40,000 ft, produces an alveolar PO2 of about 60 mm Hg (Table 3), (i.e. an
intensity of hypoxia equivalent to that produced by breathing air at an altitude of 8,000 to 10,000 ft).
Ascent to altitudes higher than 40,000 ft while breathing 100% oxygen, gives rise to significant hypoxia.
As indicated by the corresponding levels of alveolar PO2, the intensity of the hypoxia produced by
breathing 100% oxygen at 45,000 ft (Table 7) is slightly more severe than the hypoxia produced by
breathing air at 18,000 ft (Table 5). Considering hypoxia alone, the maximum altitude at which it is
acceptable to fly an unpressurized aircraft, when 100% oxygen is breathed at ambient pressure, is
40,000 ft. In the event of decompression of a pressurized aircraft, when rapid descent is initiated
immediately the pressure cabin fails, breathing 100% oxygen at ambient pressure will provide
adequate protection against severe hypoxia at cabin altitudes up to 43,000 ft. Severe hypoxia can only
be avoided on exposure to altitudes above 40,000 ft, however, by increasing the total pressure of the
gases in the lungs above the pressure of the environment, a technique termed 'positive pressure
breathing' (usually abbreviated to 'pressure breathing').
Pressure Breathing
21. Prevention of hypoxia on exposure to altitudes above 40,000 ft involves administration of 100%
oxygen while maintaining the total pressure of the alveolar gas equal to that which exists at 40,000 ft
(i.e. 141 mm Hg). This is achieved by delivering 100% oxygen to the respiratory tract at a pressure
greater than that of the environment, the technique being known as 'positive pressure breathing'.
When the altitude to which protection is required is greater than 60,000 ft, or if protection above 40,000
ft is required for longer than a few minutes, the pressure at which oxygen is delivered to the respiratory
tract is chosen so that it maintains the total pressure within the oxygen mask, and hence in the alveoli,
equal to 141 mm Hg. The positive pressures required at various altitudes to maintain this standard are
presented in Table 8. Other standards, which are discussed in later paragraphs, are also shown in
Table 8.
Reviewed Nov 15
Page 11 of 22
AP3456 -.6-4 - Physiological Effects of Altitude
Table 8 Illustrative Schedule of Positive Pressure Breathing above 40,000 ft
Positive Pressure Required:
Atmospheric Pressure
To maintain 141 mm Hg
Mask Alone
18.81 KPa absolute
Altitude (ft)
mm Hg
KPa
mm Hg
KPa
mm Hg
KPa
40,000
141
18.81
0
0.00
0
0.00
45,000
111
14.81
30
4.00
17
2.27
50,000
87
11.61
54
7.00
30
4.00
56,000
66
8.80
75
10.00
-
-
70,000
33
4.40
108
14.41
-
-
100,000
8
1.07
133
17.74
-
-
22. Positive pressure breathing, which creates pressure differentials between the respiratory tract and
other parts of the body, produces a number of disturbances, some of which limit the magnitude of the
pressure which can be applied. These disturbances also determine the counter-measures which must
be taken in order to allow the use of higher pressures.
a.
Effect of Pressure Breathing on Head and Neck.
(1) The most striking feature of breathing at high pressure using an oxygen mask is the
distension of the mouth and throat that occurs when the pressure exceeds about 10 to 15 mm Hg.
At higher pressures, the floor of the mouth and the whole of the throat are widely distended, and
above about 60 to 70 mm Hg, this distension can give rise to severe discomfort.
(2) In certain individuals, oxygen under pressure may force its way up the tear ducts, which
connect the inner corners of the eyes to the nose, and blow onto the surface of the eyes
causing spasm of the eyelids.
(3) In order to sustain breathing pressures in excess of 70 mm Hg, a pressurized helmet is
used; this applies the same pressure to the eyes and neck as is being transmitted to the
lungs. This support to the neck and throat avoids the effects described above and also
permits speech at high breathing pressures.
b.
Effect of Pressure Breathing on Respiration.
(1) Pressure breathing inflates the lungs, causing the lungs and chest to expand. In the
relaxed subject, a breathing pressure of only 20 mm Hg distends the lungs completely.
During the normal breathing cycle, inspiration is achieved by active muscular contraction,
whereas breathing out simply requires the relaxation of the muscles. In pressure breathing,
this process is reversed. Breathing in consists of a controlled relaxation of the muscles as
the gas under pressure inflates the lungs. Breathing out consists of controlled contraction of
the same muscles. Thus, the pattern of muscular contraction required during pressure
breathing differs markedly from that of normal breathing. The unusual pattern is associated
with a tendency to over-breathe. Pressure breathing is a technique that has to be learnt.
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(2) The maximum pressure which can be breathed, without counter-pressure to the chest is 30
mm Hg. So called chest counter-pressure minimizes the respiratory disturbances produced by
pressure breathing.
c.
Effects of Pressure Breathing on the Circulation. The rise of pressure within the chest,
produced by pressure breathing, has very significant effects upon the heart and circulation. The
rise of pressure in the lungs is transmitted to the blood in the heart and great vessels within the
chest and abdomen. The increase of pressure in these areas results in blood being displaced
from within the trunk into the limbs, and to the loss of the fluid part of blood out of the vessels into
the tissues of the limbs. The amount of blood displaced out of the chest and abdomen increases
as the breathing pressure increases. The amount of fluid lost into the tissues of the limb is
greater the higher the breathing pressure, and the longer the time for which it is operative. Both
the displacement of blood into the periphery and the loss of fluid into the tissues, reduce the
amount of blood available for the maintenance of the circulation. When this reduction exceeds a
critical value, the blood pressure falls and a faint occurs. There are limits, therefore, to the
magnitude and duration of pressure breathing which can be tolerated with safety. This tolerance
can be increased by applying counter-pressure to the limbs (as in the Typhoon with full coverage
anti-G trousers), so reducing the displacement of blood and the loss of circulating fluid into the
tissues.
23. In practice, pressure breathing, with or without counter-pressure to parts of the body, is used to
provide short duration protection against hypoxia during emergency exposures to altitudes above
40,000 ft, produced either by failure of cabin pressurization or ejection at high altitude. In addition to the
disturbances produced by pressure breathing, the other effects produced by decompression to high
altitude, e.g. decompression sickness (para 29), limit the duration of the exposure. Descent should be
initiated immediately decompression occurs and, provided that there is no serious structural damage to
the aircraft, carried out at the maximum possible rate. Compromises related to the maximum absolute
pressure in the lungs and the maximum breathing pressure have been accepted and proved
experimentally, thereby providing a number of high altitude protective assemblies.
a.
Pressure Breathing Mask Alone. The maximum pressure which can be breathed using a
mask alone is 30 mm Hg. The compromise set in this assembly is to provide this breathing
pressure at an altitude of 50,000 ft (Table 8). As indicated by the total pressure in the mask and
alveolar gas employed in this assembly at 50,000 ft, i.e. 30 + 87 = 117 mm Hg, it results in
considerable hypoxia at the maximum altitude at which it is used. A pressure-sealing mask used
with an oxygen regulator which provides a pressure of 30 mm Hg at 50,000 ft, will provide
protection to an altitude of 50,000 ft, provided that descent is initiated within one minute of the
start of the decompression, at a rate exceeding 10,000 ft per min.
b.
Pressure Breathing Mask, with Chest Counter Pressure Garment and Full Coverage Anti-G
Trousers.
The displacement of blood and fluid into the lower limbs produced by pressure breathing
may be greatly reduced by inflating full coverage anti-G trousers. The pressure breathing for altitude
(PBA) schedule employed in the Typhoon delivers breathing pressures up to 70 mm Hg.
.
Hyperventilation
24. The ventilation of the lungs is controlled by the respiratory centre in the brain, which, in turn, is
controlled by the partial pressure of carbon dioxide (PCO2) in the blood. A rise of PCO2 in the blood
stimulates the respiratory centre and increases ventilation of the lungs. A decrease in blood PCO2 has
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the opposite effect. The respiratory centre is extremely sensitive to small changes in PCO2 and
continuously adjusts the ventilation of the lungs to maintain the partial pressure of carbon dioxide at the
normal level. During exercise, the rate and depth of respiration increase to keep pace with the
increased rate of production of carbon dioxide by the tissues. Thus, over a wide range of physical
activity, the PCO2 of the alveolar gas remains constant at the resting value of about 40 mm Hg
(Table 4), in spite of the rate of production of carbon dioxide varying 8 to 10 fold.
25. The ventilation of the lungs may be increased out of proportion to the rate of production of carbon
dioxide, in which case the PCO2 in the alveolar gas and in the blood and tissues will be reduced below their
normal values. This condition is termed 'hyperventilation'. Hyperventilation may be produced voluntarily. It
can also be produced by anxiety, apprehension, or fear. The condition occurs commonly in student aircrew
during flying training. It is also produced by a rise of body temperature and whole body vibration at
frequencies of the order of 4 to 8 Hz. Pressure breathing may also lead to hyperventilation (see sub-para
22b(1)). Most importantly, however, hyperventilation is the bodies reflex response to hypoxia (para 11).
26. The excessive removal of carbon dioxide from the blood and tissues, which results from
hyperventilation, gives rise to the following symptoms:
a.
Tingling in the hands, the feet, and the lips.
b.
Vague feeling of unreality.
c.
Light-headedness and dizziness.
d.
Faintness.
e.
Spasm of the muscles of the hands and feet.
f.
Impaired performance.
g.
Unconsciousness.
27. Hyperventilation is a condition to be avoided. In order to reduce the likelihood of hyperventilation
occurring in flight, the following points should be observed:
a.
Learn to breathe in a normal manner, particularly when carrying out tasks which are known to
predispose to hyperventilation.
b.
Beware of the tendency to over-breathe during periods of intense concentration or tension.
c.
Do not attempt to overcome suspected hypoxia by voluntary over-breathing.
28. It is possible for individuals to confuse the symptoms of hypoxia and hyperventilation. When
symptoms are experienced at cabin altitudes at which hypoxia could occur, it should always be
assumed that the cause is hypoxia. A thorough check and recheck of oxygen equipment should be
made immediately, whilst every effort is made to breathe in a normal and controlled manner.
Decompression Sickness
29. Decompression sickness is the name given to a group of symptoms which may occur as a result of
exposure to reduced atmospheric pressure, excluding those due to hypoxia or the expansion of pre-
existing gas contained in the hollow cavities of the body. It can, therefore, occur either in an aircraft at
altitude, or in a decompression chamber. It is sometimes referred to as 'the bends', a term which is used
to describe the commonest symptoms of decompression sickness, namely, pain in the muscles or joints.
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30. Decompression sickness can occur in normal individuals who have no predisposing disease, and
there is a very wide individual variation in susceptibility. It is rare below 25,000 ft. The incidence of the
condition increases rapidly with increasing height above that altitude. The duration of exposure to low
pressure is also a very significant factor in the development of the condition. The symptoms of
decompression sickness are:
a.
Bends. The commonest severe symptom of decompression sickness is pain in a joint or limb,
the so-called 'bends'. The pain may be mild or severe. A mild pain will often develop into severe or
agonizing pain if altitude is maintained. If the pain is accompanied by pallor, sweating and nausea or
vomiting, the subject is very likely to collapse. Less frequently, the pain may disappear without
becoming severe. The pain is most likely to occur in the upper part of the arm near the shoulder, the
knee, wrist, and ankle; more than one of these areas may be affected at the same time. It usually
starts as a mild ache, rather like the after-effect of unaccustomed exercise and, if allowed to
progress, may become a deep pain spreading up and down the limb causing clumsiness and
weakness and eventually complete disablement of the limb. The early mild pain often encourages
the subject to move or rub the affected part, which only makes matters worse. On descent,
symptoms pass off around 18,000 ft to 22,000 ft, although residual stiffness and a mild ache may
persist for some time.
b.
Effects on the Skin. Itching and tingling of the skin frequently occur, but are usually
transient effects and of little significance. Localized skin rashes are sometimes observed.
c.
Chokes. 'Chokes' is the name given to a respiratory disturbance which may occur, but is a
misnomer, as the subject does not choke. It takes the form of a sore, burning feeling in the centre
of the chest, with pains on breathing in and paroxysms of coughing. The symptoms of chokes
could be described as similar to those caused by the inhalation of an irritant gas. This is not a
very common condition, but it should be taken very seriously; an immediate descent to below
18,000 ft should be started, otherwise collapse may follow. Chokes may or may not be preceded
by the bends. Although the condition is relieved by descent, there may be a residual soreness in
the chest.
d.
Neurological Symptoms. The effects on the nervous system are very varied. Neurological
symptoms should be taken seriously. Commonly, the eyes are affected in the form of a temporary
defect in the field of vision. Infrequently, there may be weakness, or even paralysis, of one or
both limbs of one side of the body. There is often a feeling of uneasiness or an inability to
concentrate. After recompression, a severe headache may develop. Steps listed in para 33
should be followed if neurological symptoms develop above 18,000 ft cabin altitude.
e.
Collapse. Collapse can occur with or without other symptoms being present. The collapse
is a typical faint, and is characterized by pallor, sweating, nausea, giddiness, and then
unconsciousness. Post decompression, collapse may occur after return to ground level and up to
five hours, or even longer, after landing. This type of collapse is usually preceded by some form
of decompression sickness at altitude, but not always. Decompression collapse is not common
but, should it occur, must be treated as a medical emergency.
31. Decompression sickness is caused by the liberation of nitrogen bubbles in the body due to
exposure to a lowered atmospheric pressure. The body is normally saturated with nitrogen, so that
there is sufficient nitrogen in solution in each tissue and fluid of the body to produce a partial pressure
of gas equal to the PN2 in the alveolar gas. When the pressure of the environment is lowered by ascent
to altitude, the nitrogen in solution in the tissues, saturated at sea level pressure, will now be in a state
of super-saturation and, under certain conditions, will come out of solution. Bubble formation is
influenced by many factors, such as movement of the tissues (hence the need to restrict movement of
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AP3456 -.6-4 - Physiological Effects of Altitude
the affected part), alterations in the circulation of body fluids, and rapid change in gas pressure. The
bubbles tend to be released in tissues with the least blood supply and greatest amount of dissolved
nitrogen. This combination of circumstances occurs principally in fatty tissues. The bubbles which are
released cause pain by pressing on nerve endings. They also pass into the circulation and can cause
disturbances in the lungs, heart and brain.
32. The factors influencing the incidence of decompression sickness are:
a.
General Factors.
(1)
Altitude. The condition rarely occurs below 25,000 ft, and even more rarely below
18,000 ft. The frequency increases with altitude above 25,000 ft.
(2)
Rate of Ascent. The range of rates of ascent which occur in aircraft does not affect the
incidence.
(3)
Duration of Exposure. The longer the duration of exposure, the greater the proportion
of individuals affected.
(4)
Exercise. Exercise, whilst at altitude, markedly increases the incidence and severity of
symptoms.
(5)
Re-exposure. Re-exposure to altitude immediately after the first exposure generally
has been considered to increase susceptibility to decompression sickness.
(6)
Hyperbaric Exposure. Exposure to breathing air at pressures above one atmosphere,
such as occurs in scuba diving, by increasing the amount of nitrogen dissolved in the tissues,
greatly increases susceptibility to the condition. Thus, after a recent dive, breathing air,
decompression sickness may occur on ascent to as low an altitude as 6,000 ft (see para 35).
b.
Personal Factors.
(1)
Age. The incidence increases with age; each decade approximately doubles the
susceptibility.
(2)
Body Weight. As has already been mentioned, fat has a higher nitrogen content than
other body tissues, so that obesity predisposes to symptoms of decompression sickness.
(3)
Recent Injury. There is some evidence to suggest that joint lesions and recent limb
injuries increase susceptibility.
33. The treatment of decompression sickness is immediate recompression, as fast as is tolerable, to
as low an altitude as possible. Except where operational considerations make maintenance of altitude
essential, descent should be made to an aircraft height at which the cabin altitude is less than 10,000
ft. In severe cases, or if symptoms persist, a landing should be made as soon as possible. If practical,
the affected individual, if suffering from severe bends, chokes, neurological disturbances, or collapse,
should be laid flat and given 100% oxygen to breathe. Medical advice should be sought immediately..
Whenever decompression sickness occurs in flight, the affected individual should receive medical
attention as soon as possible after landing. This is of great importance, since seemingly innocuous
symptoms may progress rapidly to life-threatening conditions if treatment is not instituted. It must also
be borne in mind that it may take some time to arrange transport and hyperbaric treatment for a patient
whose condition deteriorates.
34. The incidence of decompression sickness can be markedly reduced by pre-oxygenation, i.e. by
washing out the nitrogen in the body with oxygen. This is done by breathing 100% oxygen at ground
level for some time before take-off. For example, breathing oxygen at ground level for three hours will
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AP3456 -.6-4 - Physiological Effects of Altitude
protect a high percentage of subjects when exposed to 40,000 ft for three hours. Individuals who pre-
oxygenate on the ground must proceed to their aircraft and transfer to 100% oxygen on the aircraft
system, without taking a breath of atmospheric air.
35. Decompression sickness is a condition which is best avoided. The most satisfactory method of
prevention is limiting the maximum altitude to which aircrew are exposed to below 25,000 ft, by means
of pressurization of the cabin or, in unpressurized aircraft, limiting the maximum cabin altitude to
25,000 ft. The marked increase in susceptibility to decompression sickness which follows exposure to
breathing air at environmental pressures greater than one atmosphere requires that, following such an
exposure, individuals must not ascend to altitude either in an aircraft or a decompression chamber until
sufficient time has elapsed for the excess nitrogen to be eliminated from the body. The period spent at
ground level before flight should be no less than 12 hours after swimming using compressed-air
breathing apparatus, and no less than 24 hours if a depth of 10 m has been exceeded.
Vaporization of Tissue Fluids
36. A further effect of exposure to a reduced pressure is the vaporization of tissue fluids, resulting in a
quite rapid, painless swelling of the affected part. Above 63,000 ft, the total atmospheric pressure is
less than the vapour pressure of the body fluids at deep body temperature. In regions of the body
where the hydrostatic pressure of the body fluids is low, collections of water vapour could be formed.
In practice, this condition is not likely to occur until the pressure is considerably lower than the
equivalent of 63,000 ft. This condition has been observed in the hands of subjects wearing partial
pressure suits at very high altitudes (above 65,000 ft). It disappears again on descent below that
height. There is no residual disturbance of function due to this phenomenon, and it can be prevented
by applying pressure to the area concerned. In the case of the hands, for example, it can be avoided
by wearing close-fitting leather gloves.
Effect of Change of Altitude on the Ears and Sinuses
37. The head contains a number of gas-filled cavities which communicate with the nose; these are the
middle ear cavities and the nasal sinuses. The gas contained in these spaces expands and contracts on
ascent and descent and, so long as communication with the nose remains open to permit gas to flow out
of and into these cavities, no disturbances will occur. However, if free exchanges of gas in and out of
these cavities do not occur with change of altitude, a very high pressure difference can soon arise, with
painful and serious consequences. As already noted in para 3, the change of pressure for a 1,000 ft
change of height is much greater at low than at high altitude, and thus the disturbances caused in the
ears and sinuses by change of altitude occur predominantly at the lower altitudes.
38. The cavity of the middle ear is separated from the exterior by a thin diaphragm, the eardrum, and
communicates with the nose via the Eustachian tube, whose walls are soft and normally collapsed
together (Fig 4).
39. During ascent, as the ambient pressure decreases, the expanding gas in the middle ear cavity
readily escapes along the Eustachian tube, so that pressure is equalized on either side of the eardrum.
Since the anatomical structure of the tube is such that this gas can escape easily (see Fig 4),
disturbances are very rare during ascent. This passive ventilation of the middle ear may be heard as a
popping sensation in the ear.
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AP3456 -.6-4 - Physiological Effects of Altitude
6-2 Fig 1 The Human Ear
Inner Ear
Bony Wall
Cochlea
Nerve
External Canal
Ear
Middle
Drum
Ear
Cavity
Eustachian
Tube
40. During descent, the collapsed wall of the Eustachian tube tends to act as a valve, preventing gas
from flowing back into the middle ear cavity. The increase in pressure on the outside of the eardrum
progressively distorts the drum inwards as the descent continues. Gas must flow into the middle ear
cavity via the Eustachian tube during descent if the drum head is to be restored to its normal resting
position. Several actions may be employed to open the Eustachian tube and allow gas to flow into the
middle ear, such as yawning, swallowing or pushing the jaw forward. If such actions fail, pinching the
nose and blowing into it (as if blowing the nose so as to blow the fingers apart) is very effective. This is
called the 'Valsalva' manoeuvre and it must be used with some care, lest the ears become over-
inflated, resulting in discomfort which can be confused with a failure to clear the ears. Another widely
used method is to pinch the nose, close the glottis (the gap between the vocal cords), and raise the
floor of the mouth. Individuals soon find, by trial and error, the method which suits them best.
41. During a descent, the ears must be cleared constantly as difficulty is likely to occur when the
pressure difference across the eardrum is allowed to build up. This pressure build-up pushes in the
eardrum, causing pain and deafness which can become very severe as the pressure differential
increases. The condition is known as an 'ear block' or 'otitic barotrauma' (i.e. 'damage to the ear by
pressure'). As the differential across the drum reaches about 50 mm Hg, the pain is very severe and,
when it reaches approximately 90 mm Hg, it is not possible to equalize this pressure or 'clear the ears'
by voluntary effort. Further descent at this stage can cause rupture of the drum. In cases where
voluntary actions, such as those described, fail to relieve the condition, it is best to climb again until the
ears are clear and let down again at a reduced rate, being careful to keep the middle ears inflated.
42. A head cold is likely to cause congestion and swelling of the Eustachian tubes, just as the lining of
the nose is affected. Thus, it may become difficult or impossible to clear the ears. Aircrew with head
colds should not fly, unless they can clear their ears satisfactorily on the ground.
43. The nasal sinuses are cavities in the bones of the face and skull, having a lining similar to that of
the nose, with which they communicate along narrow tunnels. During ascent and descent, gas flows
freely out of and into the sinuses. In the presence of inflammation of the lining of these sinuses, as in
sinusitis or with a severe head cold, swelling may obstruct the outlets. This will cause pain, which can
be severe, during a descent. The condition is known as 'sinus barotrauma' and may be felt in the
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cheek, upper teeth, forehead, or deep in the head. In severe cases, the pain can be quite blinding, and
also accompanied by watering of the eyes. If sinus barotrauma occurs during flight, the rate of descent
should be slowed and attempts made to force gas into the sinuses by raising the pressure in the nose
by pinching the nostrils, closing the mouth, and breathing out hard. Any infection or inflammation in the
sinuses is a further reason for seeking medical advice about fitness to fly.
Abdominal Distension
44. In healthy individuals, the stomach and intestines contain a variable quantity of gas (0 to 300
millilitres). On ascent, this abdominal gas expands and normally will escape either upwards or
downwards through the mouth or anus, as the case may be. A few individuals have particular difficulty
in venting this gas, even at modest rates of ascent; this is most common amongst inexperienced
aviators. The higher the rate of ascent, the greater is the problem of expelling the gas quickly as it is
expanding. Healthy experienced aircrew may, on occasions, experience difficulty during particularly
rapid and large increases in altitude. The symptoms caused by an inability to expel this gas during
ascent vary from mild discomfort to severe pain in the abdomen, and vomiting. The incidence of
symptoms from the expansion of abdominal gas is, however, insignificant amongst experienced
aircrew, except at cabin altitudes in excess of 30,000 ft. This problem can be aggravated by intestinal
infection or the consumption of too many gas-forming foods.
Teeth
45. Healthy teeth do not contain gas. Recently filled teeth, and those affected by dental caries, may
contain small gas cavities which can give rise to toothache when climbing. If this occurs, descent will
relieve the pain. Prevention is straightforward; maintain good dental health, do not fly within 24 hours
of dental repair work, and remind the dentist that no air pockets should be left when cavities are filled.
Effects of Changes of Pressure on the Lungs
46. The lungs, being air-containing cavities, are also affected by rapid change of environmental
pressure. Only extremely high rates of decrease in the environmental pressure could, however, cause
damage to the lungs by over-expanding them to the point of rupture, because of the relatively wide
bore air passages along which the gas can escape from the lungs. In practice, very rapid
decompressions over a wide range of pressure, which could possibly give rise to lung damage, will
only occur in the event of a serious structural failure of an aircraft. It is possible, however, for lung
damage to occur if the breath is held during a wide range decompression. It is clearly important,
therefore, to ensure that intentional breath holding is avoided during practice decompression. Such an
action, particularly with inflated lungs, would carry a grave risk of lung rupture.
47. Lung damage due to rapid or explosive decompression is extremely rare, even when the
decompression occurs over a wide pressure differential.
Effects of Low Temperature
48. The effects of low temperature on the body depend on four factors:
a.
The absolute temperature.
b.
The speed of air movement.
c.
The duration of exposure.
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d.
The amount of protection.
49. As already stated at the beginning of this chapter, the temperature falls steadily with altitude
throughout the troposphere at the adiabatic lapse rate of approximately 2 ºC per 1,000 ft. In the
stratosphere, the temperature is fairly constant at about –55 ºC. Table 9 gives some typical
temperatures at various altitudes.
Table 9 Atmospheric Temperatures at Various Altitudes
Altitude (ft)
Temperature (ºC)
Sea Level
15
5,000
5
10,000
-5
15,000
-15
20,000
-25
25,000
-35
30,000
-45
35,000
-55
40,000
-55
50. Exposure to a temperature of –40 ºC, when wearing normal flying clothing, leads to gross impairment of
function after only a few minutes. Parts of the body which are bare, or only lightly clad, very soon become
cold, numb, still and functionless; this is particularly noticeable in the fingers. There is an associated dulling
of the senses, and general incapacity. If exposure to this temperature is continued, the deep body
temperature drops to a critically low level, producing a state of coma and, in time, death.
51. Exposure to a low environmental temperature in flight due, for example, to the loss of the canopy,
can become a limiting factor in deciding the altitude at which the flight can be continued. In many
cases, it may be necessary to initiate immediate descent and, even then, frostbite of the exposed
areas of the body may occur, particularly if the aggravating factor of wind-chill is present. The chances
of frostbite occurring will be greater if hypoxia is present.
52. In the event of high altitude escape, there is a marked possibility of frostbite, but even a light
covering, such as afforded to the hand by cape leather gloves, is sufficient to delay, and even prevent,
serious damage.
Cabin Pressurization
53. Aircrew operating aircraft at moderate and high altitudes are normally protected against the
effects of exposure to the environment in which the aircraft is flying by pressurization of the crew
compartment. Conditioned air is fed into the cabin and allowed to escape through discharge valves.
The opening of the discharge valves is controlled, so that the desired pressure difference is created
between the interior of the cabin and the external environment of the aircraft.
54. The human body is accustomed to sea level conditions, so it would be ideal to maintain sea level
pressure in the aircraft cabin at all times. For military aircraft, however, this is impracticable, and not
always desirable, from the point of view of weight, complexity, and the hazards arising from loss of
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pressure due to enemy action. In practice, the pressure differential, and thus the cabin altitude, is
chosen for a particular aircraft as a compromise between the physiological ideal and the proposed
performance and role of the aircraft.
55. Two major types of cabin pressure schedules are employed in military aircraft, namely high
differential and low differential. In aircraft with high differential pressure cabins, the maximum cabin
altitude is generally 8,000 ft. A differential pressure of 9 psi is required at an aircraft altitude of 50,000
ft to produce a cabin altitude of 8,000 ft. High differential pressure cabins are typically used in large
aircraft such as medium bombers, maritime reconnaissance, and transports. The crew and
passengers flying in this type of pressure cabin normally breathe cabin air throughout flight. Oxygen
equipment is fitted in order to provide protection against hypoxia in the event of a decompression. In
combat situations, when the risk of decompression is increased, some or all of the crew may use their
oxygen equipment at a cabin altitude of 6,000 to 8,000 ft, in order to ensure full protection against
hypoxia should cabin pressurization be lost. The degree of pressurization employed in the low
differential pressure schedule is such that, at the altitude ceiling of the aircraft, the cabin altitude is in
the range 20,000 to 25,000 ft, the exact value varying from one aircraft type to another. A maximum
differential pressure of approximately 5 psi is typically employed in this type of pressurization schedule.
The low pressure differential schedule is used in fighter aircraft, where the risk of failure of the
pressure cabin due to battle damage, or loss of a canopy, is higher and the large weight penalty of a
high differential pressure cabin is unacceptable. Crew operating low differential pressure cabin aircraft
use their oxygen equipment throughout flight. Some military aircraft with high differential pressure
cabins are also fitted with a cabin pressure control system, whereby a low differential pressure
schedule can be selected, as desired, in flight.
Loss of Cabin Pressure
56. The pressurization of the cabin of an aircraft may fail because air is no longer pumped into the
cabin, there is a failure in the cabin pressure control system, or a defect develops in the wall of the
cabin. In military aircraft, the jettisoning of a canopy prior to ejection is an example of the latter type of
failure. The rate at which the cabin altitude increases varies with the type of failure, the aircraft and
cabin altitudes, and the size of the opening or defect in the cabin wall. When a defect in the wall of the
pressure cabin is the cause, the final cabin altitude after loss of pressure may considerably exceed the
actual altitude of the aircraft. This additional reduction of the pressure within the cabin is due to the
external flow of air over the defect. The effect is termed 'aerodynamic suction'. Its magnitude varies
from aircraft to aircraft, with the position of the defect, and with the aircraft speed. It can result, for
example, in a cabin altitude of 50,000 ft at an aircraft altitude of 40,000 ft. Loss of cabin pressurization
does not necessarily imply loss of cabin heating since, if the failure is in the integrity of the cabin wall,
hot air will continue to enter the cabin from the engines. Large aircraft also have a considerable heat
capacity, so that a period of time may elapse before the cabin air temperature approaches that of the
external environment.
57. Failure of a pressure cabin has two distinct groups of effects upon the cabin occupants. The first
group of effects are caused by the change in pressure itself, and include lung damage and abdominal
distension. The effects in the second group are due to the exposure of the occupants to increased
altitude.
a.
Effects due to Pressure Change. The severity of the first group of effects is related to the
magnitude of the pressure change and the rate at which it occurs. Even when the loss of cabin
pressure is very rapid, the incidence of lung damage will be infinitesimally low. Following rapid
decompression, a small proportion of aircrew may suffer from abdominal distension.
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b.
Effects due to Exposure to Increased Altitude. The incidence and severity of the effects
which arise due to the exposure to increased altitude are closely related to the final cabin altitude.
The most important effect is hypoxia, and its magnitude is influenced by whether the crew are
breathing air or oxygen (see para 18). Decompression sickness is rare if the duration of the
exposure to high altitude is short (a few minutes only). If, however, hypoxia is prevented, and the
occupants of the cabin are exposed to altitudes in excess of 25,000 ft for any length of time, some
of them will develop decompression sickness (see para 29). A reduction in cabin temperature
may be associated with loss of cabin pressure. If the duration of exposure to low temperature is
short, little reduction in efficiency will occur. Once the exposure is extended beyond a few
minutes, however, serious impairment of performance and injury will occur.
58. In summary, the principle physiological hazard associated with failure of the pressure cabin of an
aircraft at high altitude, is hypoxia. If descent to low altitude is delayed, for operational or structural
reasons, then decompression sickness or the effects of low temperature, or both together, will be added
to the risk of hypoxia. The immediate action to be taken in the event of a failure of cabin pressurization at
altitude, is to ensure that oxygen is being delivered to the oxygen mask, and that the latter is adequately
sealed to the face. Whenever structural and operational considerations allow, immediate descent to as
low an altitude as possible should be carried out at the maximum practical rate. Rapid descent is
essential when a decompression results in a cabin altitude greater than 40,000 ft, since none of the
pressure breathing systems available in the Royal Air Force provides long duration protection against
hypoxia or decompression sickness.
59. Whenever a decompression results in a cabin altitude greater than 25,000 ft, descent to a cabin
altitude below this level should be carried out as soon, and as quickly, as operational considerations
allow. When passengers are being carried in transport aircraft, immediate emergency descent (so that
the cabin altitude is reduced to less than 15,000 ft (ideally 8,000 ft)) is essential, even if passenger
oxygen equipment is available, since it is unlikely that more than half the passengers will use the latter
correctly during and immediately after the decompression. Should fuel and operational considerations
make maintenance of a higher cabin altitude essential, then the re-ascent should only be performed
after the appropriate checks that the passengers are receiving oxygen have been made.
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CHAPTER 5 - PHYSIOLOGICAL EFFECTS OF ACCELERATION
Introduction
1.
Changes in either magnitude or direction of velocity (termed acceleration) may produce
considerable effects on the body. These effects depend on the:
a.
Magnitude of the acceleration.
b.
Duration of the acceleration.
c.
Direction of the acceleration.
d.
Site of action of the acceleration.
e.
Onset rate of the acceleration (sometimes called ‘jolt’).
2.
The magnitude of acceleration is usually stated in units of 'G', which is the ratio of actual acceleration to
acceleration due to earth’s gravity or 'g' (9.8 m/s2). Hence, an acceleration of 4 G is four times that due to
gravity, or 39.2 m/s2. The direction of action of acceleration is defined on a three-co-ordinate system based
on the human spine, where Z is the vertical axis, X the fore and aft axis and Y the lateral axis. Positive and
negative signs are used to specify direction along each axis such that a 'headwards' acceleration is +Gz, a
forward’s acceleration is +Gx, and a right lateral acceleration is +Gy. 'Footwards', backwards and left lateral
accelerations, therefore, become –Gz, –Gx and –Gy respectively.
3.
It is important to note that the force which is sensed by the individual is the inertial reaction. This
reaction is at all times equal in magnitude, but opposite in sign to the applied acceleration and applies
equally to every part of the body. Thus, a headwards acceleration (+Gz) tends to force the body down
onto the seat and to displace blood towards the feet.
4.
The three types of acceleration to be considered are:
a.
Linear acceleration caused by change in speed.
b.
Radial acceleration caused by change in direction.
c.
Angular acceleration caused by change in rate of rotation (ie change of speed and direction).
5.
The following is a brief summary of the chief accelerations which can occur in aviation:
a.
Linear Accelerations. Linear accelerations include:
(1) Catapult or rocket assisted take-off (+Gx).
(2) Arrested landings, barrier engagements (–Gx).
(3) Crashes, crash landings, ditching (initially –Gx and +Gz).
(4) Buffeting (predominantly ±Gz).
(5) Seat ejection (initially +Gz).
(6) Parachute opening shock and landing by parachute (predominantly +Gz).
b.
Radial Accelerations. Radial accelerations are caused by rotation about a distant axis.
They act outwards from the centre of rotation and are experienced whenever an aircraft changes
direction (predominantly +Gz).
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c.
Angular Accelerations. Angular accelerations are experienced if the rate of rotation
changes, or if a second axis of rotation is added to the first. The principal effects on the body are
those related to the vestibular apparatus (organ of balance) and it is convenient to discuss these
separately (see Volume 6, Chapter 6).
Effects of Linear Acceleration
6.
Linear accelerations result from an increase of speed (take-off). Decelerations result from a
decrease of speed (landings and crashes). The problems associated with buffeting and seat ejection
are dealt with in paras 14 to 19.
7.
During catapult-assisted take-off, an acceleration of +4 Gx may be experienced, and deceleration
of –3 Gx may occur during an arrested landing. In wheels-up landings or ditchings, the force may
exceed –10 Gx, and in crashes may exceed –25 Gx. Linear forces encountered in aviation usually last
for less than one second, though prolonged linear accelerations are imposed during the launching and
re-entry of space vehicles. The problems associated with sustained G forces will be discussed more
fully when dealing with radial accelerations.
8.
It has been shown that the human body, properly supported, can tolerate a very much greater
acceleration than most aircraft structures. In rocket sledge experiments, values of +40 Gx have been
imposed on the human body without injury. Values as great as +60 Gx have been reached with
survivable injuries. In general, it is not necessary to provide protection against accelerations higher
than ±25 Gx since values as great as this will only be attained in disastrous, uncontrolled crashes
involving massive structural disintegration of the aircraft.
9.
The problems of short duration acceleration usually concern body restraint and body posture.
These are particularly important in the use of ejection seats (paras 15-19).
10. During crash decelerations in forward-facing seats (–Gx), unrestrained occupants may be flung forward
and injured or killed by striking solid objects in front of them. Even low decelerative forces have produced
fatal results in road traffic accidents. The simplest form of restraint is the lap belt, but this is not satisfactory
as it does not prevent the body flexing at the hips, thereby permitting the head to move forward and strike
any solid object in its path. Also, this sharp forward flexion of the hips is liable to cause fractures at the lower
end of the spine. Furthermore, since the area of restraint provided by the lap belt is small, the associated
high contact pressure is liable to cause internal abdominal injuries.
11. The conventional four-point seat harness in an aircraft has both a lap belt and shoulder straps.
The restraint afforded by the lap belt across the thighs is intended to reduce both vertical and forward
movement of the hips, and the shoulder harness is designed to prevent forward flexion during –Gx
acceleration. A five-point harness has a negative G strap attaching the harness quick release fitting to
the front of the seat pan. This provides a greater reduction in vertical movement than can be achieved
with the conventional harness. The head is unrestrained and forward flexion of the neck is likely to
occur in crash decelerations. In order to prevent damage to the head, effort is made during cockpit
design to ensure a clear path for a distance of 40 cm in front of the head. Where an object intrudes, a
head-up display for example, it should be adequately padded to prevent an incapacitating head impact
in the event of a crash or barrier engagement. Further protection is provided by a well-fitting helmet.
12. Standard Service harnesses protect the wearer against forward decelerations of up to 25 G,
provided that they are properly fitted, tight, and with the lap belt as low as possible and shoulder straps
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locked. A high lap strap could allow the wearer to slip forwards underneath the harness during forward
decelerations. The negative G strap in a five-point harness prevents this occurring by anchoring the
centre point of the harness and holding the lap belt down. Seat and harness attachments must also be
stressed to ±25 Gx. In cases where there is a separate parachute quick release fitting, it should be
located higher than the seat harness quick release fitting, otherwise it could be driven back by the
second fitting and possibly cause internal injury.
13. In passenger aircraft, it is difficult to provide the occupants with a safety harness which will give
adequate restraint and, at the same time, reasonable freedom of movement and comfort. The
passenger seat is fitted with a simple lap belt to restrain the occupant in turbulence, and in the event of
other axes of acceleration occurring during crashes. A head-rest is essential to prevent neck injury
and, ideally, it should have forward projections at each side to provide lateral restraint.
Buffeting
14. Vibrations occur during flight for a number of reasons, but most significant in relation to harness
restraint is the buffeting which can occur when an aircraft flies fast in turbulent conditions, eg in cloud,
over mountains, or at low level, particularly in hot climates, or over uneven terrain. These rapidly
alternating vertical accelerations are usually of the order of ±1.5 Gz to 2 Gz, but occasionally values as
great as ±3 Gz may occur. They are governed in amplitude and frequency by the speed and wing
loading of the aircraft, as well as by the amount of turbulence. One of their effects is to hasten the
onset of fatigue in the individual, but, if of sufficient amplitude, they may make control difficult or even
cause an inadequately restrained occupant to strike their head against the cockpit canopy or cabin
roof. At certain frequencies, buffet accelerations may interfere with vision. The wearing of a protective
helmet, in addition to a properly tightened harness, prevents head injury. It is the captains’ duty to
ensure that their crew and passengers have harnesses secure when there is a possibility of flying into
turbulent conditions.
Seat Ejection
15. In order to clear high tail structures, and also give a low-level escape capability, the ejection gun
has to provide the highest possible velocity, and hence gain in altitude, without exceeding the
acceleration tolerance of the seat occupant. Early investigations showed that, not only was there a
limit to the peak acceleration which could be employed, but there was also a limit to the rate at which
this acceleration could be applied. It was established that the absolute limit of human tolerance to
ejection was +25 Gz, and that at no time must the rate of rise of G exceed 300 G per second (G/s).
16. Ejection acceleration loads depend not only upon the energy of the gun system and weight of the
seat occupant, but also upon the transmission of energy from the seat to the occupant. This
transmission is influenced by the elastic properties of equipment stowed in the seat pan, as well as by
the dynamic response of the occupant. The presence of extra cushioning material between the
occupant and the seat pan may cause the occupant to reach peak accelerations of higher than 25 Gz.
This 'dynamic overshoot' is a result of poor coupling of the occupant to the seat allowing the seat to
reach a high velocity before the occupant. The occupant then undergoes a greater acceleration to
match the seat velocity when the seat cushioning is fully compressed. It is essential that no
unauthorized equipment is placed in the seat pan, nor should the contents of survival packs or
cushions be altered in any way.
17. To overcome the limitations of performance imposed by human tolerance to acceleration, rocket-
assisted seats are used. The advantage of rocket assistance is that it permits a longer application of
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thrust, therefore achieving the necessary clearance from the aircraft with lower peak acceleration and
lower rate of onset of acceleration. Older ballistic ejection seats approached 25 Gz peak acceleration,
while a typical rocket-assisted seat has a peak acceleration of +16 Gz to +18 Gz.
18. To minimize the risk of injuries on ejection, the harness should have well-tightened lap and
negative G straps, and have the shoulder straps retracted and locked, with the occupant in the correct
ejection posture. These measures will ensure good coupling of the occupant to the seat and minimize
the chance of forward flexion of the spine during ejection.
19. After ejection, particularly at high indicated air speeds, further accelerations will be experienced,
some of which may be associated with a seat tumbling or deceleration resulting from the deployment
of stabilizing equipment. Further consideration of these matters will be found in Volume 8, Chapter 9.
Parachute Opening Shock
20. High accelerations may be experienced on parachute deployment, the opening shock load
increasing with air speed and altitude. Parachute opening shock accelerations are greater at altitude due
to two factors. Firstly, the parachute will open quicker in lower density air leading to higher accelerations.
Secondly, higher accelerations can occur due to the relative differences in the terminal velocity of the
aircrew/ejection seat and the reduced terminal velocity of the aircrew on the inflated parachute at high and
low altitudes. At 7,000 ft the opening shock load for a 7 m (24 ft) canopy is approximately 9 G, whereas at
42,000 ft this same canopy would give an opening shock load of about 32 G. An opening shock load as
high as this would almost always cause severe damage to the canopy and also to the aircrew member.
For this reason alone, it is undesirable to permit canopy deployment much above 20,000 ft, quite apart
from the fact that a delayed opening reduces the time spent at high altitude, where the problems caused
by lack of oxygen and low temperature would be significant.
Parachute Landing
21. The deceleration experienced during parachute landing is very variable, depending on the
parachute, the body weight, the landing attitude, wind velocity and the terrain. Aircrew are not usually
experienced in actual parachute landings, therefore it is important to ensure that the situation is not
aggravated by increasing the rate of descent by attempting to carry out difficult parachuting
manoeuvres near the ground. Some parachutes produce a horizontal velocity component, or 'drive', of
several metres per second. This allows a smaller canopy to give an acceptably low descent rate, and
also damps out instability so that landing should be more controlled.
Effects of +Gz Acceleration
22. As early as 1918, during test flights in a Sopwith Triplane, aviators reported visual loss and
'fainting' as a result of +Gz acceleration exposure by their aircraft.
23. Radial accelerations are most commonly experienced in turns, especially in high performance aircraft.
The formula relating centripetal acceleration (
a) to velocity (
v) and radius of turn (
r) is as follows:
v2
a =
r
As centrifugal force (
F) is equal to mass (
m) times acceleration (
a), the following formula applies:
mv 2
F =
r
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From this formula it can be seen that doubling the velocity of flight along a curved path quadruples the
force applied to the aircraft and crew, while halving the radius of turn doubles the force. This force is
felt as an increase in weight in proportion to the amount of acceleration. A 6 G acceleration is felt as a
sixfold increase in weight.
24. Under increased +Gz acceleration, the weight of the whole body, and its components (especially
the blood) is increased with the following effects:
a.
Fluid and tissues are displaced downwards. This is most apparent in the face, where the
skin can be seen to sag.
b.
Since the weight of the body may be increased many times while the power of the muscles
remains unaltered, movements become progressively more difficult. If the head is lowered, it may not
be possible to raise it again, especially if a heavy helmet is worn. Neck injuries are possible, especially
if acceleration is applied suddenly and unexpectedly, or if the head is moved or held in a rotated and/or
flexed position, such as in the 'Check 6' position. At +2.5 Gz it is almost impossible to rise from the
sitting position and unaided escape from an aircraft would be virtually impossible.
c.
A pressure gradient develops in the blood between the heart and the brain, resulting in
reduced blood pressure at head level. This can reduce the supply of oxygen to the eye and brain,
ultimately leading to G-Induced Loss of Consciousness (G-LOC). The blood pressure in the blood
vessels of the lower parts of the body rises with increasing G, and flow of blood back to the heart is
reduced. The high blood pressure in parts of the body below heart level can be sufficient to rupture
small capillaries in the feet and forearms, leaving a fine rash (‘G measles’) - this is a normal
response to accelerations greater than about 4 Gz, and does not have any long-lasting effect. Arm
pain can occur at +6 Gz and above.
25. As the level of acceleration is increased, inadequate blood pressure to supply oxygen to the retina
causes partial loss of vision ('grey-out'), followed by total loss of vision ('black-out'). Loss of vision begins
at the periphery of the visual field and gradually moves into the centre, so that the grey-out phase has
been likened to looking down a foggy tunnel. The reason for visual disturbance occurring before loss of
consciousness can be explained in simple mechanical terms. The pressure needed to supply the eye
with blood is greater than that required to supply the brain, because the eyeball has a positive internal
pressure. Thus, the fall in blood pressure at head level which results from +Gz acceleration first affects
the blood supply to the retina and produces impairment of vision.
26. Under conditions of gradual onset of acceleration, the warning presence of grey-out or black-out
allows the pilot to avoid loss of consciousness by either decreasing the amount of Gz the aircraft is
pulling, or by increasing the anti-G straining manoeuvre (sub-para 32b). Modern agile aircraft are
capable of high rates of G onset, up to 10 G/s or more. Under these conditions the brain becomes
deprived of blood and oxygen at virtually the same time as the eye so that G-LOC may occur without a
warning impairment in vision. 10-20% of all aircrew have experienced G-LOC and studies have shown
that simple recovery takes up to 15 seconds, while a further 30 seconds to 45 seconds may pass
before it is possible to appreciate the situation and take appropriate action to recover the aircraft. A
syndrome called ‘Almost Loss of Consciousness’ (A-LOC) is also recognised, in which aircrew may
show poor response to sounds (eg radio calls), an abnormal sensation in the limbs, a lack of recall,
confusion or a dream-like state, euphoria, apathy, or disorientation but without a complete loss of
consciousness. The risks associated with A-LOC are often just as great as G-LOC, as control of the
aircraft is impaired.
27. As in many other situations, the body makes some attempt to compensate for these circulatory
changes. If a level of acceleration which first produced grey-out is sustained, it is possible that vision
will return to normal. This is due to a reflex increase in blood pressure to maintain a satisfactory supply
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to the eye, heart and brain. Similarly, black-out may improve to grey-out or normal vision may be
recovered. However, this reflex is slow (7-10 seconds) and G-LOC may occur before it acts.
28. The severity of these effects is not solely dependent upon the level of acceleration; the duration of
exposure is a significant factor. Brief exposure to high levels of acceleration may cause loss of control,
or even damage to the aircraft, but will not have time to cause symptoms as both brain and eye contain
a sufficient store of oxygen to function for 3 seconds to 4 seconds in the absence of a fresh supply of
blood. Therefore, in describing levels of normal response to +Gz acceleration, it is necessary to define
both the level of acceleration and its duration.
29. For a relaxed individual with no anti-G trousers, an acceleration of +3 Gz to +4 Gz acting for 4
seconds to 6 seconds is sufficient to cause some reduction in peripheral vision. An acceleration of +4 Gz
to +5 Gz may produce black-out or even loss of consciousness. These levels can vary widely from
individual to individual, or even in the same person depending upon factors such as lack of food,
dehydration, fatigue, illness, hypoxia, or the after effects of alcohol (see para 32g). In general, the grey-
out threshold is about 0.5 to 1 G below the black-out threshold, and this, in turn, is about 0.5 to 1 G below
the threshold for unconsciousness. The range is wide, however, and some individuals can lose
consciousness at as low as + 3 Gz.
30. As G-LOC may be followed by confusion for 30 seconds to 45 seconds, the greatest risk is ground
impact, although mid air collision is also possible. Avoidance of G-LOC or A-LOC is achieved primarily
through a G straining manoeuvre that is initiated in good time to prevent the onset of grey-out or black-
out (see para 32b); waiting for the grey-out to appear and then straining to clear it is potentially risky
and may put you at risk of G-LOC, particularly at high G onset rates. Recovery from unconsciousness
is frequently associated with jerky and uncontrolled movements of the head and limbs. These
movements may interfere with the control of the aircraft. At least 50% of individuals suffering G-LOC
have no recollection of losing consciousness.
31. The distribution of blood and air within the lungs is affected by +Gz acceleration, and the efficiency of
gas transfer is impaired causing the concentration of oxygen in arterial blood to fall which can reduce mental
performance. Repeated exposures to +Gz while breathing high concentrations of oxygen may lead to a
condition of 'acceleration atelectasis' ('oxygen lung') in which the lower parts of the lungs become collapsed
and give rise to shortness of breath, cough, chest pain and difficulty in taking a deep breath. Aircraft
featuring on board oxygen generators may be more likely to cause atelectasis. The symptoms usually
disappear after a few deep breaths are taken, but occasionally persist to cause discomfort and exercise
limitation after flight.
Increasing Tolerance to G
32. Tolerance to G can be enhanced by factors that maintain blood supply to the head, by supporting
the circulation. These include:
a.
Anti-G Trousers. An anti-G suit is standard equipment in almost all UK MOD high
performance aircraft. It consists of a pair of trousers of inelastic lightweight material beneath
which bladders are inflated to apply counter-pressure to the calves, thighs and abdomen. The
bladders are inflated automatically from engine bleed air via an anti-G valve. For all aircraft types
except Typhoon, a pressure of 1.5 psi (10.3 kPa) comes in abruptly at +2 Gz (in Typhoon the
pressure rises smoothly from the baseline). Anti-G trouser pressure increases linearly with
increasing G to around 10 psi (70 kPa) at +9 Gz. Conventional five-bladder (skeletal) anti-G
trousers raise the black-out threshold of a relaxed subject by 1 G to 1.5 G. They also make
performance of the anti-G straining manoeuvre easier, and reduce the amount of fatigue
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experienced by aircrew carrying out repeated manoeuvres at high G. Full coverage anti-G
trousers (FCAGT) used in Typhoon and Lightning II feature circumferential bladders covering a
greater surface area of the lower limbs, and raise the black-out threshold by 2 G to 2.5 G. An
advanced anti-G valve giving more rapid inflation for improved protection during high G onset
rates is fitted to Typhoon and Lightning II.
b.
Anti-G Straining Manoeuvre. The 'anti-G straining manoeuvre' (AGSM) comprises 2
elements: muscle tensing and breathing strain. One of the normal mechanisms for propelling
blood along the veins and back to the heart is by the squeezing action of surrounding muscles.
Continuous tensing of the muscles in the calf and thighs throughout the G exposure (without
relaxing) is therefore beneficial and has the additional effect of raising blood pressure by
increasing the resistance to blood flow through the limbs. Raising the pressure within the chest
and abdominal cavity, by straining (attempting to force air out out against a closed throat) will raise
the blood pressure. Correct timing of the manoeuvre is essential; the strain should last no longer
than 3 seconds to 4 seconds to allow a breath to be taken and blood to move from the peripheral
veins to the heart. A longer strain may reduce blood flow back to the heart. The AGSM is a
practical skill which must be learned during centrifuge training (see sub-para d). In combination
with correctly fitted anti-G trousers, the anti-G straining manoeuvre should enable aircrew to
tolerate sustain +7 Gz to +8 Gz for 15 seconds without losing central vision, or suffering a G-LOC,
and this is extended to +9 Gz for pilots using Typhoon equipment. The individual piloting the
aircraft is likely to have a higher black-out threshold than a non-handling crew member as the pilot
is in a position to anticipate the required actions. Timing of the AGSM is critically important,
especially at high G onset rate. Muscle tensing can be started before G is applied, but the
breathing strain must not be started until G onset or G tolerance may be reduced. For high G
onset manoeuvres, it is essential that G straining is proactive, and not reactive to visual
symptoms, or G-LOC may occur without warning.
c.
Positive Pressure Breathing. In addition to full coverage anti-G trousers, Typhoon pilots are
supplied with breathing gas through the mask at a positive pressure. Pressure breathing for G
protection (PBG) cuts in at +4 Gz and rises linearly to 60 mm Hg (8 kPa) at +9 Gz. PBG increases G
tolerance in the same way as the breathing component of an anti-G straining manoeuvre, and can
reduce fatigue and extend the time spent at high G. It also makes breathing easier at high levels
of +Gz acceleration. PBG is subjectively more transparent than pressure breathing for altitude
protection and is very readily tolerated. In a PBG system, the breathing regulator delivers gas at a
pressure proportional to the applied G by responding to the outlet pressure of the anti-G suit supply
valve. This arrangement prevents pressure breathing being applied without inflation of the anti-G suit,
thus avoiding a situation which could be deleterious for G tolerance.
d.
Centrifuge Training. The G-LOC rates in air forces around the world (including the RAF)
have prompted the introduction of centrifuge training. To promote G awareness, recognize the
personal symptoms of impending G-LOC, and develop an effective anti-G straining manoeuvre, all
UK MOD fast-jet aircrew undergo centrifuge training at the ab-initio stage. Additional centrifuge
training up to +9 Gz is provided for those aircrew converting to the Typhoon life support system,
and refresher training is required for all aircrew at 5 yearly intervals.
e.
Physical Fitness. Physical conditioning may be beneficial to G tolerance and may also
reduce the risk of neck injury. The Aircrew Physical Conditioning (ACP) programme (see
Volume 6, Chapter 16) has been introduced to promote the right balance between anaerobic
training, which may increase the time for which aircrew can sustain high levels of +Gz
acceleration, and aerobic conditioning. Aerobic exercise may improve G tolerance but in some
individuals it can make matters worse, especially if the resting heart rate is below 55 beats/min.
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The ACP also features core stability and neck strengthening exercises to reduce the risk of neck
and back pain.
f.
Position. Alterations in posture can reduce the vertical heart-to-brain distance and improve the
efficiency of the circulation to the eye and brain. Pilots in World War II crouched forwards to increase
the black-out threshold by nearly 1 G, and some aircraft also featured a high rudder pedal position to be
used in combat manoeuvring. In theory, tolerance to G can be further increased by tilting the seat
backwards, or by placing the pilot in a prone or supine position. However, this is impractical due to
cockpit design, external vision and ejection problems, and current Service aircraft do not feature a seat
position which improves G tolerance.
g.
G Awareness. A number of factors can reduce G tolerance below the expected level, and it
is important for aircrew to be aware of this possibility. Some of these are within the control of the
pilot and some are not, but it is important to realise that G tolerance can vary quite extensively
from day to day. For example, tolerance may be reduced by dehydration (inadequate intake of
fluid or excessive sweating), hunger (an empty stomach and gut and lowered blood sugar).
Fatigue, either physical or mental, may reduce G tolerance, as may heat stress which diverts
blood to the skin. The after-effects of alcohol, and some medicines including those available over
the counter may reduce tolerance. Even after a week away from flying G tolerance may be below
that expected. It is important for pilots to have G awareness in mind when planning and executing
sorties that will include high G manoeuvring.
h.
G-Warm. A ‘G warm’ should be carried out prior to any high G (> 4 Gz) manoeuvring. The G-
warm provides confidence that the anti-G system is working correctly, allows a check for any day
to day variation in personal G tolerance, and provides an opportunity to focus on and practice the
AGSM. If carried out shortly before high G manoeuvring, the G-warm can improve G tolerance for
the following 3 to 5 minutes by boosting the amount of adrenaline in the blood and improving the
blood pressure response.
Effects of –Gz Acceleration
33. When the resultant of radial acceleration and gravity is directed towards the head, as in a bunt,
the body experiences 'footwards' acceleration (–Gz). This feels more unpleasant than the equivalent
positive G; even simple inversion (–1 Gz) causes engorgement of the head and neck due to the
abnormally high venous pressure. When the level of –Gz acceleration is increased, the face becomes
painfully congested, and the lower lids may droop over the eyes so that sunlight shines through them
and appears red (possibly the cause of 'red-out'). Unsupported blood vessels in the white of the eyes
may rupture due to the high pressure and the resulting red discolouration takes several days to
clear up. Negative acceleration also has marked effects on the heart, provoking a reflex triggered by
blood pressure sensors in the neck, which slows or even stops the heart for several seconds. While
this (very rarely) may cause unconsciousness directly from negative G exposure, the more important
effect in aviation is what happens if positive G is pulled immediately afterwards. This effect,
sometimes called the 'push-pull effect', causes G tolerance to be reduced by 1 G, or possibly more,
after a negative G manoeuvre (e.g. inverted spin recovery). Even short exposures in the range 0 Gz to
+1 Gz are sufficient to reduce positive G tolerance for the next 10 seconds to 15 seconds
(e.g. unloading to increase airspeed and then pulling). There is no means of protecting against the
push-pull effect other than anticipating the reduction in G tolerance it may cause and straining to make
up the deficit. Equally, there is no practicable method of protection against sustained negative G,
although manoeuvres involving –Gz are much less common than those involving +Gz, chiefly being
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confined to aerobatic display flying. The limit of tolerance for sustained negative G is in the order of –
3 Gz for 30 seconds, though –5 Gz or more may be tolerated for very brief periods (1 second to 2
seconds) in trained individuals.
Effects of ±
Gy Acceleration
34. Lateral force control has been introduced in experimental aircraft by the use of additional vertical
fins forward of the aircraft’s centre of gravity and thrust vectoring. The imposed forces are of the order
of ±1 Gy, or less, and have no significant physiological effects, though if sustained may lead to
increased fatigue of neck muscles. In the Typhoon aircraft, up to 2 Gy may be experienced briefly in
rapid roll rates at high angles of attack, but this type of manoeuvring has not been associated with any
aeromedical problems.
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CHAPTER 6 - SPECIAL SENSES
VISION
General
1.
The ability to see well is a necessary requirement in flying. The aviator is completely reliant on
sight at every stage of flight, in order to see the ground, the instruments and other objects. However,
vision is more than just an act of seeing; it depends on the proper utilisation of the eyes and then on
the correct interpretation of the visual picture by the brain.
The Eye
2.
The eye receives rays of light directly from luminous sources or reflected from objects. It then
focuses this light on the retina at the back of the eyeball, by means of the cornea at the front of the
eye, and the lens within it. Photoreceptors in the retina convert light into nerve impulses, which are
then transmitted by the optic nerve to the brain, where they are interpreted as a picture.
3.
Eyeballs are roughly spherical, and approximately 2.5 cm in diameter. They lie within the bony
orbit, suspended in fat, and are protected against damage from all directions, except at the front,
where protection is limited to that provided by the eyelids.
4.
The eyeball is filled with fluid and depends on its own internal pressure to maintain its shape and
integrity. It is composed of three skins, which are modified at the front to admit light (see Fig 1). The
outermost skin, the 'sclera', is tough, supportive and relatively free from blood vessels. It has a
transparent region at the front called the 'cornea'. The middle skin, or 'uvea', contains many blood
vessels; its prime function is nutritive. At the front, this middle skin becomes the 'ciliary body' and iris,
while at the rear it forms the 'choroid'. The innermost skin is the retina, which is light sensitive and
corresponds to the choroids in its extent. A person’s best visual acuity is obtained when an image falls on
the central area of sensory cells (the 'fovea') which is situated on a pigmented area in the centre of the
retina, called the 'macula'. The globe of the eyeball is divided into two main compartments by the lens iris
diaphragm; a large rear compartment filled with a clear jelly, called the 'vitreous', and a smaller front
chamber filled with a clear liquid, called the 'aqueous'. The circular iris contracts as light levels increase in
order to make the pupil (the opening in the iris in the centre of the eye) smaller and limit the amount of
light falling on the retina. As light levels decrease, the iris dilates to admit more light.
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6-6 Fig 1 The Human Eye
Pupil (Black Area)
Iris (Colour of Eye)
Sclera
(White of Eye)
Aqueous Humour
Cornea
(Glassy Front of Eye)
Retina
Lens
Ciliary Body
Vitreous
Uvea (Choroid)
Humour
Fovea
Optic Nerve
5.
It is conventional to compare the human eye with a camera, but this analogy is too simple. The
eye can adjust over an enormous range of brightness; it is capable of discrimination between fine
hues; and it can distinguish detail which subtends visual angles of less than 30 seconds of arc.
6.
This sophisticated visual performance is due principally to co-ordination between eye and brain.
The brain and the neural retina process visual information to improve the image falling on the retina,
adding, subtracting and comparing data, as necessary.
Visual Function
7.
It is convenient to separate the visual function into its three component senses, light, form, and colour.
8.
The eye is capable of functioning over a wide range of luminance. The luminance of an object is
a measure of its brightness; it is the product of the illumination falling on an object and the object’s
reflectance. The eye is capable of detecting light as dim as faint starlight; the maximum limit, where
discomfort is evident, is as high as bright sunlight on snow. Two visual mechanisms function over this
range. 'Scotopic' (or 'rod') vision operates over the lowest quarter of luminance; over this range the
ability to see form is poor, and colour is not perceived. Over the remainder of the range, 'photopic' (or
'cone') vision takes over, progressively giving, with increasing luminance, the advantages of good form
sharpness and the ability to discriminate colours. The transitional stage, when both rods and cones
are functioning, is known as 'mesopic' vision, and corresponds roughly to the light available under full
moonlight.
9.
The eye requires time to adjust to varying luminance because the control is a photochemical
reaction. When the eye adapts from dark to light, the adjustment is rapid, but in adapting from light to
dark, the adjustment is slower. The dark adaptation curve (Fig 2) shows the threshold luminance
required to see a light source (as a function of total darkness). It can be seen that there is not a steady
increase in sensitivity. The curve is in two portions, the initial rapid adaptation being that of the cones,
and the slower adaptation that of the rods. A further feature of rod and cone vision is their different
colour sensitivity. Rods are most sensitive to blue/green light and cones to yellow/green light (see Fig
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3). This differing colour sensitivity is evident at dusk, when red objects appear darker whereas blue
objects retain their apparent brightness.
6-6 Fig 2 Dark Adaptation Curve
0
e
c
-1
Cone Adaptation
n
a
)
2
in
m -2
m
r
Rod Adaptation
u
e
L
p -3
ld
s
o
la
h
e
d -4
s
re
n
h
a
T
(C -5
g
lo
-6
0
5
10
15
20
25
30
35
Time (mins)
6-6 Fig 3 Rod and Cone Colour Sensitivity
100
Scotopic (Night)
Photopic (Day)
Response
Response
(Peaking in
(Peaking in
80
blue-green)
yellow-green)
ity
Rods.
Cones.
s
o
60
in
m
u
L
40
e
tiv
20
la
e
R
400
500
600
700
Violet Blue Green Yellow Orange Deep Red
Wavelength m-9
10. The field of view of each eye, defined as that portion of the external world visible to the stationary
eye, extends from about 60º nasally to 75º temporally. These limits are imposed by anatomical
features, such as the bridge of the nose and the depth of recession of the eyes. On the temporal side
of each visual field there is a blind spot covering about 5º of which the observer is largely unaware.
This is where the optic nerve leaves the eye (Fig 1), and there are no photoreceptors. The fields of the
two eyes overlap by approximately 60º, where the same object is seen with both eyes; in this region
vision is binocular, so the physiological blind spot is not noticed. Helmets, visors, and aircrew
spectacles are designed to have minimal impairment on the field of view.
11. When an object is viewed it is imaged on the fovea and the surrounding macula. The fovea is a
specialised region of the retina, composed entirely of cones. Covering approximately 1º, the fovea is
where vision is sharpest, and colours are most readily seen. Peripheral to the fovea the retina is
composed of both rods and cones; the ratio of rods to cones increases, and visual resolution
decreases, with distance from the fovea.
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12. As a result of this double mechanism for light appreciation, objects in dim light are best detected
by looking 'off-centre', using the rods. Furthermore, to maintain dark adaptation it used to be
customary to wear red goggles in lighted crew rooms, and to use red cockpit lighting, since rods (unlike
cones) are insensitive to the longer red wavelengths. The advantages of preserving rod adaptation are
limited, as few flight tasks can be performed with rod vision. In most cases, the sharpness of vision
given by the cones is imperative, and the disadvantages involved with red cockpit lighting systems in
colour discrimination, the increased focusing effort required, and the distortion in the relative luminance
of coloured objects, might outweigh any theoretical advantage.
13. A valuable feature of rod vision is its ability to detect movement as an image traverses the retina.
It is useful, therefore, in search procedures at night, not to allow the rod image to stabilise within the
range of involuntary eye movements, but to scan the area of search in small arcs, inducing a moving
image of a stationary object on the retina.
14. Under good conditions, the eye can resolve detail which subtends a visual angle of 30 seconds of
arc. However, under some special circumstances, much finer resolution is possible. A single line may
be differentiated against a plain background when it subtends a visual angle as small as 0.5 seconds
of arc. This is more a measure of contrast than of resolution, but it is important in aviation, as aircraft
or wires may first be sensed by their contrast against the sky.
15. There are many factors which may influence the resolution of the eye. These include atmospheric
conditions, the optical quality and cleanliness of interposed transparencies, the requirement for
spectacles, and eye disease. The large pupillary diameters, which occur in near darkness, reduce the
depth of field of the eye, rendering the decrement caused by the need for corrective spectacles more
evident.
16. Recognition of targets is profoundly influenced by the inductive state of the retina. One part of the
retina modifies the function of another part. This is known as 'spatial induction'. In aviation, spatial
induction will enhance the recognition of aircraft against the sky. The bright sky diminishes retinal
sensitivity, and a grey aircraft therefore appears darker, with a consequent increase of the contrast
between the target and the sky. However, a stimulus on a portion of retina will also affect the function
of that portion to a subsequent stimulus. This is known as 'temporal induction’ and may reduce target
recognition. If a bright object, such as the sun, forms an image on a portion of the retina, the sensitivity
of that portion will be depressed for a considerable period of time. This may cause low contrast targets
to remain unseen.
17. Visual resolution is greatly influenced by contrast between target and background, and by the
prevailing brightness of the target. Sharpness improves with increasing luminance, up to a moderate
level, beyond which no further increase occurs. At very high luminance, sharpness may even be
impaired. The best resolution is achieved when the luminance of the target and the ambient lighting
are similar. If an aviator is placed in a dark cockpit with only a small window on the world, the
resolution of bright external targets will suffer. When cockpit illumination is increased, resolution
improves. Conversely, resolution will be impaired with a bright cockpit and a dim target.
18. Colour sense is a function of cones, and therefore of photopic (day) vision. According to the
generally accepted theory of colour vision, there are three classes of cones present at the macula, in
the ratio of 1:10:10. These cones have absorption peaks at blue, green, and red in the colour
spectrum. A combination of these three primary colours, in the correct proportions, is seen as white
light. By varying the proportions and saturation (subtraction of white light), any other colour can be
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matched. The fovea is rod-free and possesses few blue cones. As a result, if signal lights may be
seen only as point sources, it is important not to use blue, which might be seen as white.
Psychology of Vision
19. The eyes of new-born human babies can take in light for processing in the brain; they have the
sensation of seeing. However, they are unable to interpret what they are seeing until some considerable
time after birth. The interpretative aspect of sight must be learned. The learned ability to interpret visual
stimuli is called 'perception'. Thus visual sensation is innate whereas perception is learned.
20. Unfortunately, because the brain must interpret visual stimuli and give them meaning before an
object is perceived, any inadequacy of the stimulus can lead to faulty perceptions. Humans do not see
the world in the exactly same way as a camera can record it. Perceptions tend to be inaccurate, often
incomplete, distorted and usually influenced by highly personalised views of the nature of the world.
Seeing what is expected (or wanted), rather than what is actually there, is common.
21. The eye is a not a particularly good optical instrument, but the image that the brain perceives is
remarkably stable and has excellent definition. The brain uses a number of methods to reduce the
confusion of sensation, and to ensure that the visualised image is stable and consistent. Some of
these methods are:
a.
Expectancy. The brain depends on experience and memory to interpret the visual images
presented to it. The process by which memory influences perception is called 'expectancy' -
seeing what is expected. An example of this would be missing out the second "the" in the phrase
"A bird in the the hand". The second "the" is seen physically, but not perceived, as the brain
ignores it on the basis that there is normally only one "the" in this phrase.
b.
Perceptual Organization. The brain arranges groups of objects into certain patterns which
make perception easier. An example of how this is exploited in aircraft design is the layout of
cockpit instruments. These are so arranged that related instruments are placed together and are
viewed as a whole, rather than individually.
c.
Size Constancy. There is a memory store within the brain which relates known objects and
their size. Irrespective of the size of an image at the eye, the object is perceived at its known size.
This phenomenon is exploited by artists who may include a familiar object (a human, a car, or a
house, for instance) in a landscape picture to give the viewer a sense of scale.
22. Although the eye uses all of these means in an attempt to obtain a consistent and stable visual
picture, it can still give incorrect information. This occurs in some conditions of disorientation, where
either the eye misinterprets the correct information it is given, or it is given inappropriate information.
This is discussed further in paragraph 47.
23.
Perception Time. Perception time is the elapsed time between the image of an object falling on the
retina to focused central fixation and recognition. For a familiar object, this may be of the order of a second.
An unfamiliar object, viewed under adverse conditions, will have an extended perception time. This intrinsic
delay is important when considering hazard avoidance or ground target detection.
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Visual Function in Flight
24. There are several visual problems which are specific to aviation. These are outlined below:
a.
Empty Field Myopia and Night Myopia. During flight, particularly at night or in cloud, the
external scene is often featureless. Without visual cues to attract attention, the eye frequently
comes to focus at a point in space, one or two metres distant, making the aviator functionally
short-sighted. If another aircraft enters the visual field, it might not be seen, as objects at infinity
would be blurred. For this reason, it is important that aircrew periodically look at objects at virtual
infinity, such as wing tips or head-up display symbols, in order to extend their focus.
b.
Perception Time in High Speed Flight. Large distances may be travelled during the time
taken to perceive and react to objects appearing in the visual field. This problem may become
critical in the high-speed, low-level role, especially when vibration may increase pilot stress. Table
1 lists the estimated times required for the various operations from an image falling on the
peripheral retina to perception, reaction and the finish of aircraft manoeuvre. It is not possible to
reduce these periods and, indeed, they may be extended under adverse conditions. When a pilot
transfers attention from scanning the external field to reading an instrument and returns to the
external field, there is a time interval of up to 2.5 seconds, during which time the aircraft might
cover a considerable distance. This is why vital information is often presented with a head-up
display, in order that attention need not be removed from the external scene. Important
instruments are designed, sited and illuminated so that the information they give may be extracted
rapidly.
Table 1 Distance Travelled by an Aircraft whilst the Pilot is Perceiving and Reacting to an
Object Approaching in the Visual Field
Distance Travelled (nm)
Elapsed
by an Aircraft Flying at:
Stage in Avoidance of an Object
Time
250 kt
500 kt
(seconds)
(1) Time taken from image first falling on peripheral retina to
1.0
0.07
0.14
focused central fixation and recognition.
(2) Time taken for decision and subsequent action.
2.5
0.17
0.35
(3) Time taken for aircraft to change heading.
1.5
0.10
0.21
Total time elapsed
5.0
0.35
0.70
Note: The above distances must be doubled when two aircraft, travelling at the same
speed, are on a head-on collision course.
c.
Dynamic Visual Acuity. In the previous paragraphs, where visual resolution was discussed,
it was assumed that the object of interest was stationary. Where a target moves across the visual
field, the eye must track it in order to maintain its image on the part of the retina which will give the
sharpest picture (the fovea). The ocular pursuit mechanism is capable of maintaining steady
fixation on a moving target where the angular velocity does not exceed a value of about 30º per
second. At an angular velocity of about 40º per second, visual acuity may drop to half its static
value, the decrement increasing further as angular velocity increases.
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d.
Depth Perception. Both binocular and monocular cues are used to assess depth. The
binocular cues of accommodation and convergence have a limited value at the visual ranges
important in aviation. This limitation is largely due to the small distance between the two eyes of
about 6 cm, making the base of the 'rangefinder' too short. These binocular cues will provide
depth information at up to one kilometre. Stereopsis, which is produced by the slightly different
images of the object falling on the fovea of the two eyes (due to the separation of the eyes), also
gives some depth perception but only to 40 to 50 metres. Monocular cues to depth perception are
as follows:
(1)
Parallax. Head movements cause targets which are at different distances from the
observer to move in opposite directions relative to each other. The nearer target moves in
the reverse direction to the head movement.
(2)
Perspective. The property of converging parallels, such as runways and railway lines,
allows us to reconstruct the relative distances of parts of a scene.
(3)
Relative Size. Objects of known size can, by virtue of the angle they subtend, provide
information as to their distance from the observer.
(4)
Relative Motion. If two objects are moving at the same speed, parallel to the horizon (i.e. at
right angles to the viewer’s line of sight), the angular velocity of the nearer will be greater than the
angular velocity of the farther. Since angular velocity is determined by the object’s velocity and
range, a knowledge of either would enable an estimate of the other to be made.
(5)
Overlapping Contours. An object which overlaps another must be closer than the other.
(6)
Aerial Perspective. Objects at great distances appear more blue, owing to the
scattering of light by particles in the atmosphere. White lights may appear more red when
seen at a distance because the red component is less subject to scatter than the blue
component. This is a further reason to exclude blue signal lights in aviation.
Visual Illusions
25. The most important illusions in flight are those associated with the vestibular apparatus; these illusions
are dealt with later in this chapter. Only those illusions which are purely visual are included here.
26.
Autokinesis. A light, such as a star or aircraft tail light, seen against a black background, will,
after a short time lapse, appear to wander in different directions. These apparent movements occur
because the background does not provide sufficient information about the involuntary eye movements
which are occur normally. These eye movements are then interpreted as movements of the light.
27.
Flicker. The flicker produced by helicopter rotors has been found to cause epileptiform episodes.
The problem arises when the frequency is between 5 and 20 Hz, being worst at 12 Hz. Anti-collision
strobe lighting systems, which are favoured for their conspicuity, have a flash frequency of around 60
flashes per minute (ie 1 Hz) and are normally harmless.
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Vision Protection Devices in Military Aviation
28. In military aviation, vision has to be protected from several possible hazards. These are outlined
in the following paragraphs.
29.
Solar Glare. Glare from direct, reflected, or scattered sunlight causes discomfort and reduction in
visual sharpness. In transport aircraft, spectacles suffice to overcome the problem, but in high-
performance aircraft, where crews wear protective helmets, an adjustable tinted visor, integral with the
helmet, provides protection against external glare and gives an undiminished view of the flight
instruments. In the fully lowered position, the visor is capable of filtering all of the incoming light. The
amount of tint in the spectacles, or visor, is chosen to be a reasonable compromise between
attenuating high luminances, without producing a significant visual decrement. The tint is neutral, in
order to avoid affecting colour discrimination, particularly the recognition of red warning signals. As
discomfort from glare is eliminated, it is also necessary to attenuate blue light, and infra-red and ultra-
violet radiation, in order to avoid the possibility of retinal damage. The field of view is as wide as
possible, and the optical and physical properties conform to carefully calculated specifications.
Unapproved sunglasses are unlikely to satisfy these requirements.
30.
Protection of the Face against Birdstrike. The hazard of birdstrike is always present during
flight (during day or night) at low level. The majority of birdstrikes in the UK occur below 500 ft AGL.
The incidence of birdstrikes in low-level, high-speed flight is such that a strike in the cockpit area is not
an uncommon emergency. Ideally, cockpit transparencies should be strong enough to withstand bird
impact, but the cost in weight may be prohibitive. In the absence of other forms of protection, the use
of a helmet-mounted visor made of a strong transparent material, such as polycarbonate, is essential.
The visor protects much of the face as well as the eyes. Tinted and clear visors are incorporated in
current helmets to provide protection against both glare and birdstrike.
31.
Blast Protection. During a high-speed ejection, the head is exposed to high aerodynamic forces.
These may damage the face and eyes. With the visor lowered, the helmet, visor and mask are so integrated
that they remain in place throughout the ejection and provide the necessary protection.
32.
Canopy Fragmentation Devices. With aircraft designs in which there is no reasonable certainty
that the canopy would be clear of the aircraft before the ejection seat moved, explosive devices may be
fitted to shatter the transparencies and permit the seat and occupant to pass safely through. There
have been a number of occasions in which lead spatter from the explosive charges has caused
superficial damage to the face and the eyes. It is most unlikely that any such damage would result if
the visor were lowered or the eyes closed at the time of ejection.
33.
Lasers. Lasers are devices which produce intense, coherent and collimated beams of monochromatic
light, usually of small diameter. The energy density within the beam decreases slowly with increasing
distance from the laser. The eye has the ability to focus the collimated beam of some lasers, and to
concentrate the energy into small image sizes on the retina. Thus, lasers can damage eyes at a
considerable distance from the source. The applications of lasers in military aviation include ranging and
target illumination. Protection is best provided by distance. Codes of practice, such as BS EN 60825,
JSP 390 and STANAG 3606, give guidance on the method of calculating the Nominal Ocular Hazard
Distance (NOHD) – the distance within which the laser may be hazardous. The calculation is based upon
knowledge of the maximum safe corneal energy, or power density, for the particular laser system, together
with the beam divergence and maximum output of that system. Hazard distance will increase with the use of
magnifying optical instruments, e.g. binoculars or telescopes, as a result of the greater amount of radiation
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collected by the object glass. The necessity for protection of pilots from their own lasers is debatable. The
presence of of a specular reflector in the range area, orientated normal to the beam, will be very unlikely; but
should a reflector be present, its reflectivity at the laser wavelength is not likely to be high. Where it is
considered necessary, protection may be provided by goggles or visors with the requisite level of protection
at the laser wavelength. In military operations, one of the most frequently encountered lasers is the
neodymium-yag laser, a near-infrared laser operating at 1064 nanometres. This laser is widely used in
target designators, both ground and air based. Its beam is invisible, and operational lasers can cause
permanent retinal damage at distances of up to 15 to 20 nm. Because of its widespread use, and high
potential for injury, visors and spectacles are available to protect against it.
34.
Nuclear Flash. The fireball resulting from a nuclear explosion is capable of producing direct and
indirect flash blindness and, indeed, may cause eye damage. By day, the small pupillary diameter and
the optical blink reflex should prevent retinal burns from direct flash at distances at which survival is
possible. Similarly, indirect flash blindness, from scattered light within the atmosphere and the globe of
the eye itself, does not pose a problem. Temporary blindness from the image of the fireball is difficult
to avoid, but at survival distances, the irradiated area is likely to be small. Even in the worst case,
where the fireball is imaged on the macula, para-macula vision should allow all vital flight procedures to
continue. At night, when the pupil is dilated, the situation is much worse and indirect flash blindness
may deprive the aviator of all useful vision for an unacceptably long time. In short, protection against
nuclear flash is desirable by day, but vital at night. Protection devices are being developed for this
purpose.
HEARING
General
35. A good standard of hearing is important to aircrew because the recognition of auditory signals is an
integral part of their tasks. Audition is more than the act of passive listening, and involves the
interpretation by the brain of signals, often embedded in background noise. The ear receives pressure
variations, or sound waves, normally through the air, and converts these into neural impulses. For the
normal adult, the frequency-range of vibrations within the audible spectrum is 20 Hz to 10,000 Hz,
although the frequency limits of the ear can vary between 2 Hz and 20,000 Hz. Within the audible range,
the ear is most sensitive to frequencies between 750 Hz and 3,000 Hz.
36. The function of the hearing apparatus is to collect sound waves and convert them into nerve
impulses. It consists of three main parts, the outer ear, the middle ear, and the inner ear, and is shown
in Fig 4. The eardrum is in the outer wall of the middle ear cavity, separating it from the outer ear.
Sound waves are collected by the external ear and directed onto the eardrum, which vibrates.
Attached to the inner surface of the eardrum is a system of three small bones, lying in the air-filled
cavity of the middle ear, which condition the vibrations and transfers them to the fluid-filled inner ear.
The air-filled cavity of the middle ear is vented via the Eustachian tube. Temporary hearing loss can
occur when there is a pressure difference between the middle and outer ear, as may be caused by
descent from altitude (see Volume 6, Chapter 4). A common cold, respiratory infection, or severe hay
fever can cause the Eustachian tube to become blocked. A climb or descent in this condition can
result in rupture of the eardrum. This is one reason for not flying with a cold. It is the part of the inner
ear, known as the 'cochlea', which transduces vibrations into nerve impulses, essentially performing an
analysis of sound by frequency.
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6-6 Fig 4 The Human Ear
Inner Ear
Bony Wall
Cochlea
Nerve
External Canal
Ear
Middle
Drum
Ear
Cavity
Eustachian
Tube
37. More than 1% of the total power output of a jet engine is in the form of noise, ranging from the
lower limits of audibility to ultrasonic oscillations. Sound intensity is measured in decibels (dB) (a
logarithmic unit of the ratio of the measured sound intensity to a reference sound intensity). A
logarithmic formula is used to avoid an excessively large scale, since the range of responsiveness of
the human ear is very wide. The noise levels in decibels of certain familiar sounds are given in
Table 2. Note that an increase of 3 dB represents a doubling of sound intensity.
Table 2 Noise Levels of Familiar Sounds
140
Turbojet at 50ft.
Jet Takeoff at 50 ft.
120
Inside Low Level Fighter
Large Jet Landing at 50ft.
Inside Helicopter
100
Pneumatic Drill
ls
e
80
ib
c
e
Street Corner Traffic
D
Normal Speech at 3ft.
in
ity
s
60
Office
n
te
Living Room
In
40
Library
20
Broadcasting Studio
0
Reference Value
38. Intense sounds or noise can induce temporary hearing loss and produce ringing in the ears when
the noise ceases, although recovery from this is fairly rapid. The extent of temporary hearing loss is
related to the frequency of the sounds, their intensity and duration. The reduction in sensitivity is at
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frequencies higher than those of the stimulating noise. A noise at one intensity will produce the same
temporary loss of hearing as another noise at double the intensity, if the duration of the former sound is
double that of the latter. Noise-induced loss is not normally induced by sounds at below 80 dB. If
noise levels which induce temporary hearing loss are experienced regularly over a period of years,
then permanent loss of hearing is likely. Permanent loss of hearing is observed first at the higher
frequencies, with a pronounced loss at 4,000 Hz. Permanent loss of hearing can be allayed by
keeping the noise dose within specific limits. Very intense sounds can invoke special responses even
in a short time. At 120 dB, localised discomfort in the ear is experienced, 140 dB produces pain in the
ear and the eardrum may be ruptured at levels above 160 dB.
39. Sounds and voices are normally perceived within a background of unwanted noise. Sounds of
similar frequencies interfere and make hearing difficult. To offset the effects of this masking, it is
necessary to have the signal at a greater intensity than the background noise. A difference of 15 dB
will ensure accurate recognition and, as the difference increases, so will accuracy of recognition. It is
possible to mitigate the effect of a poor signal to noise ratio – hearing in a noisy environment – by using
familiar, meaningful and predictable signals or words.
40. The noise inside a jet aircraft is generated by four sources:
a.
Environmental control systems (pressurisation) and communications.
b.
Boundary layer noise, at higher IAS.
c.
Engine exhaust, although this is often inaudible over the pressurisation.
d.
Special sources, such as armament discharge.
These four sources combine to produce different noise pictures for different aircraft types. The fast jet will
show a flat noise spectrum, with a high proportion of boundary-layer noise, whereas a helicopter will show
high noise at the low frequencies because of the rotor and blade mechanisms. The wearing of properly
fitting headgear is very important because helmets, with their ear cups, can attenuate impinging noise
considerably. Wearing earplugs within the ear cups further attenuates noise, although they tend also to
reduce the audibility of intercom and radio. Increasingly aircrew are being issued with fitted ear plugs with
self-contained communication speakers but active noise reduction measures are being considered in
addition to the usual helmet ear cup protection, owing to new industrial noise protection standards and their
strict enforcement. These standards will mean that more effective noise protection mechanisms will be
required to provide adequate protection in some aircraft, as well as for many carrying out roles on the
ground. Minimising noise levels not only safeguards hearing but also reduces the stress caused by high
noise levels. Work in high noise levels increases fatigue, irritation and an accompanying risk of accident,
although there are wide differences in the stress reaction of individuals to noise.
41. People not directly involved in aviation are most likely to be disrupted by aircraft noise, so it is important
that as much of the ground running of aircraft as is possible is done away from buildings housing such
personnel. Additionally, it is advantageous to protect buildings in aircraft movement areas by such means as
double-glazing of windows. Individuals who, by nature of their work, are required to be in high noise areas
must be suitably protected by means of personal noise-excluding ear protectors.
THE SENSE OF BALANCE
General
42. The constant barrage of information coming from the specialised organs of balance in the inner
ear, which signal movement of the head and its orientation (attitude) to the Earth’s gravitational force,
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goes mostly unnoticed, unlike sight or sound. It is only when these sense organs are stimulated by
unusual patterns of linear or angular motion, as in flight, or when their function is disturbed by disease,
that the signals from these receptors give rise to disturbing sensations.
The Vestibular Apparatus
43. The inner ear is made up of the cochlea (the organ of hearing) and the vestibular apparatus (the
organ of balance). The labyrinthine structure of the vestibular apparatus is shown diagrammatically in
Fig 5. It consists of three thin-walled tubes – the semicircular canals, disposed in planes approximately
at right angles to each other. These communicate with sac-like structures called the 'otolith' organs
('utricle' and 'saccule'). The whole system is filled with fluid and is tethered within a bony cavity at the
base of the skull. The vestibular apparatus on one side of the head is a mirror image of that on the
other.
a.
Semicircular Canals – Transduction of Angular Acceleration. In each semicircular canal,
there is a swelling where the sensory cells are located. Sensory hairs from these cells pass into
the substance of a gelatinous flap (the 'cupula') which lies across the bulge (or 'ampulla') of the
canal (see Fig 6). An angular acceleration in the plane of the canal causes a deflection of the
flap, because its motion is resisted by the inertia of the ring of fluid. Deflection of the flap bends
the sensory hairs and produces a corresponding alteration of the neural signal which is
transmitted to the brain. The flap has the same density as the fluid in the canal, so it is not
deflected by linear accelerations.
6-6 Fig 5 The Vestibular Apparatus
Semicircular
Canal
Utricle
Vestibular Nerve to Brain
Auditory
Nerve
Saccule
Cochlea
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6-6 Fig 6 Section of Semicircular Canal
Nerve Transmitting
Sensory Cells
Signals from Sensory
Cells to Brain
Flap (Cupula)
(Deflected)
Ampula
Flap (Cupula)
Utricle
(Rest Position)
Angular
Acceleration
of Skull
Membranous
Motion of
Tube
Fluid Relative
to Skull
b.
Otolith Organs – Transduction of Linear Acceleration. Each otolith organs houses a plate-
like congregation of sensory hair cells, covered by a gelatinous layer that carries in its free surface a
'frosting' of calcium carbonate crystals (see Fig 7). The density of this mineral is more than twice that
of the fluid which fills the system, so it behaves as an inertial mass, restrained and supported by the
hairs of the sensory cells. Accordingly, a linear acceleration, acting in the plane of the otolithic plate,
deflects the hairs and alters the neural signal from the sensory cells. The otolithic plate, unlike the
cupula of the semicircular canal, is not heavily damped, so it conveys information to the brain about
the magnitude and direction of linear accelerations, and rate of change acceleration (jerk),
experienced by the head. Like any man-made linear accelerometer, the otolith organs are
influenced both by their orientation to the Earth’s gravitational acceleration (the gravitational vertical)
and by applied linear accelerations and, like the ball in the turn and slip indicator, they indicate the
direction of the resultant force vector. The configuration of the four otolith organs allows the direction
and magnitude of resultant linear accelerations in any axis to be sensed.
6-6 Fig 7 Section of an Otolith Organ
Calcium Carbonate
Crystals of Otolithic
Membrane
Gelatinous
Fluid
Substance
Hairs of
Sensory
Nerve Fibres
General
Sensory Cells
Cells
Leading to Brain
View
Enlarged Cross Section
Orientation on the Ground and in the Air
44. The ability of humans to determine their position, attitude and motion (ie spatial orientation), with
respect to a reference system provided by the Earth’s surface and the gravitational vertical, is
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dependent upon sensory information provided by the eyes, by the organs of balance and by other
receptors in the skin, joints and muscles which are stimulated by forces acting upon them (Fig 8).
6-6 Fig 8 Orientation
Sensory Receptors
Perception
Orientation of
Of Eyes
Head and Body in
Space (Earth Ref)
Orientation of
Integration and Interpretation
Head and Body
Of Inner Ear
of Signals Based on
relative to own
Past Experience and Expectancy
Aircraft and/or
other Aircraft
Orientation of
Of Skin, Joints
Aircraft in Space
Supporting Tissues
(Earth Ref) and/
or relative to
other Aircraft
a.
The Eyes. Both on the ground and in the air, the visual sense is pre-eminent, for it provides
a wealth of information about position, attitude and movement of the head in relation to the fixed
external environment. Even when external visual references are absent, as when flying in cloud,
the only reliable information is visual, and comes in symbolic form from the flight instruments.
b.
Other Sensory Systems. On the ground, balance and orientation to gravity can be
maintained in the absence of vision because of information provided by the vestibular apparatus,
and by the more widely distributed pressure and movement receptors located in the skin,
muscles, capsules of joints and supporting tissues. The dynamic range (sensitivity and frequency
response) of these receptor systems is nicely matched to the angular and linear motion stimuli
which occur during normal activities, (like walking and running) in a stable normogravic (1g)
environment. However, in the flight environment the body can be exposed to patterns of angular
and linear motion which are outside the functional range of these non-visual sensory systems.
Consequently, they may either fail to give adequate information, or they may give erroneous
information and lead to disorientation. Without visual cues, sustained manual control of the
attitude and flight path of an aircraft is impossible. Despite these inadequacies, the vestibular and
other acceleration-sensitive receptors do provide the aviator with information about the onset of
motion that can aid aircraft control, because the movement is sensed with less delay than the
change of position of an external visual reference or an instrument display.
Spatial Disorientation in Flight
45. There are several reasons why the task of maintaining correct spatial orientation in flight is more
difficult than when on the ground. These may be summarised as follows:
a.
In flight, angular and linear motions differ in intensity and duration from that to which humans
are functionally adapted.
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b.
The aircraft operates, and has to be controlled in, six degrees of freedom (3 linear and
3 angular). On the ground, there are normally only five degrees of freedom, and a stable
reference.
c.
The appearance of the external visual world can be difficult to interpret, especially when
visual cues are sparse or unfamiliar.
d.
When instrument references are employed, the cues are symbolic and separate; integration
and interpretation of such information is more demanding than when unambiguous visual
references are employed. For example, a glance at the visual horizon frequently enables a pilot to
assess the attitude of the aircraft in all three planes. However, two separate instruments are
needed (the attitude indicator for pitch and roll, and the turn needle for yaw) to obtain the same
information when the visual horizon is obscured.
46. False sensations (or perceptions) of attitude, position, or motion are a common experience of
flying personnel, and are a quite normal manifestation of the limitations of sensory function and
information processing. Usually, the aviator is aware that the sensation being experienced is false (i.e.
it is illusory), because it is contradicted by correct information about aircraft orientation provided by the
flight instruments; this is termed Type 2 Spatial Disorientation. Rarer, though much more serious, are
those incidents in which the pilot is not aware that the sensations are incorrect, and bases control of
the aircraft on a false perception (termed Type 1 Spatial Disorientation). This implies that control is
lost, or at least inappropriate, and so flight safety is jeopardised.
47. Many different kinds of erroneous sensation or perception, falling within the broad definition of
spatial disorientation, have been reported, and there are numerous causes. Some of the more
common types of disorientation are described and explained below.
a.
Failure to Sense Changes in Aircraft Orientation. Changes in aircraft attitude and flight
path can occur which are below the threshold detection level of the non-visual sensory systems.
Thresholds are dependent upon the intensity and duration of the motion stimulus. When the
changes are prolonged, ie more than 20 seconds, then acceleration is the important variable.
Average figures are 0.3º/sec2 for an angular movement and 0.1 m/sec2 (0.0l g) for a linear
movement. When the movement is more transient, ie 10 seconds or less, detection is determined
by the change in velocity that occurs; typical values are 1.5º/sec for angular motion and 0.3 m/sec
for linear motion. These figures come from laboratory studies, in which the subject’s task was
only to detect motion. In flight, many other factors and sensory stimuli compete for the aviator’s
attention, and so changes in attitude or velocity substantially greater than these 'threshold' values
can occur without being detected. In the absence of a visual reference, aircrew can, on occasion,
be quite unaware of an extreme change in attitude.
b.
False Sensations of Angular Motion. False sensations of angular motion are caused by:
(1)
Sustained Rotation. In general, misleading sensations of angular motion are due to
dynamic limitations of the semicircular canals which, as noted earlier, are imperfect
transducers of angular velocity. The time constant of the leaky integration of angular
acceleration is about 5 to 10 seconds. Thus, at the beginning of a rotational movement (such
as a turn or a spin), the change in angular velocity is correctly transduced, provided it
exceeds the threshold value. However, once a steady rate of turn is achieved, and there is
no longer any angular acceleration, the deflected flaps of the canals in the plane of the
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motion slowly return to their rest position and the associated sensation of turn dies away (see
Fig 9). Provided there is no appreciable change in angular velocity, the turn can continue
without any sensation of turn being evoked. Recovery from the turn is associated with an
angular acceleration in the opposite direction to that on entering the turn. The cupulae are
deflected from their rest position and will erroneously signal rotation in the opposite direction,
at a rate commensurate with the change in velocity that has occurred. This false sensation
decays somewhat more quickly than the decay of the correct sensation during the initial
phase of the turn but, whilst this is happening, the presence of inappropriate eye movements,
induced by the vestibular stimulus, can degrade vision and impair the pilot’s only reliable
source of information. The intensity of these post-rotational effects is a function of the
duration of the rotational manoeuvre and of the angular velocity achieved; accordingly,
disorientation is most likely to be a problem on recovery from prolonged, high-rate rolling or
spinning manoeuvres.
6-6 Fig 9 False Sensations of Angular Motion
Right
100
Angular
Velocity
0
+100
Flap
(Cupula)
Deflection
Sensation : Turning Left
0
Arbitrary
Sensation : Turning Right
Units
-100
0
5
10
15
20
25
30
Time (sec)
(2)
Cross-coupled Stimulation. Cross-coupled stimulation of the semicircular canals occurs
whenever an angular movement of the head is made while rotating about another axis.
However, disorientating sensations are evoked only when rotation is prolonged and semicircular
canals do not signal correctly the sustained turn. For example, if the pilot’s head is moved in
pitch at the beginning of a prolonged spin, the sensation of both head and aircraft motion will be
correct. However, if the same head movement is made 15 to 20 seconds into the spin, the
head movement will elicit an entirely illusory sensation of rotation in roll. Head movements
made during the recovery phase cause even stronger and more bizarre sensations. As a
general rule, a head movement made in one axis, after rotating for some time about an
orthogonal axis, produces an illusory sensation in the third orthogonal axis.
(3)
Middle Ear Pressure Change (Pressure Vertigo). The semicircular canals may also
be stimulated by changes of pressure in the middle ear. Characteristically, on the first rapid
ascent of a sortie, there is a sudden onset of a false sensation of turning (ie vertigo), which is
associated with the venting of air from the middle ear. This disorientating sensation usually
dies away within 15 to 20 seconds, although initially it can be quite intense, and be
accompanied by blurring of vision and apparent movement of the visual scene. The same
symptoms may also be produced if an over-pressure in a middle ear is achieved when the
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ears are 'cleared' by a too forceful 'Valsalva' manoeuvre. Usually, the disability is associated
with impaired middle ear ventilation, due to a common cold or other respiratory tract infection,
and it is another reason for not flying when affected by these common ailments.
(4)
Effect of Alcohol. Alcohol modifies vestibular function and increases the likelihood of
disorientation. The vertigo which accompanies a change in position of the head with respect
to gravity is the best-known effect of alcohol. However, it is not generally appreciated that
such a 'positional vertigo' can be induced many hours after the blood alcohol level has
returned to zero. While in the presence of high g forces, the abnormal response may be
elicited for up to two days after the consumption of alcohol. Alcohol, and certain other drugs,
also tends to increase the visual disturbances produced by erroneous semicircular canal
signals, as, for example, on recovery from a prolonged spin. Normally, these inappropriate
eye movements are suppressed within a few seconds (2 to 5) but, when intoxicated, the
ability to suppress the movements is impaired, so vision may be blurred for a substantially
longer time (15 to 20 seconds). This increase of eye movement occurs at quite low blood
alcohol levels (10 to 20 mg/100 ml) though, unlike the positional vertigo, it does not persist
after the blood alcohol has returned to zero.
c.
Misleading Attitude Sensations – Terminology.
(1)
Sustained Linear Accelerations – Somatogravic Illusion. In the presence of the
constant acceleration of Earth’s gravity, the otolith organs, and the other gravitational indicators,
provide information which allows the orientation of the head and body to be sensed with
accuracy. Furthermore, the brain is able to distinguish changes of attitude from transient linear
accelerations. However, perceptual errors arise when the imposed linear acceleration or
deceleration is sustained, as in an aircraft when power is applied, or dive brakes are operated
(see Fig 10a-c). In such circumstances, the resultant of the imposed acceleration and gravity is
accepted as the vertical reference, so there is an erroneous perception of attitude which
increases the longer the acceleration is sustained. The false sensation of pitch-up on
accelerating is the more serious, for if a pitch-down corrective response is made, the radial
acceleration of the induced bunt causes a larger deviation of the resultant vector, and the
illusion is intensified. Likewise, the failure to sense accurately the angle of bank during a turn is
also due to the resultant of the radial and gravitational accelerations being accepted as the
vertical; for in a co-ordinated turn, the resultant vector remains normal to the aircraft’s
longitudinal axis and aligned with the long axis of the pilot’s head and body (see Fig 10d).
(2)
The Leans. A false sensation of roll attitude is one of the commonest illusions
experienced by aircrew. It usually occurs on recovery from a prolonged turn, or from a
previously undetected banked attitude, to straight and level flight. In both of these conditions,
the affected aviator feels that the aircraft is straight and level before it rolls out. The change
in bank on roll out is made within a few seconds and is sensed by the semicircular canals.
This vestibular information is interpreted as roll from the wings-level attitude to one of bank in
a direction opposite to that which existed before recovery was initiated. The curious feature
of 'the leans' is that it may persist for many minutes, even though instruments indicate level
flight. Characteristically, the illusion disappears as soon as an unambiguous external visual
reference is present.
(3)
Effect of Head Movement. The disorientating sensations produced when head movements
are made in a turning aircraft are not solely due to a cross-coupled stimulation of the semicircular
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canals. The presence of a linear acceleration greater than 1g means that the otoliths will also be
stimulated in an atypical manner when the head is moved. The principal effect on moving the
head under high g is to generate an otolithic signal, which corresponds to a greater change in
attitude, relative to the acceleration vector, than has actually occurred. The semicircular canals
and receptors in the neck signal the angular movement of the head with little error, and so there is
a mismatch which is interpreted as a change of attitude of the aircraft in the plane and direction of
the head movement. At higher accelerations (5 to 6g), a sensation of tumbling, as well as of a
change in attitude, can accompany the head movement. In high performance aircraft, appreciable
g forces are developed at low rates of turn. As the angular rates are close to the threshold for the
semicircular canals, the intensity of the cross-coupled stimulus accompanying the head movement
is insignificant, and so any disorientating sensations are most probably caused by otolithic
mechanisms.
6-6 Fig 10 Misleading Attitude Sensations
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(4)
Somatogyral Illusion. The name 'somatogyral' is derived from '
soma' (meaning body)
and 'gyral' (meaning turning). The illusion is a false sense of rotation which persists after
rotation has stopped. The false sensation of rotation is felt in the opposite direction to the
original rotation. This may occur after spin recovery, when a powerful sensation of rotation in
the opposite direction can develop, particularly if spin recovery occurs in cloud or at night.
(5)
Coriolis Illusion. The coriolis illusion can cause an intense, unpleasant sensation of rotation.
It is caused by head movements during sustained rotation. Consider rotation in the yaw plane. If
the head is moved to look down, one canal will be taken out of the plane of rotation, giving it a
deceleration stimulus, while another canal will enter the plane of rotation, giving it an acceleration
stimulus. The result is an illusory sensation of rotation which can be intense and is often
associated with nausea. The illusion can be avoided by minimising head movements when
undergoing significant angular acceleration. Regrettably, aircraft manufacturers have not always
understood this phenomenon and in some high-performance aircraft large head movements are
required to locate frequently used instruments and switches.
(6)
'G-Excess' Illusion. The G excess illusion occurs as a result of head movements made
in an abnormal G environment. Under increased G, the response of the otolith is
disproportionately high compared with information derived from visual cues and semi-circular
canals. The illusion may be one of rotation, or a less specific sensation of disorientation
which may be difficult to describe in terms of change in attitude and motion. The sensation,
nevertheless, may be powerful, particularly when the head is moved quickly. Although the
exact nature of this illusion is controversial, it is suggested that looking up and into a turn can
give the illusion of being under-banked and nose-up, resulting in an inappropriate over-bank
and nose-down attitude.
d.
Errors in the Perception of Visual Cues. Although many of the disorientating sensations
experienced by aircrew are caused by inadequate vestibular signals, spatial disorientation may
also arise because of errors or deficiencies in the aviator’s perception of visual cues.
(1)
External Visual Cues. Disorientation is likely to occur when the pilot attempts to use
external visual cues, rather than referring to the instruments (both performance and attitude),
in those conditions where visibility is impaired, or where there is a paucity of external cues.
During flight over featureless terrain, such as sand or snow or over a waveless sea,
judgement of height is likely to be difficult or misleading. Similar difficulties arise when
attempting to maintain hover or to land on terrain which is poorly illuminated, or indicated by
an inadequate array of lights, thus reducing visual references. In addition, 'the leans' is often
experienced when formation flying in cloud or in hazy conditions. Even when visual cues are
largely unambiguous, they may be misinterpreted because they differ from those which the
aviator expects to be present. One example is the use of a cloud top as a horizontal
reference. Cloud tops are commonly horizontal, but on the rare occasion when they are not,
this visual cue is erroneous and the pilot who accepts it will have a false perception of aircraft
attitude. Errors in the perception of height and distance also occur when ground features are
not of the expected size. These range from gross features, like the aspect ratio of a runway,
to finer detail, such as the size of trees and shrubs, or even surface texture. Less commonly,
there is gross misinterpretation of external visual cues; the acceptance that the lights of a
fishing fleet are stars and that the aircraft is in an inverted attitude, is an example.
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(2)
Instrument Cues. Errors in the perception of the symbolic cues displayed by the
aircraft instruments are occasionally responsible for disorientation. Instruments can fail,
albeit rarely, without any indication of failure being represented or detected by the aviator.
More common is the situation in which there is a breakdown of the normal instrument scan
and of the perceptual integration of the various elements of the head-up or head-down
display. Attention is focused on one instrument, to the exclusion of the others, and the pilot
fails to obtain a comprehensive perception of the attitude and flight path of the aircraft. This
'coning of attention' is more likely to occur at times of high workload and high arousal, such
as during an aircraft emergency.
Prevention of Disorientation
48. Knowledge of the causes of spatial disorientation, and of the flight conditions in which it is likely to
occur, should lead either to the avoidance of provocative flight environments and manoeuvres, or,
when this is impracticable, to the exercise of special care in such situations.
49. Illusory sensations are much more likely to be experienced, and to distract the pilot, when visual
cues are inadequate. Therefore, a high degree of proficiency at instrument flying is essential if the
aviator is to correctly resolve conflicting sensory cues and maintain proper control of the aircraft.
Proficiency, in this context, implies:
a.
A high standard of instrument flying.
b.
Being in current practice.
c.
Having an intimate knowledge of the specific aircraft, and relevant instrument procedures.
50. Any prolonged period of ground duty leads to a loss of skill in operating the aircraft and a heightened
susceptibility to disorientating sensations. Aircrew should, therefore, be particularly vigilant on return to
flying duties after a ground tour, when a properly planned and supervised period of refresher training is
essential. Even after a week or two without flying there is some loss of habituation to the motion stimuli of
flight. Accordingly, on return from leave it is desirable that the first flight should not be a demanding IMC
sortie.
51. Advice on preventative measures may be summarised as follows:
a.
Do not allow control of the aircraft to be based at any time on 'seat of the pants' sensations,
even when temporarily deprived of visual cues.
b.
Do not unnecessarily mix flying by instruments with flying by external visual cues.
c.
Aim to make an early transition to instruments in poor visibility; once on instruments, stay on
instruments until external cues are unambiguous.
d.
Maintain a high proficiency at instrument flying.
e.
Avoid unnecessary manoeuvres of aircraft or head movements which are known to induce
disorientation.
f.
Be particularly vigilant in high-risk situations in order to maintain intellectual command of the
orientation and position of the aircraft. These high-risk situations include:
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(1) Night flying.
(2) Flying in poor visibility.
(3) Landing at unfamiliar airfields.
(4) Flying when ground cues are obscured or absent such as with snow or sand.
(5) Flying in formation.
(6) Air-to-air refuelling, particularly in adverse weather conditions.
g.
Do not fly:
(1) With an upper respiratory tract infection.
(2) When under the influence of drugs or alcohol.
(3) When mentally or physically debilitated.
h.
After a period off flying, the first sortie should be a simple day VMC one.
i.
Remember, experience does not confer immunity.
Coping with Disorientation
52. A minor, but persistent, disorientating sensation, such as the Leans, may be dispelled by a
redirection of attention to other aspects of the flying task, provided that the correct orientation of the
aircraft has been established, and instrument references have been cross-checked. Some aircrew find
that a quick shake of the head is effective, although it is important that such head manoeuvres be
made only when the aircraft is established in straight and level flight.
53. If there are strong illusory sensations, and difficulty in establishing orientation and control of the
aircraft, the following procedures are recommended:
a.
Transfer to instruments and regain straight and level flight with the power set for cruise
speed.
b.
Establish a selective radial scan for straight and level; check altitude and compare with the
safety altitude. Climb above safety altitude if necessary.
c.
Avoid using external visual references until they are unambiguous; trust the instruments.
d.
Seek help if severe disorientation persists. Consider handing control to another pilot on the
flight deck; ensure that air traffic control is aware of your predicament. Try to find better weather.
e.
If control cannot be regained, abandon the aircraft with safe ground clearance. Do not leave
it too late.
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Conclusion
54. Remember, nearly all disorientation is a normal response to the unnatural environment of flight.
Any alarming flight incidents should be discussed with colleagues, including the Station Medical
Officer. What may appear to have been an unusual experience might turn out to have been
commonplace.
AIR SICKNESS
General
55. Air sickness, like other forms of motion sickness (e.g. car sickness, sea sickness or space sickness) is
not a pathological condition but is the normal response of the human body to certain motion stimuli.
Typically, on exposure to provocative motion, there is initially a slight feeling of malaise, then nausea of
increasing severity and eventually, vomiting. These symptoms are commonly accompanied by feelings of
warmth, sweating and pallor, and more variably, by headache, dizziness, increased salivation, drowsiness,
apathy or depressed mood. This collection of signs and symptoms constitutes the motion sickness
syndrome and, if caused in flight by motion of the aircraft, is called 'airsickness'.
Causal Mechanisms
56. Why humans react in this curious way on being exposed to particular motion stimuli is not known;
there is, however, a reasonable understanding of what makes them 'motion sick', and why certain
types of motion induce sickness while others do not. The current concept is that individuals develop
motion sickness when the various sense organs that signal body motion provide discordant
information. The essential feature of this discord is a mismatch between the motion information
provided by the eyes and the inner ear, and the information that is 'expected' by the central nervous
system (see Fig 11).
6-6 Fig 11 Mismatch
Stimuli (Input)
Receptors
Brain Mechanisms
Responses
(Output)
Neural
Updates Neural
Store
Store (Adaptation)
of
‘Expected
Signals’
Eyes
Neural
Mis-
Semi-
Centres
Motion
Motion
Match
Circular
Controlling
Comparator
Sickness
Stimuli
Canals
Signs &
Signal
Syndrome
Symptoms
of Motion
Sickness
(Pallor,
Sweating,
Otoliths
Nausea,
Vomiting,
Drowsiness,
Apathy, etc.)
57. Various types of 'mismatch' can be identified. Most important is the mismatch of signals from the
vestibular apparatus of the inner ear, in which the semicircular canals and the otoliths do not provide
concordant information. For example, when head movements are made in an aircraft which is turning,
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both the semicircular canals and the otoliths can provide erroneous and incompatible signals which are
likely to differ substantially from those generated by the same head movement in a normal 1g
environment. Likewise, low frequency (below 0.5 Hz) linear accelerations (such as occur in flight
through turbulence, repeated high rate turns and aerobatic manoeuvres) can also generate conflicting
vestibular signals, and hence be a potent cause of motion sickness.
58. The mismatch of visual and vestibular information can also be an important causal factor. For
example, personnel who cannot see out of the aircraft in which they are travelling are more likely to
suffer from airsickness than those with a good external visual reference. This is because, in those
people without an external view, the motion sensed by the inertial receptors of the vestibular apparatus
is not accompanied by any visual motion cues. Sickness can also be induced by purely visual motion
in the absence of any motion of the individual, as in some simulators which have a convincing external
visual display but no motion of the simulator cockpit.
59. Anxiety, and the presence of environmental features, such as the smell of the aircraft or
manoeuvres which have previously caused sickness, may increase susceptibility to motion sickness in
some individuals. However, in general these factors are of secondary importance.
Factors Affecting Susceptibility
60. There are very large differences between individuals in their response to provocative motion
stimuli. Some are never sick; others might succumb within minutes – perhaps on exposure to only
mild turbulence; only those without a functioning vestibular system are truly immune. There are also
considerable differences in the way people adapt to repeated, or prolonged, exposure to provocative
motion, as well as differences in the retention of adaptation following exposure.
61. Air sickness is most likely to occur on initial exposure to an unfamiliar motion; thus it is seen most
frequently in student aircrew during the initial phases of flying training, with recurrence on first
experiencing the more provocative flight manoeuvres such as spinning, high-rate turns and aerobatics.
With continuing flight experience, the majority of students adapt and air sickness is no longer a
problem. However, a few do not develop protective adaptation, or are very slow to adapt, and training
can be impaired by continuing sickness.
62. The retention of adaptation is also highly variable. In a few individuals, it is lost within days; more
commonly, the decay of adaptation is relatively slow. On return to flying (which can be from a
fortnight’s leave to a ground tour lasting years), many aircrew find that their tolerance to provocative
motion has decreased. Fortunately, re-adaptation usually proceeds more rapidly than the initial
adaptation. Adaptation can be highly specific: it is not uncommon for flying personnel who have
adapted to the motion of one type of aircraft to suffer from airsickness on transfer to another type with
different motion characteristics. Pilots may also experience malaise when flying as a passenger but
not when they are in control of the aircraft.
Prevention
63. Air sickness can be prevented, or at least the onset of symptoms delayed, by a number of
methods; however, those available to aircrew are limited by operational constraints. Head movement
should be reduced to a minimum, and good restraint of the body ensured. Provision of a good external
visual reference is advantageous, as is involvement in a task, provided this does not involve additional
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head movements or introduce conflicting visual cues (such as might happen when reading a book or
map in turbulence).
64. A number of drugs increase tolerance to provocative motion, though there are considerable
differences between individuals in the efficacy of a particular drug and the incidence of side effects.
Unfortunately, all of these drugs are sedative and can impair performance, so they should not be used
by pilots when in command of an aircraft, or by other aircrew who have a critical role to play during
flight. They are, however, valuable in allaying symptoms in passengers, and a short course of anti-
motion sickness drugs can help student aircrew to tolerate aircraft motion while acquiring protective
adaptation – Nature’s own cure.
65. No medication should be taken by aircrew prior to flying without consultation with a Military
Aviation Medical Examiner (MAME). If motion sickness persists, in spite of efforts by the aviator and
medical staff to overcome it, referral to the RAF Centre of Aviation Medicine for desensitisation training
should be considered.
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AP3456 - 6-7 - Thermal Physiology
CHAPTER 7 - THERMAL PHYSIOLOGY
Introduction
1.
JSP 539 Version 2.2 - 'Climatic Illness and Injury in the Armed Forces: Force Protection and Initial
Medical Treatment' provides a useful reference point for further information regarding the prevention
and management of heat and cold injuries.
THERMAL STRESS IN AVIATION
General
2.
Thermal stress arises from an imbalance between an individual’s metabolic heat production and
the net result of their heat exchange with their environment. Factors influencing the latter can be
divided into three main groups:
a.
Thermal Environment. Aircraft operate over a wide range of thermal environments meaning
that, for much of the time, crews are directly exposed to local climatic conditions. This is also
important during survival following a crash, ditching or ejection. In-flight, cabin conditioning offers
protection from the outside however occupants of rotary aircraft, operating with the doors open,
face the risk of heat or cold stress due to exposure to the external environment.
b.
Aircraft Factors. Sources of heat include the avionics systems, but also aerodynamic
friction associated with high-speed flight. With an effective ECS however these are generally
negated.
c.
Aircrew Factors. Metabolic heat production may increase by two to three times during
demanding flight activity when compared to sedentary levels. For rear crew undertaking physical
activity, such as loading, this may be even higher. Vigorous physical exercise may raise
metabolic heat production by up to 25 times more than at rest. Flying clothing as well as
additional protective equipment, such as helmets and body armour, will generally interfere with
heat loss processes increasing the thermal burden further.
Human Heat Exchange
3.
Central (core) body temperature is maintained around 37 ºC and is essential for proper enzyme
and nerve function. Although humans can cope with fluctuations above or below this temperature,
such changes can affect physical and mental performance. Regulation of core body temperature,
otherwise known as thermoregulation, is achieved through a number of means including behavioural
responses to temperature change, changes in blood flow to the core and skin, sweating and shivering.
4.
Heat can be gained from or lost to the environment through a number of processes, these being:
a.
Conduction. This describes heat exchange between two solid surfaces in direct contact or at
solid-fluid interfaces. This is of particular importance following cold water immersion (e.g. following
ejection or ditching) as water conducts heat away from the body 25 times more readily than air
significantly increasing the rate of cooling of core body temperature and the onset of hypothermia.
To remain comfortable for any period of time following water immersion, the water temperature
needs to remain around 34 to 35 ºC as cooling is inevitable at temperatures below this.
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AP3456 - 6-7 - Thermal Physiology
b.
Convection. This is mass transfer of heat by movement within a fluid medium (normally air
or water) where molecules retain their heat energy while moving within the confines of the
medium. In hot environments, the convective effect of wind will help to cool while in the cold,
leading to wind chill and an increased risk of cold injury. Following water immersion, convection
due to water turbulence in a sea state will increase the rate of heat loss and therefore
hypothermia.
c.
Evaporation. When water evaporates from a surface, energy is absorbed during the
transition from the liquid to the gaseous state. This is how heat is lost through the evaporation of
sweat from the skin’s surface. When the ambient temperature exceeds the mean skin
temperature (33 ºC) the evaporation of sweat becomes the sole means of heat loss. The
presence of wind will increase evaporation thereby adding to the heat loss associated with
convection in a hot environment. With increasing humidity, evaporation of sweat decreases and
the risk of heat illness increases.
d.
Radiation.
All objects possessing heat emit thermal radiation. The thermal energy from
solar radiation can become trapped within an aircraft canopy resulting in the ‘greenhouse effect’.
Thermal Effects of Clothing
5.
Aircrew are normally clothed in multi-layer clothing with warm air becoming trapped between
layers and within the clothing fibres themselves to provide insulation. Ingress of water or wind will
reduce this insulation as will physical exertion that induces an exchange of air beneath the clothing with
the ambient air, a phenomenon known as the ‘bellows effect’.
6.
Open-weave, highly permeable materials (e.g. knitted inner coverall) will trap air within the weave
which is then heated by the body. This insulation is soon lost however if exposed to wind which can
easily penetrate the weave and replace the warm air with cooler air. More impermeable materials may
protect against this effect however have the disadvantage of trapping perspiration which then dampens
insulating layers thereby reducing their insulating effect.
HOT ENVIRONMENTS
Effects of Heat
7.
Without heat loss processes, core body temperature would increase by about 1 ºC/hr owing to
metabolic heat production. There are a number of effects of excessive heat on the human body.
Thermal discomfort may lead to distraction. There is plenty of research on the association of dehydration
and mental performance. Some studies suggest an impairment of cognitive function at levels as low as
2% dehydration. Levels of 1 to 2% dehydration are commonly seen in aircrew on single sorties.
Dehydration will also lead to a reduction in sweating, in order to preserve fluid, which can further impair
heat loss. Aerobic performance is reduced in the heat and even mild heat stress may lead to a
degradation in memory, attention and vigilance as well as reasoning and decision-making.
Sunburn
8.
Even milder degrees of sunburn can cause sufficient damage to interfere with the delivery of
sweat to the skin surface thereby compromising this route of heat loss.
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AP3456 - 6-7 - Thermal Physiology
Heat Illness
9.
This is a spectrum of illness caused by a rise in core body temperature. Although traditionally
subdivided into heat syncope, heat exhaustion and heat stroke, it can be difficult to define a precise
separation, except for heat stroke. Most cases will occur in temperate climates.
10. There are several factors that may increase an individual’s risk of developing heat illness. These
include general health (e.g. obesity, poor physical fitness or nutrition), lifestyle (e.g. sleep deprivation,
alcohol) and previous episodes of heat illness.
Signs and Symptoms of Heat Illness
11. These depend on severity and include:
a.
Thirst
b.
Headache
c.
Dizziness
d.
Agitation
e.
Nausea and vomiting
f.
Weakness
g.
Poor coordination
h.
Staggering
i.
Confusion
j.
Collapse
12.
Heat Stroke. This is a medical emergency and has a high mortality rate if not recognised and
treated promptly. Core body temperature has exceeded 40 ºC and the individual often suddenly
collapses, convulses or becomes delirious. An individual with heat stroke will be hot but dry, rather
than sweaty as seen with heat illness. This is due to the cessation of sweating which means that the
ability to control body temperature has been lost.
13.
Treatment of Heat Illness. Core body temperature is an unreliable guide to the severity of heat
illness. If heat illness is suspected the following actions should be carried out:
a.
Remove the casualty from the heat.
b.
Lay them down in the shade and raise their legs.
c.
Remove clothing, wet them down and fan them to encourage heat loss (‘strip, spray, fan’).
d.
Administer cool oral fluids (if conscious).
e.
Consider evacuation (even if apparent recovery).
f.
If heat stroke is suspected, the casualty should be cooled rapidly by whatever means
possible. They should be immediately evacuated, and cooling measures should not be interrupted
during their transfer for more definitive medical care.
14.
Prevention Of Heat Illness. Most cases of heat illness should be preventable through the
application of simple measures. These include:
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AP3456 - 6-7 - Thermal Physiology
a.
Pre-deployment Training. A 6-week pre-deployment training programme incorporating an
initial 3 to 4 weeks to improve aerobic fitness. This will reduce the time to acclimatisation.
b.
Risk Assessment. This should include the use of a heat stress index (e.g. WBGT) to
determine appropriate levels of physical activity on the ground in order to minimise the risk of
developing heat stress. This is of particular relevance to those new in theatre who may not have
had time to fully acclimatise.
c,
Rest Periods.
Fifteen minutes of rest during every hour of heat exposure has been
shown to dramatically reduce the incidence of exertional heat stress in the Israeli Defence Force.
d.
Water Discipline. This is the most important factor in preventing heat illness. As previously
stated, dehydration leads to a reduction in sweating and hence heat loss. Thirst should not be
used as a guide to rehydration, appearing at approximately 2% dehydration. Urine colour should
be used as a guide to adequate fluid intake aiming to maintain a ‘straw-coloured’ urine. Further
guidance to water requirements can be found in JSP 539 Chapter 2 Annex B.
e.
Environment. The period prior to takeoff or between sorties can be a critical time for aircrew
with respect to heat stress. Air-conditioned buildings and transport of aircrew to their aircraft will
minimise thermal exposure. Adequate hydration facilities should be provided to prevent
dehydration. Aircraft should be parked out of direct sunlight or sun shades used with time spent
‘in-cockpit’ on the ground minimised to that which is necessary. Alternative crews may be used
for pre-flight inspections.
Acclimatisation
15. This process involves repeated exercise over a two-week period to raise and maintain an elevated
core temperature for at least an hour each day. It should ideally be carried out in-theatre although an
alternative would be to acclimatise in conditions that replicate the theatre environment including
elements such as temperature, humidity etc. Partial acclimatisation (approximately 75%) is normally
achieved after about 8 days. Full acclimatisation will generally be lost after spending 14 days or more
in a cooler environment.
16. The purpose of acclimatisation is to make sweating more efficient. An acclimatised individual will
sweat more with sweating occurring sooner and at a lower skin temperature for the same level of
activity when compared to an un-acclimatised individual. Less salt will be lost in the sweat produced.
Personnel will need to increase their water intake to account for this increased sweating during the
acclimatisation period.
Clothing for Hot Conditions
17. The requirements of aircrew clothing as well as additional protective equipment (e.g. CBA) and
the need for cockpit integration means that military clothing assemblies may not meet the ideal design
features for clothing suitable for hot conditions. Where practicable however, aircrew can minimise
thermal burden by simple measures such as the removal of extra clothing layers or opening clothing to
allow heat loss.
18. During off-duty periods, aircrew can reduce the risk of heat stress through the use of clothing
which is:
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AP3456 - 6-7 - Thermal Physiology
a.
Lightweight and open-weave (to minimise insulation and facilitate heat loss).
b.
Light-coloured (to reflect radiant heat).
c.
Loose-fitting (to facilitate the ‘bellows effect’ of air exchange with movement).
d.
Absorbable (e.g. linen, cotton) and vapour-permeable (to facilitate evaporative heat loss).
COLD ENVIRONMENTS
Effects of Cold
19. Exposure to cold conditions in the absence of adequate thermal protection may result in either
peripheral cold injury or hypothermia. There are several factors that may influence individual
susceptibility, and these include ethnicity, health and lifestyle (e.g. physical fitness, nutrition, concurrent
illness), inappropriate clothing and a history of cold-related problems.
Wind Chill
20. Wind cannot lower the ambient temperature however the presence of wind in a cold setting will
make it feel colder than it is. This is known as wind chill. Table 1 shows the cooling effect of wind chill
at different temperatures (SAT = still air temperature). The equivalent chill temperature is the ambient
temperature needed to produce the same effect on bare skin in the absence of wind. The chart is of
little use in predicting the time to hypothermia.
Table 1 The Cooling Effect of Wind Chill
Equivalent chill temperature (oC)
SAT (oC)
4
-1
-7
-12
-18
-23
-29
-34
-40
-46
0
4
-1
-7
-12
-18
-23
-29
-34
-40
-46
5
2
-4
-12
-15
-21
-26
-32
-37
-43
-48
)
h
p
10
-1
-9
-15
-23
-29
-34
-37
-51
-57
-62
(m
d
15
-4
-12
-21
-29
-34
-43
-51
-57
-65
-73
e
e
p
20
-7
-15
-23
-32
-37
-46
-54
-62
-71
-79
s
d
in
25
-9
-18
-26
-34
-43
-51
-59
-68
-76
-84
w
d
30
-12
-18
-29
-34
-46
-54
-62
-71
-79
-87
re
u
s
a
35
-12
-21
-29
-37
-46
-54
-62
-73
-82
-90
e
M
40
-12
-21
-29
-37
-48
-57
-65
-73
-82
-90
Danger – risk of cold
Freezing within one
Freezing within 30
injury
minute
seconds
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Peripheral Cold Injury
21. Exposure to sub-zero conditions is likely to result in freezing cold injury (FCI) commonly
recognised as frostnip or frostbite. Prolonged exposure to wet conditions in more temperate conditions
is more likely to result in non-freezing cold injury (NFCI) e.g. immersion foot.
a.
Frostbite.
This involves freezing of the tissues. The term frostnip refers to brief freezing
which resolves completely within 30 minutes of re-warming. An early symptom of frostbite is
numbness. If appropriate action is not taken at this stage, frostbite will worsen leading to swelling
and blistering. Tissue loss becomes more likely with deeper progression. Once frostbite has
been recognised, the following steps should be taken:
(1) The individual should be removed from the cold to prevent further progression and the
affected area padded to prevent further injury. Rubbing or applying pressure should be
avoided as these may worsen any tissue damage.
(2) In order to avoid increased tissue damage associated with thawing and re-freezing, re-
warming in field conditions should only be undertaken if evacuation is delayed. Walking on a
painless, frozen foot will cause less damage than attempts to walk on a thawing, painful one.
(3) The individual should be evacuated.
b.
Non-freezing Cold Injury (NFCI).
This may develop within hours when tissue is exposed
to wet conditions in temperatures as mild as 5 to 10 ºC. As the name suggests, there is no tissue
freezing however NFCI can result in long term persistent pain and ‘cold sensitivity’ if unrecognised
and managed appropriately. Initial treatment involves pain relief and keeping the area dry and
warm while awaiting further medical assessment.
Hypothermia
22. Core body temperature has fallen below 35 ºC resulting in impaired function of mental and
physical processes. The severity of symptoms depends on the time hypothermia has taken to develop
and the level to which core temperature has fallen. At a core temperature of around 28 ºC, cardiac
arrest is likely to occur.
23. The progressive signs and symptoms of hypothermia include:
a.
Feeling intensely cold, strong shivering
b.
Subtle changes e.g. tiredness
c.
Mental confusion, slurred speech
d.
Poor coordination
e.
Limb rigidity
f.
Reduced conscious level
g.
Death
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AP3456 - 6-7 - Thermal Physiology
24.
Initial Management of Hypothermia. Whether hypothermia has developed slowly (e.g. survival
on land) or more rapidly (e.g. water immersion), the priority is to reduce further heat loss and re-warm
the casualty. On land, shelter should be sought or erected. At sea, the priority is to get out of the
water and into a life raft.
a.
Wet clothing should be removed and replaced with dry clothing, if available. If not, wet
clothes should be left on and covered with waterproof material and any available extra insulation.
Warm, sweet drinks should be administered to a conscious casualty. Alcohol has no place in
management as it increases skin blood flow and hence the risk of further heat loss.
b.
The presence of shivering indicates that hypothermia is likely to be mild. Once core body
temperature has fallen below 32 ºC, indicating moderate or severe hypothermia, shivering stops.
In these cases, it is important to avoid excessive handling or rapid re-warming of a casualty as
this may precipitate irregular heart rhythms leading to cardiac arrest.
Clothing for Cold Conditions
27. The aim is to achieve insulation by trapping warm air between clothing layers while excluding the
ingress of wind and water. The advantage of a layering system is that layers can be added or removed
as dictated by environmental conditions and work requirements.
28.
Windproof/Waterproof Layers. Clothing insulation is reduced by 30% in a 9 mph wind therefore
in windy conditions; the addition of an external windproof outer layer will serve to reduce this
convective heat loss. A waterproof layer, often combined with wind proofing, will protect against the
50% loss of insulation that can be seen with wetting of clothing. ‘Breathable’ fabrics (e.g. Gore-Tex)
are impermeable to water in liquid form however allow water vapour to pass through.
29.
Head, Hands and Feet.
The head, hands and feet present special problems in the cold. Heat
loss from the head can exceed 50% of the metabolic heat production. Aircrew generally wear
protective flying helmets but, as these may be lost when an aircraft is abandoned, survival kits should
contain additional head protection. Footwear should be designed with climatic conditions in mind,
providing adequate insulation and a waterproof layer. Good hand protection in the cold is generally
incompatible with the maintenance of sufficient sensitivity and dexterity so a compromise must be
sought. Mittens are best when the still air temperature falls below about -10 ºC.
30.
Immersion Coveralls. In itself, an immersion coverall does not provide insulation. However, by
preventing the ingress of water following water immersion, it protects the insulation provided by
underlying clothing. Although it will delay the onset of hypothermia it is not designed for prolonged
immersion however ‘buys time’ to get out of the water. It will also provide protection against cold shock
following entry into cold water. Although there may be the temptation to alter the seals for comfort, this
will compromise the watertight integrity of the coverall. As little as a 500 ml leak into the coverall can
reduce underlying insulation by 30%.
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31. A useful mnemonic for the desirable properties of clothing for cold weather is:
Clean (so that it will not mat down or become greasy and so lose its insulating properties)
Open weave
Layered
Dry
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AP3456 - 6-8 - Noxious Substances in Aviation
CHAPTER 8 - NOXIOUS SUBSTANCES IN AVIATION
Introduction
1.
Noxious substances may be defined as those which are capable of producing a temporary or
permanent adverse effect on an individual’s health, well-being, or performance. They pose particular
problems in aviation since even a minor decrement in the performance of aircrew is a flight safety
hazard. Moreover, the effects of exposure to a noxious substance may be greatly increased in the
presence of physiological stresses of flight such as 'G', cold, or hypoxia. Thus, an exposure to a
noxious substance at a concentration which would have little or no effect on a man on the ground may
produce a hazardous situation in flight. The following paragraphs outline the ways in which exposure
to noxious substances may occur and the possible effects of such exposures, the precautions and
remedial actions required, and the major groups of noxious substances important in aviation.
Noxious Substances
2.
The number of noxious substances which may be encountered in aviation is very large and grows
as new materials are introduced. Some substances, such as aircraft consumables like fuels and
lubricants, are noxious in themselves; protection against these is by preventing their contact with flight
or ground crews. Toxic hazards may also result from the decomposition of normally harmless
materials, such as occurs during a fire. Noxious substances may exist in any physical form; they may
be solids, liquids, gases, vapours, or aerosols (finely divided solid or liquid particles suspended in a
gas, or in air).
Routes of Entry to the Body
3.
Noxious substances may gain access to the body by one or more of the following routes:
a.
Inhalation. In aviation, as in ground working environments, the most common route of entry of a
noxious substance to the body is the inhalation of a gas, vapour, or aerosol. Substances absorbed
through the lungs rapidly reach all parts of the body via the bloodstream. The contaminant may be
present in the cockpit or cabin air or, very much less frequently, in the oxygen supply.
b.
Ingestion. Ingestion of noxious substances occurs less frequently than inhalation. However,
one problem that arises all too frequently is illness as a result of consuming food or drink which is
contaminated either with a toxic substance or food poisoning organisms. It is vital for aircrew to take
all possible precautions to avoid this risk, particularly when operating away from their home base in
areas where local hygiene standards may be questionable or poor. Food poisoning may be totally
incapacitating, and the onset of symptoms may be sudden. Noxious substances may also be
ingested if a person eats or smokes with hands contaminated by a toxic substance. Rarely, a
noxious substance may be inadvertently swallowed, should a splash enter the open mouth.
c.
Skin Absorption. Corrosive or irritant substances will cause a local effect if they come into
contact with skin, but many substances such as solvents are able to pass unnoticed through intact
skin, then to be transported to all parts of the body in the bloodstream. This may occur if noxious
substances are not cleansed rapidly from the skin, or if contaminated clothing remains in contact
with the skin. This hazard is not confined to liquids; solid substances may dissolve in sweat and
then be absorbed.
d.
Inoculation. Contamination of the eye with a toxic dust or liquid, or exposure to a toxic gas,
may result in absorption into the eye. The eye is a very sensitive organ and is often affected
before other parts of the body, resulting in discomfort, and impaired vision.
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AP3456 - 6-8 - Noxious Substances in Aviation
e.
Injection. Noxious substances may be inadvertently injected through the skin. This may
occur if a wound is produced by a contaminated object, or if a fine jet of liquid at high pressure hits
the skin. The latter may occur, for example, as a result of a leak from a hydraulic system.
Effects of Noxious Substances
4.
Noxious substances which gain access to the body may produce a localized effect such as irritation
or inflammation of exposed skin or eyes, or more generalized symptoms such as headache, or
disturbance or loss of consciousness. Some substances produce both local and generalized effects. The
effects may be 'acute' (appearing rapidly after exposure begins and often resolving rapidly afterwards), or
'chronic' (resulting in long term illness or disability). Some substances produce both an acute and a
chronic effect. Following exposure, there may be a 'latent interval' before any effects become manifest.
Latent intervals of hours or even days are not uncommon, but they may be extremely long, measured in
years. In most cases, a substance will exert its major effect on one organ or physiological system, known
as the 'target' organ or system for that particular noxious substance.
5.
The most immediate flight safety concern is the acute effect of an exposure. This may range from
a minor annoyance to a major, possibly life-threatening disturbance. Most in-flight exposures have
been the result of contamination of the cockpit or cabin atmosphere and this may often be recognized
by the presence of smoke or an unusual odour. It is, however, possible for a colourless and odourless
gas (such as carbon monoxide) to impair performance without the subject being aware of its presence,
or of the developing impairment. Aircrew who become aware that their thought processes or actions
are becoming slow or inaccurate, or who notice a performance decrement in another crew member
should consider this possibility. A further problem is posed by substances that exert an effect after a
latent interval. An individual may attach little significance to a toxic exposure at the time, only to
become unwell some time later. In this context, aircrew must also remember that they may be
impaired by exposure to a toxic substance whilst off-duty, particularly during leisure activities such as
car maintenance or household 'd-i-y'. In addition, the onset of disabling food poisoning symptoms may
be delayed by hours or days following the causative meal.
Control and Protection
6.
To minimize the risk of exposure to noxious substances, the materials used in aircraft
construction and the consumables used during operations are assessed to ensure that the safest
practicable options are chosen. Design also aims to provide adequate containment or segregation of
toxic materials to prevent aircrew exposure. Work practices employed during aircraft servicing are
assessed and controlled to protect both aircrew and ground personnel. Protective clothing and
equipment is provided where it is not otherwise possible to remove or control the hazard. It is essential
that all personnel follow duly authorized procedures to ensure their safety and that of others.
7.
Aircrew must remain alert to the dangers posed by noxious substances both on and off duty and
take precautions to avoid contact or exposure. Any incident resulting in subjective symptoms must be
reported and the individual must seek medical advice before flying again. In addition, aircrew should
seek medical advice following any but the most trivial contact with a known toxic substance even if no
symptoms result at the time of contact, in view of the possibility of delayed reaction. The risk of food
poisoning has been mentioned above; aircrew must minimize this risk by scrupulous attention to food
hygiene and food hygiene guidance.
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AP3456 - 6-8 - Noxious Substances in Aviation
Cockpit or Cabin Contamination During Flight
8.
The actions to be taken, should contamination of the crew compartment be recognized or
suspected during flight, vary according to the aircraft type, oxygen system or equipment available, the
crew position and the flight conditions. The aim is to prevent or reduce inhalation of or contact with any
noxious substance which may be present. Aircrew must be fully conversant with the drill for their
specific aircraft and role, as detailed in Flight Reference Cards. Required actions may include, as
appropriate:
a.
Manual selection of 100% oxygen and safety pressure on demand regulators, the latter to
prevent inward leakage of contaminated cockpit air.
b.
The use of portable oxygen sets by rear crews.
c.
Protection of the eyes by use of visors or goggles.
d.
Covering exposed skin where possible.
e.
Depressurising the aircraft if it is safe to do so; this may require reducing altitude.
f.
Increasing ventilation by any safe and practicable means.
g.
Declaring the emergency in order that medical and other services are immediately available
on landing.
Noxious Substances Encountered in Flight
9.
The following paragraphs highlight the major classes of noxious substances which may be
encountered by aircrew. Detailed description of individual substances is not practicable in this chapter;
the intention is to draw attention to the wide range of substances and to important practical
considerations. Individual substances are mentioned for illustrative purposes; further specific information
is available in engineering and health and safety instructions and publications. Flight safety, health and
safety, medical and engineering staffs should also be approached for specific guidance.
Fuels and Propellants
10. The risk of exposure to aircraft fuels is greater for servicing personnel than for aircrew. However, in
certain situations, some aircrew may perform or closely supervise refuelling operations, posing a risk of
contamination of skin or clothing. Moreover, a spill during refuelling, particularly in still air and warm weather,
can result in significant vapour contamination of the cabin or cockpit. With a few specialized exceptions,
aviation fuels are basically hydrocarbons (gasolines or mixtures of gasolines, kerosenes and aromatics) with
various additives to modify physical properties or to improve combustion characteristics. All can irritate the
skin or eyes, but the principal hazard is inhalation of the vapour, when the severity of the effect will depend
on the concentration and duration of exposure. Exposure to vapour concentrations above 0.05% may
produce detectable effects, particularly if prolonged. The main effect is on the nervous system, dulling both
the senses and awareness; the dulling effect on the sense of smell may result in loss of awareness of the
continued danger. A low dose of the vapour, such as breathing 0.2% for 30 minutes, generally causes
dizziness, nausea, and headache. Higher concentrations may irritate the eyes and produce signs akin to
drunkenness, or even unconsciousness, convulsions and death. The various additives may include
compounds of lead, aromatic organic substances such as xylene and aniline, and other toxic substances
such as the glycol ethers used as fuel systems icing inhibitors. Many of these additives may be absorbed by
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AP3456 - 6-8 - Noxious Substances in Aviation
inhalation and some will pass through intact skin, when they are capable of producing a wide range of acute
and chronic problems.
11. Specialized fuels and propellants are employed when there is a need for a high energy output
from a given mass of fuel, for example in rockets, missiles, small auxiliary power units and to start
some turbine engines. The highly reactive substances required by these applications are usually also
highly toxic. Examples are aniline, hydrazine and isopropyl nitrate (AVPIN). These are burnt, either in
air, or with liquid oxygen or an oxidizing agent such as hydrogen peroxide or fuming nitric acid,
resulting in the production of hazardous exhaust gases. In addition to toxicity, these fuels and
oxidizers pose handling problems. For example, hydrazine may burn spontaneously on exposure to
air, liquid oxygen may cause frostbite or produce ignition or detonation of some organic materials, and
many of these substances require special containment. Aircrew who are at any risk of exposure to
these materials must be aware of the specific hazard and of the emergency actions to take in the event
of a leak or of personal contamination.
Combustion and Pyrolysis Products
12. Toxic products may be evolved not only if substances are actually burnt, but also if they are
merely overheated. 'Pyrolysis' is a broad term embracing both situations. The principal hazards are
posed to aircrew by engine exhaust gases, fires on aircraft and systems failures resulting in
overheating of components. The greatest single cause of aviation incidents involving noxious
substances is a pyrolysis product, namely carbon monoxide. In view of its importance, this is
considered separately below.
13. Aircrew may be exposed to engine exhaust gases in a variety of ways. In the past, this has
occurred relatively frequently as a result of defective bulkhead sealing in single piston aircraft. Exhaust
gases are passed through heat exchangers in some aircraft types, to provide cabin heating. A defect
in the heat exchanger may result in contamination of the cabin hot air supply with exhaust gases.
Engine running within hardened aircraft shelters may result in a build-up of exhaust gases, despite the
measures taken to provide ventilation. Helicopters, particularly when hovering close to the ground,
may draw in exhaust gases from their own efflux. Fixed wing aircraft are not immune from this
problem, which may occur during certain conditions of flight or if there is a defect in the cabin wall close
to the point of efflux.
14. In addition to carbon monoxide, exhaust gases contain other noxious components such as unburnt
hydrocarbons, carbon dioxide, oxides of nitrogen, and aldehydes. Aldehydes, present in significant
concentrations in jet exhausts, and oxides of nitrogen, are highly irritant and may produce soreness of the
eyes and impaired vision, as well as sore throat and coughing. Aircrew, particularly those operating from
hardened aircraft shelters, must minimize their exposure to exhaust gases prior to flight.
15. Aircraft fires produce a highly noxious smoke containing a huge variety of substances, including
carbon monoxide. This is despite the efforts made to reduce the potential hazard by selection of safe
materials in aircraft construction. The smoke is characteristically highly irritant to the eyes and
respiratory system. It is also asphyxiant and narcotic, capable of causing rapid dulling and loss of
consciousness. The importance of prompt and correct application of the appropriate aircraft-specific
emergency drills on suspicion of an aircraft fire cannot be overstressed.
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Carbon Monoxide
16. Carbon monoxide is an odourless, colourless gas which is present in the smoke from almost all
aircraft fires and in aircraft exhaust gases, particularly from piston engines where concentrations of up
to 9% may be encountered. To put this in context, the long-term exposure limit for workers on the
ground is 0.005% (50 parts per million).
17. Inhaled carbon monoxide passes easily into the bloodstream where it enters the red blood cells and
binds to the haemoglobin, thus preventing the carriage of oxygen from the lungs to the tissues, so that the
tissues become hypoxic. Unfortunately, carbon monoxide binds to haemoglobin much more strongly than
oxygen and the resulting compound, carboxyhaemoglobin, is more stable than the equivalent compound
with oxygen. In numerical terms, carbon monoxide’s affinity for haemoglobin is over 200 times as great
as that of oxygen. This means that even a very low concentration of carbon monoxide in the inspired air
will result in a progressive build-up of carboxyhaemoglobin to harmful levels. Increased rate and depth of
breathing as a result of exercise or hypoxia will increase the rate of carboxyhaemoglobin build-up. For
example, breathing 0.1% carbon monoxide for 1 hour whilst at rest will result in approximately 20% of the
body’s haemoglobin being converted to carboxyhaemoglobin. Taking light exercise during this period
would raise the percentage converted to around 40%.
18. As carboxyhaemoglobin builds up during an exposure, the effects of carbon monoxide poisoning
appear and increase in severity. Tissues most sensitive to hypoxia, such as the nervous system, are
the first to be affected. The symptoms which occur when various percentages of the body’s
haemoglobin (Hb) are converted to carboxyhaemoglobin (HbCO) in an individual breathing air at sea
level are summarized in Table 1.
19. The Table only describes likely symptoms at sea level. At altitude, the effects of a given level of
carboxyhaemoglobin will be markedly increased by any hypoxia which exists as a result of the reduced
partial pressure of oxygen in the inhaled air. However, the effect of a given concentration of carbon
monoxide in the inhaled air will be reduced because of the lower partial pressure it exerts at altitude. A
further point of particular relevance to aircrew is that mental performance has been shown to be
impaired at carboxyhaemoglobin levels as low as 5%, although the individual may feel perfectly well.
At levels of 10% or more, aircrew performance is affected to the extent that flight safety is significantly
degraded.
6-8 Table 1 The Effects of Concentrations of Carbon Monoxide Poisoning
% Hb converted to
Symptoms likely at Sea Level
HbCO
0 to 10
None noticeable.
10 to 20
Slight frontal headache.
Throbbing headache. Breathlessness on exertion.
20 to 30
Possible nausea and weakness.
Severe headache. Weakness. Dizziness. Dimness of vision.
30 to 40
Nausea and vomiting. Breathlessness at rest. Possible collapse.
Increasing likelihood of collapse. Increasing pulse rate. Irregular breathing.
Over 40
Collapse. Convulsions. Respiratory failure. Death.
20. The immediate treatment of a victim of carbon monoxide poisoning consists of restoration of a
safe breathing supply, preferably 100% oxygen if available, and rest at room temperature. Overheating
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should be avoided. If the victim is not breathing, artificial respiration will be required, possibly for a
lengthy period. Medical attention should be obtained as soon as possible.
Fire Extinguishing Agents
21. The ideal fire extinguishing agent would be effective, in as small a bulk as possible, against all
types of fire (including liquid and electrical fires). It would also be non-toxic and safe for use in
confined spaces or where high voltages are exposed. Further, it should not yield toxic pyrolysis
products when applied to a fire. Unfortunately, such an agent does not exist. Substances used in
aircraft, either in fixed fire suppression systems or in hand-held extinguishers, are selected to be
effective against the types of fire likely to be encountered and to offer the best compromise between
effectiveness and safety. Aircraft fire extinguishers or fixed systems commonly use water (with glycol
added as an anti-freeze), water mist, or carbon dioxide gas. Cabin extinguishers may contain dry
powder for use on electrical fires and foam may be used particularly to counter fires in aircraft on the
ground. Most halogenated hydrocarbons (halons) are no longer permitted under the Montreal Protocol
because they are known to deplete the Ozone layer. Alternative noxious substances introduced
include Nitrogen, the inert gas Argon and some other agents termed halocarbons.
a.
Water/Glycol Mixtures. Water/glycol mixtures have the advantage that they are virtually
non-toxic. Use of such mixtures may become noxious if used on liquid or electrical fires against
which they are ineffective and considerably hazardous.
b.
Water Mist. Water mist is variously described as water fog or fine water spray. It works,
primarily by the rapid absorption and dissipation of heat from the fire. The small droplets ensure
that the process is more effective than with conventional sprinklers or water sprays. Therefore,
relatively small quantities of water are required. Unlike conventional water extinguishers, water
mist can be effective against liquid fuel fires and is likely to cause minimal damage to electrical or
other equipment. Exposure of equipment to freezing temperatures may restrict its application
although benign additives may be included which alleviate this problem. Although a water mist
discharge may reduce visibility, its cooling effect will significantly enhance survivability in a fire
inside an occupied enclosure. Water mist may be used in fixed or portable applications.
c.
Carbon Dioxide. Carbon dioxide is an asphyxiant gas which extinguishes fires by excluding
oxygen. It is of limited use against fires of combustible solids such as paper or cloth. It is
unsuitable for use in confined spaces, since breathing concentrations above 2% causes adverse
symptoms. The initial effect is to produce laboured breathing, but, if the concentration rises to 3 to
10%, increasingly severe sensory disturbances and dizziness occur. Breathing 10% carbon
dioxide may result in unconsciousness in as little as one minute.
d.
Foam. Foams, which are available in many types and compositions, are particularly effective
against liquid fuel fires where they form a barrier between the fuel and its oxygen supply. Foams
are suitable for fixed and hand-held applications, but the composition is corrosive.
e.
Dry Powder. Dry powder extinguishants are available in different compositions and are
effective against most fire types. These are not generally noxious as such but risks in breathing in
the powder must be considered. Contamination and clean-up after discharge can be
problematical. Dry powder extinguishers can be the most effective, on a weight basis, and are
more usually confined to portable applications.
f.
Nitrogen/Inert Gas Blends. Nitrogen, the inert gas Argon, and blends of both with, or
without, small amounts of carbon dioxide are effective extinguishing agents against all types of
fire. They extinguish fires by reducing the oxygen concentration inside an enclosure to
below 15%. Provided the oxygen concentration remains above 12%, occupants can survive
without significant adverse effects, caused by the agent, for a reasonable period. The gases are
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stored in heavy high-pressure cylinders. Since quite large quantities are required, this tends to
preclude use in hand-held containers.
g.
Halocarbon Agents. Following the restrictions on the general use of halons, three halocarbon
agents falling within two categories have been introduced as suitable extinguishing media. The two
categories are Hydrofluorocarbons (HFC) - within which Heptafluoropropane and Trifluoromethane
have been approved - and Perfluorocarbons (PFC) containing the approved agent Perfluorobutane.
These agents work on all fire types by inhibiting the chemical mechanism of the fire through heat
absorption. They are less effective than the halons which they replace and require about twice as much
agent to achieve the same effect as a halon. The process of extinguishing a fire with halocarbons
produces highly toxic and corrosive products, primarily hydrogen fluoride (HF) and this is one reason
why these substances can be used only in controlled fixed systems. However, incidents have occurred
where fire suppression systems have been inadvertently triggered on the ground, particularly during
servicing, resulting in the exposure of servicing and other personnel in the area and this must be
considered in the maintenance environment.
Other Aircraft Systems
22. A number of aircraft ancillary systems contain or use noxious substances.
a.
Hydraulic Systems. Hydraulic systems utilize fluids at very high pressures. In normal
circumstances, the fluid is completely contained, but a leak in a hydraulic line may result in an
atomized jet of fluid entering the cockpit or cabin. A very small defect in a hydraulic line or union
may be invisible to the naked eye, as may be the jet of fluid leaking from it, but the velocity of the
jet may enable it to pierce skin. However, the most likely effect is the creation within the aircraft of
an aerosol of microscopic fluid droplets which may be inhaled. A wide variety of hydraulic fluids is
in use and their toxicity is variable. In general, the greatest risk is of inhalation of an aerosol or
vapour, though some can be absorbed through intact skin. Many are irritant to the eyes and
respiratory passages, some produce nausea or drowsiness. Medical advice must always be
sought following exposure, since some hydraulic fluids contain substances such as glycol ethers
which can produce serious long-term effects.
b.
Lubrication Systems. Rarely, lubricating oil may gain access to an aircraft as a result of a
mechanical defect permitting oil to mix with the air in the compressor stage of a turbine engine,
upstream of the bleed providing cabin-conditioning air. In this case, the oil may be in the form of a
fine oil mist or vapour which may be irritant to the eyes and nose. If this is inhaled, the result may
be headache, nausea, and vomiting, followed by serious inflammation of the lungs, which may not
occur for some hours. Another possible source of exposure is oil in contact with hot engine parts,
when pyrolysis will result in the generation of a highly irritant smoke.
c.
De-icing Systems. De-icing fluids consist of various mixtures of alcohols and glycols with
water. Exposure to de-icing fluid in an aircraft is usually due to the fracture of a pipe carrying the
fluid, permitting a fine spray to enter the interior. Although the fluids concerned are not very toxic,
the result of breathing an aerosol or vapour may be irritation of the eyes and nose, and possibly
headache and nausea.
d.
Refrigeration Systems. Refrigeration systems in aircraft contain similar substances to
those in domestic refrigerators, namely hydrochlorofluorocarbons (HCFC). They may be
anaesthetic in high concentration and some are capable of causing liver damage, but the principal
risk to aircrew is posed by the dangerous pyrolysis products, such as chlorine, fluorine, and
phosgene, which may be evolved in the event of a fire in the equipment.
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Suspected Contamination of Oxygen Supply
23. Stringent quality control ensures that the purity of aircraft oxygen supplies is very high; incidents of
contamination are very rare. However, the presence of an odour in the breathing supply must suggest
the possibility. Most incidents are the result of the presence of a contaminant in the hoses or other
components of the oxygen system, rather than in the supplied oxygen itself. This may be the result of
faulty servicing technique such as inadequate purging following the use of degreasing agents.
Although a detectable odour is the result, the quantity of contaminant present is not normally significant
in toxicological terms. However, an unusual odour must never be ignored; the aircraft specific drills as
detailed in the Flight Reference Cards must always be performed. These may include the use of an
alternative oxygen supply where possible, or descent to an altitude where oxygen is not required. It is
vital that the problem is reported to the ground, so that immediate medical treatment and investigation
is available on landing. The oxygen system, including the mask and hose or PEC must be quarantined
for full engineering investigation.
Ozone
24. Ozone, a tri-atomic form of oxygen, is formed at high altitude by the action of solar ultra-violet light
on molecular oxygen. The concentration rises above 40,000 ft to a maximum of approximately 10
parts per million (ppm)(by volume) at 100,000 ft. Ozone is a highly irritant gas; exposure to one ppm
for one hour will cause serious eye discomfort and coughing. Higher concentration will cause
dangerous inflammation of the lungs. Fortunately for aviators, ozone decomposes on heating and little
survives passage through the compressor stage of an aircraft engine and the cabin conditioning
system. If this were not the case, the concentration in the cabin of an aircraft flying at 60,000 ft would
be of the order of 4 ppm. In practice, concentrations inside aircraft at high altitude are normally below
0.1 ppm and rarely exceed 0.2 ppm.
Cargo
25. Many hazardous substances are carried by air. Regulations specify the maximum quantities of
specific substances which may be carried by an aircraft, together with specifications for safe packing,
handling, and stowage. It is important that aircrew are aware of the potential hazards posed by
dangerous air cargo items, the necessary precautions, and the immediate actions to be taken in the
event of a leakage or other emergency. Passengers cannot be expected to be aware of the hazards
posed by many everyday items when they are taken on board an aircraft. Loadmasters and others
must remain vigilant in their duties, to prevent passengers from inadvertently creating a hazard by
bringing noxious, or potentially noxious, substances on board in their luggage, cabin baggage or in
their pockets. Posters and other publicity media should be used to raise travellers’ awareness of the
types of items which constitute dangerous air cargo.
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CHAPTER 9 - PHYSIOLOGICAL AND PSYCHOLOGICAL EFFECTS OF LOW FLYING
Introduction
1.
Aircrew obtain most information about the external world by means of vision, and the demands to
process visual information are particularly severe at low altitude. Consider, for example, the task of
searching for a waypoint whilst maintaining a visually judged altitude and avoiding wires, birds, and
other obstacles. These activities may have to be undertaken in unfavourable conditions that impair
performance, for example excessive turbulence, noise or heat.
2.
This chapter describes human limitations that should be recognised by all those engaged in low
flying. Emphasis is placed on high-speed, low-level flight which is essential for the success of
missions within defended territory. However, the discussion is relevant to other aspects of low flying,
and encompasses both fixed- and rotary-wing operations. The reader is referred to Volume 6,
Chapter 6 for supplementary information concerning the special senses.
3.
Since low flying restricts the margin of error available to aircrew, minimum acceptable heights are
specified in the Military Aviation Authority (MAA) Regulatory Article (RA) 2330 and may be qualified in
Command, Group, or Station Orders. Continuous flying at heights below 100 ft produces an
increased flight safety hazard which must be balanced against the increased chances of survival in
the face of enemy defences.
Vision
4.
Man’s visual system was not designed to cope with the unnatural demands of flight. The major
visual difficulties likely to be encountered by aircrew are discussed below.
5.
Loss of Visual Reference. One of the most fundamental visual problems during low-level flight
is the loss of the external visual reference in adverse weather conditions. At 540 kt, the pilot may have
insufficient time to respond to a hazard, even with a visual range of 2 nm. Clearly, the decision to
carry out a low-level abort must be made as early as possible, with the pilot’s scan established on
head-down instruments very soon after beginning the appropriate abort procedure.
6.
Dynamic Visual Acuity. During flight, the relative angular velocity of a ground-based
object depends upon aircraft height and speed, the range of the object and its bearing to the line of
flight. Since only objects on an aircraft’s track have no angular velocity, the resolution of detail
in the external scene depends largely upon dynamic visual acuity, rather than the static acuity
measured in routine eye tests. Dynamic visual acuity is poorer than static acuity because the image of
a target followed by the eye does not consistently fall on the area of greatest visual acuity in the centre
of the retina; moreover, the higher the angular velocity, the more likely there is to be some movement
of the target relative to the retina. Visual acuity declines as target velocity increases; it is halved for a
target moving at 40 degrees per second, and reduced to about one-third for a target moving at 80
degrees per second. This presents a particular problem for the pilot flying at high speed and low
level.
7.
Vibration and Vision. Problems of dynamic visual acuity are not confined to the resolution
of external objects; the reading of cockpit instrumentation is disrupted by aircraft vibration:
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a.
Low Frequency Vibration. The high level of turbulence often encountered at low altitude
is likely to induce buffeting of the airframe structure, subjecting the occupants’ bodies to low-
frequency vibration. Up to about 2 Hz, the head and body move together. However, in the range
of 3 Hz to 4 Hz, and particularly if the vibration occurs in the head-foot (z) axis of the body, the
head moves independently of the body, interfering only slightly with perception of the external
world but disrupting the ability to read instruments. For vibration at frequencies up to 10 Hz,
some stabilisation of the eye is provided by automatic eye movements compensatory to the
direction of head movement, but residual movement of instrument displays relative to the eye can
be expected. If the frequency of this movement exceeds 1 Hz, the pursuit reflex (responsible
for tracking moving objects with the eye) breaks down, substantially reducing visual
acuity. Moreover, the automatic eye movements described above are almost impossible to
suppress, and can create difficulties when using helmet-mounted displays that move with the
head.
b.
High Frequency Vibration. As the frequency of vibration increases beyond the critical
range of 3 Hz to 4 Hz, head movements become progressively smaller. Since most helicopter
vibration occurs at relatively high frequencies (typically in the range 12 Hz to 18 Hz), rotary-
wing aircrew are less likely to be adversely affected than their fixed-wing colleagues.
Nevertheless, during the hover, difficulties in reading the radar altimeter and other important
displays may be experienced.
c.
Severe Vibration. Head movements during particularly severe vibration can be reduced
by removing the head from the head-rest, and preferably the back from the back-rest. Further
alleviation of this problem will be provided by improved seating and developments in space-
stabilised displays.
8.
NBC Equipment and Vision. The Aircrew Respirator No 5, if correctly fitted, imposes
little restriction upon the visual field. However, its use impedes head movement, and may
therefore interfere with look-out. Misting of the visor does not occur under normal circumstances,
but may be experienced if there is a failure of the blown air supply, if the mask is incorrectly fitted, or if
there is an extreme temperature differential between the internal and external surfaces of the visor.
Since aircrew clad in the NBC Aircrew Equipment Assembly are likely to sweat profusely after heavy
physical exercise, their activities outside the cockpit must be undertaken at a more leisurely pace.
9.
Night Flying. Night flying has a number of adverse effects on vision:
a.
Deprived of visual cues at 'infinity' (i.e. beyond about 6 metres), aircrew may
experience temporary short-sightedness, whereby the eye focuses to between 1 and 2 metres, or
even less, if there is a prominent canopy frame in near vision. This degrades visual look-out by
blurring and reducing the contrast of distant objects, and by making them appear smaller and
hence more distant. There is evidence that it may be possible to train aircrew to gain voluntary
control over the focal distance of the eye; a more immediate means of alleviating this problem is
to divert the gaze periodically towards a relatively distant feature, such as a wing-tip.
b.
Apparent motion of light sources may occur during night flying. If a stationary point of light is
observed in an otherwise dark visual field, it often appears to move. During flight, there is the
further problem of more systematic illusory motion produced by changes in the force
environment. Forward linear acceleration causes an apparent upward shift, and forward linear
deceleration an apparent downward shift, of objects in the visual field. Angular acceleration is
also a source of difficulty; during recovery from a sustained turning manoeuvre, objects appear to
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rotate in the opposite direction. These effects are a potential source of spatial disorientation.
The pilot may perceive the apparent motion to indicate a change of aircraft attitude. Similarly, the
pilot may interpret a stationary light as another aircraft, and take avoiding action. Little action can
be taken to prevent the occurrence of the illusions caused by motion. However, illusory
movement of a stationary light can be reduced by frequent changes in the direction of gaze; a
change in the aircraft’s flightpath can also help to reduce the illusion.
c.
At night, the pilot’s ability to fly at low level is enhanced by electronic aids such as night
vision goggles (NVGs) (see Volume 7, Chapter 17). There are, however, disadvantages
associated with the use of NVGs. They provide a restricted field of view, and they produce an
image of relatively low contrast and resolution that lacks cues to depth. In addition, NVGs add
weight to the helmet and are a potential hazard in the event of ejection.
10.
Attitude Indicators.
a.
The fovea of the eye is an area of high visual acuity with the capacity for good colour vision,
whereas the more peripheral retina offers greater sensitivity to light and movement. This
distinction reflects the existence of two separate visual systems. The foveal system answers the
question "what?" (i.e. it enables us to identify objects); the peripheral system answers the question
"where?" (i.e. it informs us of our orientation in space, often without our awareness that we are
using this information). Thus, it is possible to walk around a room, unconsciously avoiding
obstacles, while devoting attention to reading a book.
b.
The peripheral system is better suited to processing attitude information. The natural
horizon stimulates this system, and hence provides powerful cues to orientation, even if attention
is directed elsewhere. The attitude indicator presents the same information, but in a much less
compelling form. This instrument must be viewed directly, using the foveal visual system, and so
attitude must be interpreted rather than effortlessly apprehended, increasing both workload and
the likelihood of disorientation.
c.
There have been several attempts to develop attitude indicators that present information to
the peripheral visual system. The Peripheral Vision Display (PVD), formerly known as the
Malcolm Horizon, represents one such approach. In this system, a laser is used to project a bar
of light across the instrument panel; the bar remains parallel with the horizon. Other possible
solutions include displays mounted on the canopy arch.
Perception
11. During the process of perception, incoming sensory information is interpreted in the light of past
experience. Aircrew’s 'mental models' of the environment therefore depend not only on sensory
information, but also on what they expect to perceive.
12.
Illusions. An illusion occurs whenever a percept does not correspond to reality. Various types
of illusion, each potentially hazardous, may be experienced during low level flight.
a.
Aircrew will use various cues to estimate distance and height. Two common cues are:
(1) The size of the image on the eye of a ground-based object of known size.
(2) The texture of the terrain (unless, for example, it is covered by snow or sand).
Both of these cues depend upon assumptions concerning the size of ground-based features. If these
assumptions are false, misinterpretation of height will ensue. For example, a pilot who interprets
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bushes as more distant trees will over-estimate the aircraft’s height. This particular problem is
prevalent in northern latitudes, towards the tundra, where plants are smaller due to reasons of climate.
Care should be taken when the terrain is unfamiliar, eg during detachment to foreign regions. Careful
briefing and pre-flight planning greatly reduce the probability of such illusions occurring.
b.
Descent below the intended altitude may also be induced by the phenomenon of adaptation
to motion. The motion detectors on the retina generally fire at a rate proportional to the angular
velocity of objects in the visual field. However, at a steady speed, their rate of firing gradually
declines. During a subsequent manoeuvre, the pilot may tend to compensate for this reduced
sensitivity by losing height and so increasing visual angular velocity.
c.
During a low-level abort procedure, the inertial force associated with aircraft acceleration
combines with that of gravity to produce a resultant that is displaced from the vertical (Fig 1a).
This apparent shift in the direction of the gravitational vertical creates a compelling illusory
impression of positive pitch (Fig 1b). Reduction of aircraft pitch in response to this illusion
presents an obvious threat to flight safety; moreover, it further displaces the resultant and so,
paradoxically, increases the magnitude of the apparent positive pitch. The pilot who has made
the decision to abort in good time, and who has become established on instruments, is unlikely to
be influenced by this illusion during the abort procedure.
6-9 Fig 1 False Vertical Due to Acceleration
Fig 1a Actual Att itude
Fig 1b Perceived Attitude
Acceleration
Inertial Force
due to Acceleration
I
(I)
Force of
g
Gravity
Resultant
R
(g)
(R)
13.
Collision Course Geometry. Two aircraft on a collision course, each flying at an independent
constant speed, will maintain a constant bearing relative to each other (see Fig 2). The relative bearing
(a) will depend on speeds, and relative tracks. Each aircraft will therefore present a static image to the
crew of the other aircraft, and be hard to detect when distant (i.e. small). In such situations, detection is
often extremely late unless the aircrew look directly towards the other aircraft. The difficulty of visual
acquisition has been demonstrated experimentally. Aircrew failed to acquire a light aircraft on almost half
of the occasions on which an interception had been deliberately engineered. Good visual look-out, with
efficient search of the entire visual field, is the only means of reducing the probability of mid-air collisions.
Every effort should be made to minimise the dwell-times between eye movements, since detection
performance decreases with the duration of individual fixations.
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6-9 Fig 2 Aircraft on Constant Relative Bearing
Impact
Aircraft B
a
g
rin
a
e
B
tive
la
e
R
Angle a will depend
on individual aircraft
a
speeds and relative tracks
Aircraft A
Human Performance
14. Like a computer, man’s information-processing system has a finite amount of processing
resources. The everyday term 'attention' refers to the allocation of these resources to particular
information sources or activities.
15. In general, it is possible successfully to divide attention between simultaneous activities, provided
that their total demands do not exceed the available resources. However, excessive demand may
easily be experienced during low-level flight. Under these conditions, it is necessary either to shed
some of the load completely or at least to defer the completion of activities of lesser importance until
the workload peak has passed. The ability to respond efficiently during periods of high workload
requires awareness of the priority that must be assigned to competing demands for attention; effective
training and practice are necessary to enable individuals gain and retain such ability.
16. Training has the further important function of reducing the incidence of excessive workload
(see Volume 6, Chapter 1). As an activity is practised, its demands upon the limited mental
resources decline. Eventually, automatic routines are established that control sequences of action
without the need for conscious intervention. Paradoxically however the delegation of activities to
automatic control can lead to error, particularly if the demands upon mental resources from other
sources are high. Errors made by skilled aircrew tend to involve sequences of action that are
internally coherent, but inappropriate to the circumstances – conscious awareness is often triggered
only when unintended consequences become apparent. The reduced margin for error during
low flying increases the need to ensure that actions are monitored as frequently as possible.
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17
. Interaction with Electronic Aids. The electronic aids fitted to modern military aircraft
greatly reduce mental workload. Head-up displays (HUDs) minimise the need to make large eye
and head movements to take in the outside world and the cockpit interior, and facilitate shifts of
attention between these sources of information. Terrain avoidance or terrain following guidance
similarly simplifies the aircraft control task. However, these benefits carry potential penalties.
a.
Although they reduce the overall level of aircrew workload, the monitoring component is
increased since, in the event of a system malfunction or damage, the aircrew must be able to
take over. Furthermore, as avionics technology advances, more and more systems tend to be
crammed into the cockpit, so exacerbating the situation. Experimental evidence suggests that
reversion from HUD to cockpit instruments is unlikely to be accomplished in less than 3 seconds
and may take considerably longer. During this transition period, 'seat of the pants' sensations
concerning the behaviour of the aircraft may be illusory.
b.
The low frequency fluctuations in z-axis acceleration associated with non-pilot-induced
terrain following may lead to fatigue and to motion sickness.
18.
Reaction Time. If an individual is expecting a single, clearly defined, signal to which a
simple response must be made, the reaction time may be little more than one-tenth of a second.
However, under less favourable conditions, reaction time is greatly increased. The time to respond to
an unexpected emergency during flight, for example, is likely to fall within the range of 2 to 7 seconds.
a.
Aircrew factors influencing reaction time include:
(1)
Preparedness.
Reaction time is shorter when warning is given of the
imminent occurrence of the signal.
(2)
Age and Fitness. Reaction time decreases until about age 25, thereafter remaining
relatively constant until a gradual increase is observed after age 60. Physical fitness
appears to be associated with faster responses.
(3)
Stress and Anxiety. Stress and anxiety can increase arousal. This may
decrease reaction time, but may also reduce accuracy.
(4)
Experience. Reaction time, even to very simple signals, decreases with practice.
An important role of simulator training is therefore to provide experience in responding to
emergencies during flight.
b.
Environmental influences on reaction time include:
(1)
Retinal Position. In general, reaction time to a visual signal increases with its distance
from the centre of the visual field. Important warning signals are therefore presented as
centrally as possible.
(2)
Intensity of the Signal. Reaction time decreases as the intensity of the signal
increases.
(3)
Sensory Modality. Reaction time depends upon the sense organ to which the signal is
presented. For example, individuals respond more quickly to an auditory than to a visual
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AP3456 - 6-9 - Physiological and Psychological Effects of Low Flying
signal. A further advantage of auditory signals is their ability to attract attention regardless of
the individual’s direction of gaze. Such signals are therefore commonly used to present
warnings to aircrew.
(4)
Signal and Response Characteristics. Reaction time increases with the number of
possible signals that may be presented, and the number of possible responses to these signals.
(5)
Workload. Reaction time to a signal is likely to increase as a function of workload.
Stress and Arousal
19. The term 'arousal' refers to the individual’s level of alertness, extending from deep sleep to a state of
frantic excitement. For any task, there is a particular level of arousal that produces peak performance.
As the difficulty of the task increases, so the optimal level of arousal decreases (Fig 3). Many adverse
environmental conditions increase or decrease the arousal level, and so affect performance.
6-9 Fig 3 Relationship Between Arousal, Performance and Task Difficulty
Difficult Task
Easy Task
e
c
n
a
rm
rfo
e
P
Level of Arousal
20.
Over-arousal. A number of arousing agents can be identified that may impair efficiency
during low-level flight:
a.
Workload. The high workload associated with low-level flight increases the arousal level.
b.
Environmental Factors. Stressors such as heat and noise also have arousing effects.
At low levels, uncomfortable levels of heat may be experienced because of limited cockpit cooling.
21.
The Effect of Over-arousal. Since over-arousal compromises flight safety, its effects must
be recognised. The most important of these are listed below:
a.
Lessening of Calculative Powers. Activities involving the storage and manipulation of
information are more greatly impaired by over-arousal than activities simply requiring throughput
of information. Calculations of fuel reserves, for example, may be more severely disrupted than
the control of aircraft attitude.
b.
Attentional Effects. Under normal conditions, mental resources may be allocated
voluntarily to various aspects of the flying task. However, over-arousal creates a focusing of
attention on particular components of the task; it may, for example, induce aircrew to allocate a
disproportionate amount of resources to a relatively minor malfunction.
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AP3456 - 6-9 - Physiological and Psychological Effects of Low Flying
c.
Impaired Functional Field of View. A phenomenon related to over-arousal is shrinkage of
the area of the visual field from which information may be extracted. Consequently, visual look-out
may be seriously impaired, with a reduced probability of detecting traffic in the visual periphery.
d.
Speed Versus Accuracy. Over-arousal tends to encourage individuals to respond rapidly
but to sacrifice accuracy. It may therefore degrade the quality of decision-making.
e.
Reduction of Mental Resources. Over-arousal reduces the mental resources available
for the performance of tasks.
f.
Reliance on 'Automatic' Behaviour. Changes in the amount and distribution of mental
resources encourage the delegation of well-practised activities to automatic control, and present
fewer opportunities to monitor their progress.
g.
Emergencies. Several accidents involving aircraft malfunction have been found to
be attributable more directly to the associated state of over-arousal than to the original
emergency.
22.
Under-arousal. Fatigue, generated by sleep loss or by prolonged work, may seriously impair the
efficiency of aircrew performance. The state of under-arousal associated with fatigue has several
consequences:
a.
Lessening of Routine Task Performance. Contrary to the effects of over-arousal,
routine activities involving little information storage are more likely to be disrupted by fatigue than
more intellectually demanding tasks.
b.
Behavioural Lapses. Periodic lapses in performance occur, accompanied by changes
in brain activity that indicate decreased alertness and receptiveness to stimuli.
c.
Attentional Effects
. Fatigue impairs the ability to pay particular attention to important aspects
of the task. This effect is the opposite of that of over-arousal, discussed in sub-para 21b.
d.
Loss of Speed and Accuracy. Both speed and accuracy of work may be reduced under
fatigue.
23.
Combinations of Stressors. The combined effects of stressors cannot necessarily be
estimated simply by summing their effects in isolation. Stressors that decrease arousal tend to
counteract the effects of arousing stressors. For example, both sleep loss and noise impair
performance, but a sleep-deprived individual may be more efficient in a noisy environment than in a
quiet one.
Air Sickness
24. Air sickness is considered in some detail in Volume 6, Chapter 6. In the present context, it
should be noted that high-speed, low-level flight creates powerful 'streaming' of ground-based features
in peripheral vision, which, together with motion in the z axis, may provoke air sickness in individuals
previously unaffected at medium level.
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Summary
25. Modern aircraft technology has helped to alleviate the difficulties associated with low flying.
Nevertheless, training and experience, together with careful pre-flight planning, are essential to permit
aircrew to cope with the considerable demands that remain.
26. Even when flying in good weather, the instruments should be monitored for correct functioning; this
will mean that a swift and assured transition to instruments can be made if bad weather is then
encountered. Once the decision to abort from low level has been made safety altitude should be reached
as quickly as possible, having established a robust instrument scan. This means that actions such as
frequency changes should be deferred until this the aircraft is safely established at or above safety
altitude and the pilot flying on instruments..
27. Adherence to the correct abort procedures will minimise the risk of spatial disorientation during or
after the manoeuvre.
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AP3456 - 6-10 - Helicopter Environmental Effects
CHAPTER 10 - HELICOPTER ENVIRONMENTAL EFFECTS
Introduction
1.
The environmental problems encountered by the helicopter crew are not new; most of them are
found to some degree in other aircraft types. In the helicopter not only are all these problems present
simultaneously but also the helicopter frequently has to be flown 'hands on' for long periods of time.
(Only those with autostabilizers or autopilots can be flown hands off and this precludes most light
helicopters.) Accident risk is higher than in conventional aircraft, escape possibilities are limited and, in
some helicopters, crash survivability is poor.
Temperature
2.
Even within the European theatre, a helicopter may have to operate in ambient temperatures from
−30 ºC to +45 ºC. Even in the UK, the seasonal temperature change may exceed 30 ºC and extremes
of temperature produce additional challenges for the crew.
3.
Cold Environments.
a.
Aircraft Performance. In cold environments, cabin conditioning, which consists of
strategically placed warm air vents, may be inadequate for roles which require the frequent
opening of doors. These vents derive their heat from hot air bled from the engines, which results
in reduced aircraft performance. The effect may be insignificant but in certain conditions that
small loss of power can be important, for example in low speed flight and when the aircraft is
heavy.
b.
Comfort and Crew Performance. Cold conditions produce similar symptoms and
performance decrements for aircrew as for troops on the ground. However, numb fingers and
toes, shivering and discomfort are likely to be more hazardous in the flight environment, but
aircrew are unable to move around sufficiently to keep warm. These effects can be mitigated by
insulated clothing but this can produce bulk and discomfort that can also affect performance;
insulated gloves can significantly reduce the manual dexterity needed to operate an aircraft,.
Electrically heated socks and gloves may be used where appropriate power supplies are available, to
augment the existing cabin conditioning system and cold weather flying clothing.
4.
Hot Environments. In hot environments the temperature within a helicopter can be higher than
ambient due to the greenhouse effect of the large area of transparency. Even in warm conditions
helicopter crews experience significant heat loads. This is exacerbated by the requirement to wear at
least minimum clothing of 2 layers with long sleeves and underlayers (for flame protection in the event of
an accident), a helmet and load carrying equipment or body armour that is impervious to water vapour
and air flow. The result is that the ability of helicopter crew to lose heat through conduction, convection,
evaporation (of sweat) and radiation is severely limited. Consequently, helicopter crew are at risk of a rise
in core temperature and a reduction in performance. Personal conditioning systems, consisting of liquid
filled tubed garments supplied from ice packs or thermoelectric devices, have been developed, proven in
the laboratory and trialled successfully by helicopter aircrews, but they have not yet been brought into
mainstream operations. In recent operational theatres the crews have dealt with the heat by good
acclimatisation, excellent hydration and various means of reducing heat load pre-flight and between
sorties.
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Noise
5.
Sources of Noise. Ambient noise levels of 115 dB inside a helicopter are quite common. Some
of the noise is aerodynamic and some from avionics or avionic cooling, but most comes from the
power train i.e. engine, gearbox, and rotor blades. Another important source of noise is in the
communications system. As the majority of the noise is of low frequency, conventional microphones
are frequently unsuitable. The majority of helicopters use boom microphones and these can cause
considerable distortion of speech, although their signal/noise ratio is good.
6.
Effects of Noise. Loud noises can cause numerous problems. In the short term there can be
difficulty in communicating and over a period of hours exposure to loud noise can cause additional
fatigue. In the medium term, loud noise can cause a temporary reduction in hearing, a temporary
threshold shift, although this should recover over several hours or days. In the long term, such
exposures can cause a permanent threshold shift producing Noise Induced Hearing Loss (NIHL) with
significant reduction in the ability to hear, especially in the speech frequencies. Hence, it is important
that helicopter aircrew are protected against noise for both their performance and their health.
7.
Noise Protection. Under European and UK legislation, employers must take steps to protect
workers from noise at a level of 80 dB(A) averaged over an 8 hour working day (LEP,d) and they must
provide additional protection if the level exceeds 85 dB(A) LEP,d. Furthermore, workers must not be
exposed to a noise level exceeding 87 dB(A) LEP,d. A hierarchy of protection from noise exists that
commences with removal of the noise source, replacement of the source with quieter equipment,
shielding of the source, shielding from the source etc. However, all of these are very difficult in aircraft
so we are generally left with personal protective equipment as the solution.
8.
Personal Protection from Noise. In recent decades the main protection from noise has been
the ear cups in the flying helmet, or headset, possibly with a microphone that assists in reducing
transmitted sound from the cockpit. However, there are now other methods that involve either in-ear
communication devices (IECD) or various forms of active noise reduction.
a.
Microphones. A voice-operated electronic switch is employed in some systems to improve
the overall signal/noise ratio and intelligibility by turning the microphone off when the wearer is not
speaking. Such a system should incorporate individual automatic level controls, so that the switch
will operate at a given point above the local noise level. However, even if the signal/noise ratio
from the microphone is good, it is not possible to produce an acceptable ratio at the ear simply by
presenting the signal at a very high level; the ear becomes non-linear in its response at high level,
and hearing loss can result. It is therefore vitally important to maximize the attenuation provided
by the helmet.
b.
Ear Cups. The ear capsules provide a physical barrier against the passage of sound to the
ear via the normal air conduction pathway. These can be very effective, with some non-aviation
types producing up to 45 dB attenuation. Protection is derived from differing aspects of the
design: the shell material is important in attenuation at frequency ranges of 400-2000 Hz; the
filling material protects more in the high frequencies, above 2 kHz; whilst the cup volume is the
limiting factor at low frequencies (below 400 Hz). Consequently, since aviation ear cups are
limited by the need to fit beneath the helmet, their volume is limited and the best types will
currently only provide around 27 dB attenuation.
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AP3456 - 6-10 - Helicopter Environmental Effects
c.
In Ear Communication Devices. In order to improve hearing protection further, some
aircrew would wear expanded acoustic resin (EAR) ear plugs inside the ear cups of their helmet
(‘double protection’). Whilst this increased protection against external noise, it also reduced the
level of communications volume from the intercom and radio, because the speaker was external
to the ear plug. This could be countered by increasing the audio volume but there was a loss in
speech intelligibility even though overall noise exposure was reduced. In recent years, the
principle of double protection has been used but with miniature speakers feeding the
communications sound to the inside of the ear plug. Such devices are termed in-ear commination
devices (IECD) and they can provide excellent levels of hearing protection. They have the benefit
of double protection but also the audio is presented against a lower background noise level so the
contribution of communications, which can be significant, is reduced. There are a variety of
designs with some using EAR type plugs and others using silicone or a resin material. However,
all will require connections to the audio system and they must integrate adequately with the other
head worn items, as well as providing suitable levels of comfort.
d.
Active Noise Reduction. Active Noise Reduction (ANR) uses a microphone in the headphone
which detects all ambient noise at the ear. A processor then generates and introduces a sound signal
that is 180 degrees out of phase which, when added to the signal sent to the earphone, cancels out the
ambient noise. In turn, this allows the input signal from the communications system to be heard more
clearly. Despite great promise, early analogue ANR suffered from considerable limitations from
processing time and frequency limits, offering little protection against sounds above approximately 500
Hz. More recently, digital ANR has been able to expand the frequency range up to approximately 1000
Hz. Currently, protection levels are around 15 dB which is adequate for some platforms in combination
with a helmet or headset. In the future, it is likely that adaptive digital ANR will be introduced. This
should be able to provide protection at specific frequencies, tuned to each aircraft platform, e.g. the
propeller frequencies of turboprop aircraft, and by targeting the highest peaks of exposure, it should be
able to provide significant protection.
Vibration
9.
Helicopter aircrew are exposed to vibration with significant linear and angular acceleration
components in the three orthogonal axes. Vibration is present throughout a sortie from start to shut
down, but it changes with phase of flight. Vibration increases with airspeed, all up mass and transition to
the hover. It will also be exacerbated by turbulence. The dominant vibration frequency is a function of
rotor speed and the number of rotor blades, and ranges from 12 to 18 Hz (dependent on aircraft type).
The magnitude of the linear vibration is usually greatest in the gz (vertical) axis and can be of the order of
6 m/s2 (0.6g) in some aircraft during certain phases of flight.
10. Vibration can give rise to the following:
a.
Difficulty in reading aircraft instruments.
b.
Difficulty in reading hand-held maps and charts.
c.
Impaired ability to make fine positioning and control movements.
d.
Generalized discomfort and early onset of fatigue.
e.
Specific symptoms e.g. teeth chatter, flutter of facial muscles, and aggravation of backache.
Thus, vibration may impair operational effectiveness of helicopter aircrew by degrading performance,
increasing workload and engendering fatigue.
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Comfort and Controls
11. Helicopters are notorious for providing poor comfort for their crew. Sources of discomfort include:
a.
Control Geometry. By current convention, helicopters are controlled using a cyclic stick, for
directional and speed control; a collective lever, for power control; and yaw pedals for
aerodynamic balance in flight plus directional control in the hover. The cyclic stick is situated
between the pilot’s legs and operated using the right hand; the collective lever is to the left of the
pilot’s seat and operated with the left hand; and the yaw pedals are operated by the feet in the
same way as rudder pedals in fixed wing aircraft. Unfortunately, the actual positions of these
controls, plus the need to maintain a lookout to the sides and rear, mean that pilots tend to sit
leaning forward in the so-called ‘helicopter hunch’. Consequently, the spine is forward flexed at
best and at worst it also has some rotation and lateral flexion.
b.
Seat Design and Adjustment. Seat design varies in quality but, almost invariably, gives
little back support. Seat pans and cushions vary from flat hard cushions to contoured shapes
giving good support. In general, the seat pan should be contoured, should be raked and should
also be long enough to support the thighs when the hips and knees are flexed to operate the
controls. This is essential to reduce the pressure on the buttocks and ischial tuberosities during
longer sorties. If armour is required for the crew, it should be designed into the seats rather than
being added on as an afterthought. Retro-fitted armour can encroach on the space for the
occupant and result in a limited ability to move in the seat to reduce pressure on affected areas.
Seat adjustment varies from vertical only, to horizontal only, or a combination of both, with or
without yaw pedal adjustment. However, the ranges of adjustment are often too little for those at
the extremes of the anthropometric distribution.
c.
Aircrew Equipment Assemblies. The clothing and equipment worn by aircrew (the aircrew
equipment assembly (AEA)) can add to discomfort. It is essential that clothing fits correctly and is
designed not to have excessive bulk leading to folds that can cause pressure on the tissue
beneath. Ideally clothing would be designed to fit optimally when sitting in the position adopted
during flight. Equipment adds bulk and can also add large amounts of weight; these can reduce
mobility and add pressure to the areas in contact with the seat. Head mounted mass also adds to
discomfort due to spinal loading especially when the spine is flexed forward. This is a particular
problem for rear crew who have to support the weight of helmet, NVG, counterbalance weight etc.
whilst leaning over the tailgate, looking through holes in the floor or through bubble windows,
where the additional weight is support by the musculature rather than down an upright spinal
column.
d.
Flight Durations. In the past, helicopters had limited fight durations and had to shut down
for refuel. Consequently, the crew had the ability to unstrap and exit the aircraft periodically.
Without internal or external ferry tanks, flight endurance is typically around 2 hours. More modern
aircraft are able to accept running refuels so they can re-fuel with rotors stopped but engines
running. This is advantageous in reducing time on the ground and increasing availability of the
aircraft, but it means that aircrew cannot get out. Hence, crews can potentially spend 5 or 6 hours,
or possibly more, strapped into the seat.
e.
Environmental Factors. As mentioned in earlier sections, helicopters are frequently too hot,
or too cold; they are always noisy and they always vibrate. Such environmental factors add to the
likelihood of discomfort and stresses on the crew.
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These factors result in complaints of discomfort being common in helicopter crew. Low back pain is
most common in front seat crew but complaints of pain in the mid or upper back, or neck are not
unusual. For rear crew neck pain is more frequent. Such problems can be mitigated by good
equipment design, lumbar supports and more recently, a physiotherapy led aircrew (physical)
conditioning programme has commenced.
12. These ergonomic issues are exacerbated by the need, in most helicopters, to keep the hands and
feet on the controls throughout flight. In normal flight modes the forces on controls are very light due to
hydraulic assistance systems. However, in certain emergency situations the pilot will lose hydraulic
power, resulting in very heavy forces needing to be applied to the controls. More modern helicopters
have been equipped with flight control systems previously only seen in large multi-engine fixed wing
aircraft. They now have flight control systems and autopilots that allow pilots to remove their hands
and feet from the controls periodically to reduce strain on the limbs and back.
Disorientation
13. Helicopter aircrew experience illusory sensations of aircraft motion and attitude which are, in
general, similar to those reported by pilots of fixed wing aircraft. However, because the helicopter
lacks inherent aerodynamic stability, recovery from an abnormal attitude or control error precipitated by
disorientation, must be actively pursued by the pilot. In addition, because flight is commonly at low
altitude, there may be little time in which to make the necessary recovery action.
14. When in the hover, the pilot has to maintain attitude in pitch, yaw, and roll, as in a fixed wing aircraft,
but also has to minimize translational motion in three orthogonal axes. Without reliable visual cues,
particularly at night, the pilot is unable to maintain accurate and stable hover, because the angular and
linear motion stimuli may be below the threshold for detection by his sensory system. Thus, disorientation
commonly occurs when the pilot attempts to hover in conditions where external visual references are
degraded, or absent, as when flying at night, or in cloud, fog, snow, dust or smoke. Some aircraft have
instruments to assist in such conditions, but the scan can be disturbed and disorientation ensues when
the pilot transfers from instrument to external visual reference and vice versa. Difficulties can be
compounded by vibration which may impair visibility of the aircraft instruments. Some modern helicopters
provide automatic systems for take-off to hover to overcome these problems but in the majority of aircraft
pilots must control a hover by visual reference to the external environment.
15. In order to maintain accurate hover without using instruments, the pilot must have a stable and
discernable ground reference. At night, a single light on the ground is inadequate for the correct
appreciation of height, and it may give rise to a false perception of motion due to the 'auto kinetic
illusion'. Flight near moving light sources (e.g. on a motor vehicle or on another aircraft) can also
disorientate because of error in the appreciation of relative motion. Even when the ground is
illuminated by lights on the helicopter, problems can arise if only a small area of ground is picked out
by a narrow beam of light; furthermore, at heights, typically greater than 100 ft, ground texture is lost
and other visual cues should be employed. When hovering over the sea, the wave pattern generated
by rotor downdraught can, by its relative motion, produce an illusory sensation of backward motion.
Likewise, when at low altitude over water or snow, the movement of spray or snow downwards through
the rotor can be interpreted as ascent of the helicopter. These visual problems are compounded by
the use of night vision goggles.
16. Blade flicker is more commonly a cause of distraction and irritation than disorientation, though at
times the repetitively moving pattern of light within the cockpit does give rise to an illusory sensation of
rotation (vertigo) in the opposite direction to that of the visual stimulus. A more frequently reported
cause of disorientation and distraction when flying in cloud, fog, rain, snow etc. is the backscatter of
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AP3456 - 6-10 - Helicopter Environmental Effects
light from the helicopter’s anti-collision beacon into the cockpit. In such flight conditions, the reflected
light, apart from giving inappropriate visual motion stimuli, degrades visibility of external visual cues
and may necessitate transfer to flight instruments. Under these conditions, the anti-collision light
should be switched off. Flicker induced epilepsy is a very rare condition and susceptible individuals are
not accepted for aircrew training.
17. Illusions reported by helicopter aircrew attributable to physiological limitations of inner ear
(vestibular) mechanisms are similar in character and incidence to those described by pilots of fixed
wing aircraft. The 'leans' (a false sensation of roll attitude) is by far the most common, though an
illusory sensation of turn on recovery from a sustained turn is also a frequent occurrence. In addition,
false sensations of turn and attitude change are evoked when a head movement is made in a
helicopter which is turning or when the aviator is exposed to an abnormal force environment (i.e. linear
acceleration other than 1g). Typically, these varied manifestations of vestibular disorientation are
experienced only when flying on instruments or when flying by marginal external visual references as at
night or in poor visibility conditions.
18. Whilst all of the disorientation phenomena listed above can, and do, occur, more recent data show
that the bulk of helicopter disorientation accidents in recent years are due to very high crew workloads.
As aircraft have been designed with greater amounts of operational equipment, such as weapons,
observation aids etc., the workload on crews has increased, despite the automation of many flight and
engine systems previously monitored by the crew. This challenges the ability to appropriately divide
attention and can detract from the flying task. As a result, the typical military helicopter spatial
disorientation accident is less one of classical vestibular or visual illusions, but more one of a hard-
pressed crew flying a system intensive aircraft (often at night using night vision devices) failing to
detect a dangerous flight path.
19. Feelings of detachment, isolation, and estrangement (the 'Break-off' phenomenon) are
experienced by helicopter pilots, typically, during the more monotonous phase of a sortie when flying
solo in conditions where external visual references are not well defined (e.g. smooth sea, hazy
indistinct horizon) and there are few cues of relative motion. The flight environment conducive to the
induction of sensations characteristic of the 'Break-off' phenomenon is similar to that found in fixed
wing aircraft, though, notably, in helicopters 'Break-off' is not confined to high altitude flight but can
occur quite low (500 ft agl). The commonest sensation is one of being 'suspended in space' or
'balanced on a knife edge'; though the feeling of detachment can be more severe, and the aviator may
even feel that he is separated from the aircraft. Coupled with such 'dissociative' sensations there is
frequently a heightened awareness of changes in aircraft orientation, though frank disorientation with
illusory sensations of attitude and motion are quite rare.
Accidents
20.
Accident Risk. Overall, the risk of fatalities in helicopter accidents is higher than in fixed wing
aircraft. This is due to their flight environment and the characteristics of the aircraft. Half of helicopter
accidents have occurred at heights of less than 100 ft and a further 30% between 100 ft and 500 ft.
Flying so close to the ground means that the risk of controlled flight into terrain (including obstructions
such as trees, telegraph cables, power lines and other wires) is high. In addition, if an emergency
occurs there may be very little time to respond and control it before reaching the ground. These
heights preclude the use of escape systems such as conventional parachutes, so military helicopter
crews have to ride their aircraft to the ground Consequently, military helicopter crews spend much time
learning to manage emergencies, control a descent in autorotation (without power to the rotor blades)
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and then land it safely. Whilst all will be able to do this competently in a training situation, they all
accept that operational circumstances will be likely to make it much more difficult.
21.
Impact Survivability. Helicopters have tended to be built to low standards of crash impact
survivability, much lower than fixed wing aircraft. In recent years much has been done to improve this
situation. The design aims have been to:
a.
De-lethalise the cockpit and cabin. Aircraft fuselages are stronger to prevent deformation
of the occupant space and prevent intrusion of components such a gearboxes and engines that
will cause injury. Structures within the ‘strike envelope’ of flailing limbs can also be removed or
modified to prevent injury.
b.
Improve restraint. Occupants that are properly restrained will be less likely to be thrown
around inside the fuselage, or ejected from it, and injuries will be reduced. The fitting of effective
harnesses, either 5 point or 4 point, has assisted.
c.
Attenuate Energy. Impact energy can now be absorbed by energy attenuating
undercarriage and seating that absorbs energy, preventing its transmission to the occupants.
d.
Prevent fire. The risk of post-crash fire can be reduced by, for example, using self-sealing
fuel tanks and fuel lines that seal when disrupted; inertially operated fire extinguishers that
activate when exposed to impact decelerations; and engines mounted out board of the fuselage
that will break clear in an accident.
Although many modern aircraft incorporate some of these features, very few will have all of them. For
older aircraft the situation is worse due to the cost or impossibility of retro-fitting such features.
Consequently, many helicopter crew still fly aircraft with limited crashworthy features.
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AP3456 - 6-11 - Sleep, Wakefulness and Circadian Rhythms
CHAPTER 11 - SLEEP, WAKEFULNESS AND CIRCADIAN RHYTHMS
Sleep
1.
Since disturbed sleep is frequently experienced by aircrew, some knowledge of the sleep-
wakefulness continuum is helpful in understanding the changes in sleep associated with air operations.
Sleep can be classified into various stages (see Fig 1). These stages may be used to indicate the depth
and continuity of sleep. They are also used when considering the changes that take place when sleep
occurs at unusual times or when an individual is exposed to a time zone transition.
2.
From Figure 1, there are four stages of sleep. In addition, there is a stage, known as REM (rapid
eye movement) sleep, in between 'Awake' and 'Stage 1'. The numbered stages are known collectively
as 'non-REM sleep' and increase in depth (or intensity) as the number increases. A healthy young
adult normally passes quickly from wakefulness through the lighter stages (1 and 2) into the deeper
stages (3 and 4) in which the brain waves slow down. Between 70 and 90 minutes after the onset of
sleep, the first period of REM sleep occurs. It is followed by further non-REM stages, and then another
episode of REM sleep. These cycles of non-REM and REM sleep last about 100 minutes, and their
content alters as the night proceeds. Later sleep cycles have less slow brain wave sleep, and periods
of REM sleep become longer towards the end of the night. The sleep pattern of a young adult is
shown in Figure 1. Typically, about 50% of the night is occupied by stage 2 sleep, 20% by slow wave
sleep, 25% by REM sleep, and 5% by stage 1 drowsy sleep and minor awakenings. However, the
nightly amounts of the various stages of sleep are related to age. In middle age, there is much less
slow brain wave sleep and an increase in wakefulness during the night. It may, therefore, be much
more difficult for middle-aged individuals to achieve acceptable sleep when the normal pattern of sleep
and wakefulness is altered.
6-11 Fig 1 Sleep Pattern of a Young Adult
Sleep Stages
4
3
p
e
2
le
S
f
o
1
th
p
e
D
REM
Awake
1
2
3
4
5
6
7
8
Sleep Onset
Hours
Latency
Sleep Period Time
Circadian Rhythms
3.
Many biological processes vary with respect to time in a periodic and regular manner. In humans,
the commonly observed phenomena which oscillate once around the length of the solar day (24 hours)
are known as 'Circadian Rhythms'. Such rhythms are free-running, self-sustaining oscillations with a
periodicity between 23 and 26 hours, but they are normally entrained to a 24 hour cycle by
environmental synchronizers, known as 'zeitgebers'. The principal zeitgebers are light and darkness,
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though others, such as meals and social activities also have an influence. Many variables, including
body temperature and alertness, demonstrate circadian periodicity, though the timing of maximum
(acrophase) and minimum (nadir) values differs between rhythms. With the increased use of night
vision enhancing technology, flying during hours of darkness is becoming almost the norm. Therefore,
consideration of disturbances in circadian rhythms is even more important than in the past.
4.
On most tasks, if not all, performance rises during the day to a peak or plateau around 1800 hours
and falls to a minimum usually between 0300 and 0600 (Fig 2).
6-11 Fig 2 Circadian Rhythm of Performance
Circadian Rhythm
+
e
c
n
a
Midnight
rm
Midday
Midday
rfo
e
P
-
0500 hrs
Performance over long periods of time may also be influenced by an interaction with the circadian system.
With increasing time on task, after an initial improvement, the level of performance falls (Fig 3).
6-11 Fig 3 Change in Performance with Time on Task
+
e
c
n
a
Time on Task (Hours)
rm
rfo
0
4
8
12
16
e
P
-
Performance
5.
The extent of this degradation may depend on the stage of the circadian cycle with which it
coincides. For example, during a 16-hour period of duty commencing at 1400 hours, very low levels of
performance occur during the latter part of the duty period coinciding with the circadian trough in
performance (Fig 4). On the other hand, if duty commences around 0200 hours, it is likely that
performance will be maintained due to the favourable influence of the circadian rhythm during the latter
part of the duty period (Fig 5). It would appear that increasing levels of alertness during the day partly
compensate for the effect of prolonged work, whereas the natural increase in sleepiness at night may
add to the problem.
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AP3456 - 6-11 - Sleep, Wakefulness and Circadian Rhythms
6-11 Fig 4 Resultant Performance Graph for Duty Commencing at 1400 Hours
+
e
c
n
1400 hrs
a
Midnight
rm
rfo
Midday
Midday
e
P
-
0600 hrs
Resultant
6-11 Fig 5 Resultant Performance Graph for Duty Commencing at 0200 Hours
+
e
c
n
a
Midnight
Midnight
rm
Midday
rfo
e
Resultant
P
-
1800 hrs
Disturbed Sleep
6.
Disturbed sleep over a day or two may arise from a change of surroundings or from difficulty in
coping with an unusual pattern of work. It is in this context that aircrew may experience sleep
difficulties. Changes in sleeping environment, and rest at unusual times, are part of day-to-day life for
many aircrew. Even limited alterations disturb some individuals, particularly if they occur suddenly.
Similarly, work by day and rest at night are in harmony with the normal pattern of sleep and
wakefulness. Those who work unusual hours and have to cope with time zone changes are likely to be
out of phase with this natural rhythm.
7.
Disturbed sleep is one of the major consequences of shift work. The night worker is forced to rest
during the day, not only out of phase with the normal rhythm of sleep and wakefulness, but also when
environmental factors such as noise and light do not favour sleep. There are also higher ambient
temperatures and social influences, which may disturb even the most tired morning sleeper. It is
estimated that 50% of shift workers suffer from sleep disturbance, whereas in day workers the figure is
between 5% and 20%. During operations in which sustained effort is required, especially during night
operations, daytime sleep may be inadequate in quality and quantity to the point that serious sleep debt
accrues. In these circumstances, any measures which improve the quality of daytime sleep can be
crucial. The measures outlined in para 17 should not be viewed as luxuries, but as measures which
improve operational effectiveness and mitigate risk.
8.
Adaptation to night work takes several consecutive days and, during this time, sleep taken during
the day may be shorter because the individual is unable to stay asleep. Problems are also associated
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AP3456 - 6-11 - Sleep, Wakefulness and Circadian Rhythms
with morning shift work. Sleep before an early morning shift may be curtailed, and individuals may be
unable to compensate by commencing their sleep earlier. Shift workers often compensate for lost
sleep by napping, and by extending sleep on days off.
9.
Some irregularity of sleep is inherent in most air operations. Duty hours may encroach on the
normal nocturnal sleep period, and some periods may be shortened. The mean duration of sleep over
several months may be around 8 hours, but the range may extend from as little as 5 hours to as many
as 9 hours. Prolongation of some sleep periods is the essential compensation to duty hours which
encroach on early morning and late evening sleep.
Fatigue
10. ICAO defines Fatigue as :
“A physiological state of reduced mental or physical performance capability, resulting from sleep
loss or extended wakefulness, circadian phase or workload (mental and/or physical activity) that
can impair alertness and ability to perform safety related duties”.
11. In simplistic terms fatigue is an experience of physical or mental weariness that results in reduced
alertness. The major cause of fatigue is not having obtained adequate rest and recovery from previous
activities. Fatigue largely results from inadequate quantity or quality of sleep. This is because both the
quantities and quality of sleep are of equal importance to recover from fatigue and maintaining normal
alertness and performance. Furthermore, the effects of fatigue can be exacerbated by exposure to
harsh environments (Operations) and prolonged mental or physical work.
12. Inadequate sleep (quality and quantity) over a series of nights causes a sleep debt which results
in increased fatigue that can sometimes be worse than a single night of inadequate sleep. Sleep debt
can only be recovered with adequate recovery and sleep. When personnel work outside the normal
routine Monday to Friday hours e.g.: 0800 to 1700, this can limit the opportunity for sleep and recovery
in each twenty-four-hour period. This is partly due to the disruption of the circadian rhythms.
13. Fatigue-related symptoms can be divided into three categories: physical, mental and emotional.
Table 1 depicts examples of each of these types of fatigue. If a person is experiencing three or more of
the symptoms outlined below, there is an increased chance that they are experiencing some level of
fatigue or reduced alertness. It should be remembered that fatigue may not be the only cause of the
symptoms presented below but if they occur together, it is a good indication that an individual is
fatigued. Personnel who present three or more symptoms in a short period of time are likely to be
experiencing fatigue-related impairment.
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AP3456 - 6-11 - Sleep, Wakefulness and Circadian Rhythms
Table 1 Categories of Sleep Related Symptoms
Physical Symptoms
Mental Symptoms
Emotional Symptoms
Yawning
Irrational decision making
Quieter or withdrawn than usual
Heavy eyelids
Irrational reactions
Lacking energy
Rubbing Eyes
Illogical reactions
Lacking in motivation to do a
Head drooping
Difficulty concentrating on tasks
task well
Microsleeps
Lapses in attention
Irritable or grumpy with peers,
Difficulty remembering what they are family or friends
doing
Failure to communicate important info
Failure to anticipate events or actions
Accidently doing the wrong thing
14. The result of fatigue is reduced alertness which may have a negative impact on performance. The
fatigue associated with tiredness and reduced alertness is different from physical fatigue or weariness
which is caused by long hard physical work. In this case, fatigue may be more accurately defined as
mental fatigue although it certainly affects physical performance as well, especially tasks that require
hand-eye coordination, rapid reaction times and fine motor skills. Other skills that are impaired by
fatigue include attention, vigilance, concentration, communication, and decision-making. Impairment
can lead to fatigue-related errors, which in turn can lead to incidents or accidents.
15. It is vital that supervisors understand their shift system, in which our personnel should be given
sufficient time to sleep, rest and to have family and social time away from work. A fatigue friendly
environment equates to a normal routine Monday to Friday hours eg: 0800 – 1700 shift. Yet even
allowing a shift pattern to finish by 2200 would still provide satisfactory opportunity for sleep and rest
without having to waken excessively early the following day. However, whilst this provides sufficient
time for sleep, there is little or no time for socialising or spending time with families or friends thus
leading to social isolation and low mood, both of which can lead to fatigue. Therefore, providing ample
opportunity for balancing sleep, social and family time should be taken into account when choosing a
particular shift pattern. Shifts should also have a definitive end of duty time, as the absence of a
definitive end of duty time creates uncertainty which can act as a stressor within the body that may lead
to mental fatigue. This is particularly key on some engineering night shifts where personnel commence
duty at 1630 and have no definitive end of duty time. DH’s should therefore ensure that for normal
situations an end of duty time is defined. Furthermore, DH’s should ensure that their ADS/orders where
fatigue is covered includes maximum permissible consecutive duties and details mandatory rest
periods. These duties should clearly articulate the process for deviation away from the norm, where an
increased consecutive period is required for operational reasons.
Time Zone Changes
16. The disturbance of sleep which occurs after a change of time zone is often referred to as jet lag
and is of particular importance to aircrew. Trans meridian flights lead to desynchronization of circadian
rhythms from those of the environment. Fatigue may occur at inappropriate times of the day, while the
ensuing need to synchronize rhythms to a new time zone leads to sleep disturbance. Individuals may
have difficulty in falling asleep when it is the local time for rest, and there may be spontaneous
awakenings during the night or early awakening during the morning.
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AP3456 - 6-11 - Sleep, Wakefulness and Circadian Rhythms
17. Circadian rhythms are usually entrained by cues in the environment, but desynchronization can
arise in a number of ways. The zeitgebers in the environment may change their period length, become
weakened or disappear completely. Conflict may also arise when rest and activity patterns are out of
phase with the environmental synchronizers, such as with shift work or after a trans meridian flight.
Adaptation of sleep and wakefulness after a change in the work-rest cycle, or after a time zone
change, may take several days.
18. In managing this there are a few considerations:
a.
Number of time zones crossed.
b.
The adaptation is usually faster following westward travel than eastward travel across
the same number of time zones. This is because most people have a circadian body clock
that has a cycle slightly longer than 24-hours, therefore it is easier to adapt to a westward
shift. After a westward flight, aircrew should usually fall asleep quickly as the local time for
rest is well into the 'night' of the individual’s natural rhythm for sleep and wakefulness.
c.
After eastward flights across 6 or more time zones, the circadian body clock may adapt
by shifting in the opposite direction, i.e. shifting 18 time zones west rather than 6 time zones
east. When this occurs, some rhythms shift eastward and others westward and overall
adaptation maybe slowed. After an eastward flight, the onset of sleep after retiring may be
delayed over several days as individuals attempt to sleep earlier in their previously established
sleep-wakefulness cycle. However, many eastward journeys are overnight, and the loss of
sleep during the flight duty period, and possibly during the day of arrival, may combine to
overcome any difficulties in falling asleep on the first night in the new location.
d.
Adaptation is usually faster when the circadian body clock is more exposed to the time
cues that it needs in the new time zone. Therefore, the earlier you can adopt the eating and
sleeping cycle in the new time zone, the less effect jet lag will have upon you.
e.
Increased fatigue levels are likely if a person does not adopt to the new time zone and
continues to eat and sleep under their previous time zone. This may result in degraded
performance on mental and physical tasks and mood changes, and minor digestive system upset.
19. Operationally, the main problem posed by trans meridian flights is coping with the effect of time
zone changes rather than adapting to them. Some aircrew are involved in repeated crossing of time
zones, or in North-South operations, which involve night flights. In these circumstances, sleep
becomes irregular over an extended period with respect to both duration and time of day. With 24
hours stand-down periods, a long sleep immediately after a flight could mean that aircrew are then
awake for too long before the next duty begins; so crews often split their sleep into two parts. Sleep is
restricted immediately after the flight, and a further sleep of 3 to 4 hours is taken shortly before the next
duty. During long-range flights, which extend wakefulness beyond 16 hours, and when crew
composition and duties permit, naps of up to one hour may be extremely helpful in reducing fatigue. In
addition, they probably assist in the adaptation to new time zones, particularly with westward flights
when the day is lengthened.
20. Whilst Jet Lag is well recognised and can be acclimatised to; Shift Lag is less well appreciated.
One cannot acclimatise to shift lag due to the multiple social and environmental cues that affect our
circadian rhythms. Night shifts will thus remain the most dangerous working environment in regards
fatigues’ effect.
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AP3456 - 6-11 - Sleep, Wakefulness and Circadian Rhythms
Transport Operations
21. The management of aircrew coping with irregularity of work and rest is complex; the key is in the
design of duty schedules. Long-range operations, incompatible with acceptable sleep, could prejudice
mission accomplishment as well as flight safety (as could intense short-range schedules). Therefore,
duty hours are an extremely important issue. Normally, workloads must allow crews to achieve an
acceptable pattern and duration of sleep. Practically, this means time off-duty of sufficient duration to
allow eight hours for sleep, in addition to time for taking meals, bathing, transport to and from duty, and
some time for relaxation. Nevertheless, the characteristics of a particular period of work could also
lead to reduced effectiveness, especially when individuals are expected to be continuously on task for
long periods of time. In this context, the scheduling of duty should, as much as possible, avoid the
marked falls in performance associated with prolonged periods of duty terminating in the early hours of
the morning, since this superimposes a fatigued state on the circadian nadir (see Fig 4).
22. There is a cumulative effect of irregular work, and a critical factor in achieving adequate sleep is
the limit to duty hours over a number of days. A small increase in hours may convert an acceptable
schedule to an unacceptable one. Conversely, a small reduction in the overall number of duty hours
may have a beneficial effect, allowing an additional sleep period, or greater flexibility in the choice of
time to sleep, in a particular rest period.
23. The available duty hours compatible with sleep do not increase linearly with the number of days of
the schedule, due to the cumulative effect of the irregularity of sleep. The duty hours which fully rested
aircrew, operating worldwide routes, can cope with in the first 7 days of a complex schedule cannot be
maintained in the next 7 days. This effect must be considered in the scheduling of aircrew, as the rate
of working is the basis for any system of flight time limitations.
24. The European Working Time Directive (EWTD) has defined the scheduling described above
based upon the evidence in the first three sections. Whilst not exhaustive the following provides a
broad summary of the requirements of the EWTD with respect to normal shift working.
a. Shifts not exceeding 8 hours can work the shift pattern for 6 days followed by a day off.
b. 10-hour shifts should not work more than 5 consecutive days and have 3 days off.
c. 12-hour shift should not work more than 4 consecutive days and have 2 days off.
d. It is accepted that operational pressures may necessitate adoption of different shift patterns
from those above. Such necessity falls within the ‘inevitable conflicts’ exemption to EWTD.
25. Risk assessment is the primary tool for managing fatigue on operations, but a typical ‘rule of
thumb’ risk level can be determined from the ratio of sleep to wake time in the previous 48-hour period.
This is explored in more detail in AP8000 Leaflet 8213 - Fatigue Management. Mitigation action should
be taken in proportion to how far the forced local conditions differ from the guidelines in this leaflet and
2008DIN01-050.
26. In coping with irregularity of work, short periods of sleep seem to be very useful. A period of sleep of
around 4 hours duration, in the evening, leads to a sustained improvement in performance overnight. On
the other hand, naps of about an hour, while they may reduce the tendency to fall asleep, have a
beneficial but limited effect on performance when an individual has already become tired. This would
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AP3456 - 6-11 - Sleep, Wakefulness and Circadian Rhythms
suggest that, as far as performance is concerned, the strategy of sleeping for a few hours before
overnight duty is more beneficial than attempting to overcome the effects of sleep loss by naps.
Management of Sleep Disturbance
27. Some aircrew find it difficult to cope with irregularity of work and rest. This is particularly likely in
middle age when sleep begins to deteriorate (Para 2). Sleep disturbance may be a reflection of illness
or personal difficulties and, if these are suspected, then medical advice should be sought. In the
absence of such causes, there is much that individuals can do themselves to optimize their sleep.
Exercise, avoiding heavy meals and limiting the consumption of alcohol and caffeine may help. Any
incremental improvement in sleeping conditions will enhance the quality of what sleep aircrew are able
to obtain. These enhancing factors include:
a.
Darkness.
b.
Quiet.
c.
Cool ambient temperature.
d.
Comfort (sleep in the reclining position with mattress, blankets and pillows).
These factors are all the more important when sleep is required during day-time in preparation for night-
time duties, since obtaining quality sleep during the day is always more difficult than during the night.
28. In the event of persistent difficulties after attention to sleep habits, then hypnotic medications
(drugs which induce sleep) may be useful. There is unequivocal evidence of disturbed sleep in air
operations and, for this reason, the use of hypnotic medications at specific points in the schedule is
warranted. These medications are most useful to enhance daytime sleep, or to promote sleep at
appropriate times after trans meridian travel. The medications should be tested 'on the ground', and a
medical practitioner should help to identify when their use is likely to be most beneficial. In the United
Kingdom, Temazepam is the medication of choice for aircrew. There should be an interval of 8 hours
between ingestion and commencement of duty.. If hypnotic medications are used, then alcohol must
be avoided.
29. Some air forces have used stimulant medications to mitigate the effects of fatigue. Their use is
controversial. They are not currently used in flying operations in the UK armed services. The UK
approach has been to ensure adequate rest with the appropriate use of hypnotic medications to induce
sleep if necessary, as described in paragraph 18.
30. Caffeine is the most widely used stimulant. Caffeine has the effect of perking you up by blocking
adenosine reception in the brain. Adenosine can suppress nerve cell activity and may be involved in
the sleep/wake cycle. True caffeine effects require 9 days of caffeine abstinence prior to use. Caffeine
from drinks (e.g. coffee) are usually absorbed within 45 minutes of consumption and the affects can
last for up to 6 hours. Therefore, caffeine consumption is not recommended close to periods of sleep.
The use of caffeine needs to be carefully managed as taking it too often increases the body’s
tolerance, therefore reducing the effect from the same quantity. Caffeine also needs to be used
strategically to ensure maximum benefit:
a. Avoid caffeinated drinks/food when not tired.
b. Avoid caffeinated drinks/food in the morning, as the body is waking up naturally and will feel
more awake as the morning progresses. Using caffeine to speed this process simply increases an
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AP3456 - 6-11 - Sleep, Wakefulness and Circadian Rhythms
individual’s tolerance. The exception to this is when required to rise earlier than normal or in need
of an extra boost.
c. Avoid caffeinated drinks/food within 3 hours of bedtime.
d. Be aware of how long it takes for caffeine to take and how long the effect will last.
e. Be aware how much caffeine you are consuming.
f. Be aware that fatigue is symptomatic of caffeine withdrawal
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AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
CHAPTER 12 - OXYGEN AND AIRCREW EQUIPMENT ASSEMBLIES
Introduction
1.
Breathing ambient air on ascent to altitude produces a progressive fall in the partial pressure of
oxygen in the lungs PO Above 8,000 ft the P will be at levels which are insufficient to meet the body’s
2
O2
requirements for oxygen and hypoxia will develop. This most serious of hazards must be prevented in
flight and one method of so doing is to provide an artificial pressure environment, ie a pressurized
cabin. The alternative method is to provide a source of added oxygen so as to maintain the PO at
2
ground level equivalent at all altitudes. In most military flying a highly pressurized cabin (High
Differential Cabin) is inappropriate for several reasons and so both the methods are combined. The
cabin is pressurized to a certain degree (Low Differential Cabin) and any shortfall in oxygen required is
met by a supplementary source in the aircraft. Oxygen systems in one form or another are fitted to all
RAF aircraft which operate at actual or cabin altitudes in excess of 8,000 ft.
2.
The physiological and operational requirements for aircraft oxygen systems may be summarized thus:
a.
Oxygen Concentration. Oxygen would be most simply and conveniently delivered as 100%
oxygen at all altitudes. This, however, has several disadvantages not least of which are those of
cost, weight and bulk; particularly since 100% oxygen is not required physiologically until a cabin
altitude of 34,000 ft is reached. Furthermore, ear discomfort and deafness may develop as a
result of reabsorption of oxygen from the middle ear cavity, frequently some time after landing
(Delayed Otitic Barotrauma or 'Oxygen Ear'). Difficulty in breathing, chest discomfort and cough
may occur after flights in high performance aircraft during which high g manoeuvres have been
performed while breathing 100% oxygen and wearing G-trousers. Also, breathing 100% oxygen
for long periods (12 to 16 hours) so irritates the respiratory tract that chest discomfort may result.
Finally, there is an increased risk of fire if 100% oxygen is used. For the reasons given above
aircraft oxygen systems aim to provide a progressive increase in oxygen concentration (Airmix) in
the inspired gas which is directly proportional to the fall in PO experienced during ascent, and
2
which maintains the lung PO at the ground level equivalent of approximately 100 mm Hg. Fig 1
2
illustrates the concentration of oxygen required to achieve this. In practice, this aim is achieved by
providing an increase in inspired oxygen concentration from ground level, until at about 30,000 ft
most oxygen systems are delivering 100% oxygen. The delivery of 100% at 30,000 ft, rather than
at 33,700 ft as theoretically required, allows a safety margin. l00% oxygen will continue to prevent
hypoxia up to 40,000 ft but above this altitude, pressure breathing is required to provide continued
protection.
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AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
6-12 Fig 1 Relationship between Altitude and the Concentration of Oxygen Required to Maintain
Ground Level Equivalent
Lung P = 100 mm Hg
02
100
80
)
(%
n
60
tio
tra
n
e
c
40
n
o
C
n
e
g
20
y
x
O
0
10
20
30
40
Altitude (ft × 1,000)
b.
Adequate Nitrogen Concentration. Nitrogen must be present in sufficient quantity to
prevent the occurrence of 'Oxygen Ear' or 'Oxygen Lung'. Thus, 40% nitrogen or more is normally
required in a breathing system until altitude/oxygen requirements dictate otherwise.
c.
Adequate Ventilation and Flow. The system must be capable of delivering up to 60 litres
per minute along with instantaneous peak inspiratory flows of 200 litres per minute.
d.
Minimal Resistance to Breathing. Resistance due to valves and turbulent flow throughout
the system, caused by uneven surfaces, branches and changes in internal diameters must be
minimized to prevent disturbances to respiratory rhythm. Ideally, the flow characteristics should
be such as to produce no noticeable resistance to breathing.
e.
Temperature. The inspired gas should be within ±5 °C of cockpit ambient temperature.
f.
Safety Pressure. Inward leaks around the facemask seal or from hose connections must be
countered. This is accomplished by providing a small positive overpressure in the mask to ensure
that any leaks are outward.
g.
Protection against Toxic Fumes and Decompression Sickness. A facility for selecting 100%
oxygen at any time and at any altitude is necessary in the event of toxic fumes appearing in the cabin or
when decompression sickness is liable to develop or has done so (cabin altitudes above 18,000 ft).
h.
Indication of Supply and Flow. Indications of both supply and flow must be available to the
user at all times as a check of correct function.
i.
Evaluation of Integrity. Where possible fail-safe methods of operation should be used (eg
the crew member should be unable to breathe through the mask until it is correctly connected to
the rest of the system) together with the means to check emergency functions (eg manual test of
mask seal and pressure breathing facilities).
j.
Convenience. As much of the system as possible should be automatic, and the drills to
cope with a failure should be simple. Failures must be immediately and clearly indicated.
k.
Duplication. In aircraft with low differential pressure cabins, there should be a back-up
system in the event of main system failure. There is no need for such an Emergency Oxygen
supply in aircraft with high differential cabins where the cabin itself provides the primary protection
against hypoxia and the oxygen equipment is only used if cabin pressurization fails, or toxic fumes
contaminate the cabin.
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AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
l.
Provision for High Altitude Escape. A separate emergency oxygen supply is needed in aircraft
fitted with ejection seats or from which bale-out is possible. This supply, fitted either to the seat or to the
personal parachute pack, is usually the same as the back-up supply referred to at k above.
m.
Independence from Environment. The environment extremes sustained in flight must not
impair the performance of the oxygen equipment. This is particularly so in regard to low
temperatures, accelerations (aircraft manoeuvres and windblast on escape) and atmospheric
pressure changes.
3.
Oxygen systems have been progressively refined over the years. The subject has become
increasingly complex and aircraft specific. Because of this, the following account is necessarily of a
general nature.
4.
In broad terms, any aircraft oxygen system consists of two parts: a supply or store of oxygen and
a means of delivering it to the user (regulator, hose and face mask).
AIRCRAFT OXYGEN SUPPLY/STORAGE
General
5.
Oxygen is most usually obtained from an on-board store which is replenished whilst the aircraft is
on the ground. Some systems however, use the on-board generation of oxygen by molecular sieve
oxygen concentrators (MSOCs). Usually oxygen is stored either as a gas at high pressure or as a
liquid at low temperature.
6.
Whatever the source, the gas supplied to the system must be of a certain high standard. Thus, it
must contain at least 99.5% oxygen, be odourless and virtually free of any toxic substances (eg the
carbon monoxide concentration must be less than 0.002%). The maximum allowable levels for various
hydrocarbons are specified in relation to the type of storage system used since this will influence the
potential contamination hazard. To avoid the risk of ice formation at low temperatures the water
content must not exceed 0.005 mg per litre of oxygen at Standard Temperature and Pressure (STP), ie
0 °C, 760 mm Hg (1013.2 mb).
Gaseous Storage
7.
In gaseous storage systems, the oxygen is held in cylinders mounted outside the pressure cabin.
Commonly used sizes are 750 litre and 2250 litre cylinders at normal ambient temperature and
pressure (NTP), ie 15 °C and 760 mm Hg (1013.2 mb). The cylinders are specially strengthened and
may be wire wound to prevent fragmentation. They are filled to a pressure of 1800 pounds per square
inch (psi); the pressure is stepped down by reducing valves before entering the next part of the system
and there is usually a duplication of pipework and non-return valves as protection against a single leak
emptying the whole system. A typical gaseous storage system is shown at Fig 2.
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AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
6-12 Fig 2 A Typical Gaseous Oxygen Storage System
Filter
Contents Gauge
Non-return
Valves
Cylinder Valve
Pressure Reducing
Valve
Charging
Filter
Valve
High Pressure
Pressure Cabin
Delivery
Oxygen Cylinder
Wall
8.
There are three advantages of such a system. It is relatively simple, oxygen is not lost by venting
when not in use, and it can be used immediately after filling. However, the cylinders are bulky and
heavy and consequently, this system is unsuitable as a primary aircraft oxygen supply when weight and
space are at a premium.
Liquid Storage
9.
The problems of weight and bulk are greatly reduced by storing oxygen as a liquid under low
pressure. Liquid Oxygen (LOX) vaporizes at –183 °C at normal atmospheric pressure, each litre of
liquid yielding 840 litres of gaseous oxygen (NTP). This expansion ratio for LOX is almost seven times
greater than that for gaseous oxygen stored at 1800 psi. Such systems therefore occupy about half
the space and are half as heavy as the high-pressure gaseous systems, as shown in the comparison
at Table 1. Between 3.5 and 25 litres of LOX can be carried depending on aircraft type and crew
requirements.
Table 1 Comparison of Gaseous and Liquid Storage Systems Each Yielding 3000 Litres (NTP)
Oxygen
Storage System
Weight of Charged System (kg)
Space Occupied by System (l)
High
Pressure
Cylinder
19
52
containing gas at 1800 psi
Liquid Oxygen Converter
8
25
containing 3.5 litres
10. The double-walled insulated container - essentially a stainless steel vacuum flask - its control
valves, and connecting pipework are collectively known as a LOX Converter. It is divided into two
parts: one is insulated and contains the liquid; the other is uninsulated and contains the gas. A typical
LOX Converter is pictured at Fig 3. The converter may be permanently mounted in the aircraft or be
removable for rapid replacement.
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AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
6-12 Fig 3 A Typical LOX Converter
11. The converter is charged from a ground LOX dispenser. LOX entering container evaporates and
eventually cools the internal walls to –183 °C. The container then rapidly fills with LOX.
12. When the charging hose is disconnected the top and bottom of the container are connected by a
length of uninsulated pipe which includes a pressure build-up coil and a pressure closing valve. During
the liquid phase, LOX from the bottom of the container vaporizes into the build-up coil and passes as
gas back into the top. The surface of the LOX is warmed by the gas so that its vapour pressure rises.
When the operating pressure of the converter is reached (between 70 and 300 psi) the pressure
closing valve shuts and the flow of LOX into the build-up coil ceases.
13. The heat leak into the container raises the pressure in the converter until the pressure opening
valve operates to allow gas to be fed into the delivery pipe. During this phase (the gas phase), any
demand by the user is met by a flow of gas from the top of the container in preference to a flow from
the liquid phase. A differential check valve allows the passage of liquid only when the pressure in the
delivery line falls below the converter pressure by a pre-determined value.
14. The converter contents are monitored by electrical capacitance and displayed in the cockpit and at
the charging point. The pressure of gaseous oxygen being delivered is also displayed in the cockpit.
15. The LOX system is compact, of low weight and the container will not explode if damaged.
Unfortunately, evaporation and venting losses mean that the converter needs to be recharged at frequent
intervals. In addition, LOX takes a long time to stabilize once in the converter and it may be upset if the
container is agitated, as for example, by aerobatics. For this reason combat aircraft require the addition
of a stabilizing chamber which ensures that on charging the liquid in the container is at a temperature at
which its vapour pressure equals the normal operating pressure. LOX is prone to contamination by toxic
materials and great care must be taken to prevent the build-up of contaminants.
Molecular Sieve Oxygen Concentrators (MSOC)
16. Most of the problems of LOX systems can be overcome by the onboard production of oxygen by the
pressure swing adsorption method, using a molecular sieve. A molecular sieve is a synthetically
produced porous material and if the pores are of a suitable size gas molecules are able to pass through
them. Generally the adsorption of a molecule depends upon its polarity and its size; clearly if a molecule
is larger than the pore size it cannot pass through the sieve. Careful design ensures that if air is passed
Reviewed Nov 15
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AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
through the sieve under pressure most of its nitrogen content will be adsorbed leaving behind an oxygen-
enriched product gas. (Because the adsorption characteristics of argon are similar to those of oxygen the
product gas will comprise 95% oxygen and 5% argon). Over time, the sieve bed becomes saturated with
nitrogen which needs to be purged to prevent it from appearing in the product gas. Removal of nitrogen
from the sieve is achieved by depressurizing the bed to ambient pressure followed by back-purging with a
portion of the product gas, a process known as pressure-swing adsorption.
17.
Functional MSOC System. Fig 4 shows the operation of a simple two-bed MSOC. The flow of
gas into and out of the beds is controlled by valves which are cycled automatically. Each bed in turn is
pressurized with conditioned bleed air from the engine at a pressure of 138 to 414 kPa (20 to 60 psi)
for a product gas flow of 10 to 30 litres (NTP) per minute. Air consumption is typically of the order of
200 to 300 litres (NTP) per minute. As the compressed air flows through one bed, nitrogen is
adsorbed by the sieve material and the product gas, rich in oxygen, flows to the plenum. Before the
pressurized bed becomes saturated with nitrogen, the valves controlling pressure cycling operate to
vent the front end of the bed to ambient. Pressure within the bed falls rapidly and a large purge flow of
gas containing a high concentration of oxygen (from the product gas being produced from the other
bed) is passed back through the bed to enhance nitrogen and contaminant desorption. The
combination of reduced pressure and back-purging results in complete flushing of nitrogen from the
bed so that the sieve material is ready once more to concentrate oxygen during its next pressurization
cycle. The pressure swing cycle can be explained with reference to Fig 4. MSOC bed A is
pressurized, via line 3, and delivers oxygen-rich gas to the plenum. A large bleed flow of product gas
is diverged to purge nitrogen from bed B, via the purge orifice and line 1. Bed B is now pressurized via
line 2 when the control valve rotates. Line 4 then becomes the purge route from bed A via the purge
orifice.
6-12 Fig 4 Simple Two-bed MSOC
Control Valve
1
Molecular Sieve
Bed B
2
Purge Orifice
Conditioned
m
u
Oxygen-enriched
Bleed Air
n
Product Gas
le
3
P
Molecular Sieve
4
Bed A
Nitrogen-rich Exhaust Overboard
18.
Limitations in the Use of MSOCs in Aircraft. Extensive in-flight experience has shown the
MSOC to be a very efficient filter of contaminants, including engine oil and hydraulic fluid molecules
from engine bleed air, as well as vapour, allowing oxygen to be concentrated in suitable quantities.
However, a separate gaseous supply is still required in case of engine failure or crew ejection.
Moreover, not all MSOCs are able to meet the requirement to provide the 100% oxygen needed to
protect against hypoxia following a rapid decompression from a cabin altitude at or above 20,000 ft so
that a backup supply of 100% oxygen must therefore be supplied.
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AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
OXYGEN DELIVERY
General
19. From whichever source the oxygen derives, the easiest way in which it can reach the aircrew is by a
Continuous Flow System. Since the flow does not vary with the demand of the user, such a system tends
to be inefficient and wasteful. However, it is simple and was used to provide the earliest methods of
oxygen delivery - indeed continuous flow systems are still sometimes used to provide a bail-out and
emergency oxygen supply (para 30). Historically, the next development was the use of a reservoir
interposed between the regulating device and the face mask, and designed to prevent too much wastage
of gas. This disadvantage is more thoroughly overcome by Pressure Demand Systems in which the flow
of gas from the regulator varies directly with the inspiratory demand of the user. In addition, the extra
facilities required (Airmix, Safety Pressure, Pressure Breathing etc) can be provided.
20. Most aircraft use pressure demand systems, and the principles underlying the design and function
of pressure demand regulators are essentially the same whether the regulators be panel-mounted,
man-mounted or seat-mounted.
Panel-mounted Pressure Demand Regulators
21. The regulator consists of a demand valve, which incorporates a pressure-reducing valve, a breathing
diaphragm, and a lever mechanism. This is shown diagrammatically at Fig 5. When the user breathes
in, a fall in pressure in the mask is transmitted to the regulator where the reduction is sensed by the
breathing diaphragm. The diaphragm moves inwards and causes the lever mechanism to open the
demand valve. When the user breathes out, pressure builds up in the regulator as oxygen continues to
flow into it but is not demanded, the diaphragm moves back and the demand valve closes. The regulator
also includes refinements in the form of Automatic Functions and Manual Selections.
6-12 Fig 5 Oxygen Pressure Demand Regulator
Oxygen
Source
Airmix
Aneroid
Demand
Valve
Passage
Venturi
Pivot
To User
Diaphragm
Pressure
Pressure
Safety
Safety
Breathing
Breathing
Pressure
Pressure
Aneroid
Spring
Spring
Aneroid
The Automatic Functions are:
a.
Airmix. In order to deliver air which is progressively enriched with oxygen on ascent, a
venturi tube is fitted downstream of the demand valve. Opening into the venturi is a passage
linked to a chamber which incorporates an aneroid capsule and a non-return valve. As oxygen
flows through the venturi at high velocity, a fall in pressure causes cabin air to be sucked through
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AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
the chamber and passage. Air, mixed with oxygen, is thus delivered to the user (Airmix). As
altitude is increased, the aneroid capsule expands, gradually closing off the orifice and so
reducing the amount of air mixing with the oxygen. There is a progressive increase in the
concentration of oxygen reaching the user until, at about 30,000 ft, 100% oxygen passes to the
mask, the orifice being completely shut.
b.
Safety Pressure. At cabin altitudes above 8,000 ft, the risk of hypoxia as a result of inward
leaks in the system (especially with an ill-fitting mask) is prevented by Safety Pressure. This is
produced by applying a spring force of 2 mm Hg to the underside of the breathing diaphragm.
This opens the demand valve until an equal pressure is built up within the system to overcome the
spring. The pressure within the mask is thus kept above ambient throughout inspiration. The
spring is prevented from acting on the breathing diaphragm by an aneroid until the cabin altitude
exceeds safety pressure height, a height which varies from regulator to regulator.
c.
Pressure Breathing. Positive pressure breathing above a cabin altitude of 40,000 ft is
achieved by applying a further spring force to the underside of the breathing diaphragm. It is
prevented from acting below 40,000 ft by a pressure breathing aneroid which encloses the safety
pressure capsule. At 40,000 ft, the pressure breathing aneroid allows further expansion of the
inner aneroid and so a large force is applied to the diaphragm. This force is related to cabin
altitude by further gradual expansion of the pressure breathing aneroid. The regulator will provide
protection to an altitude of 50,000 ft at which time it will be delivering 30 mm Hg positive pressure
to the user.
22. The indications of flow and contents are:
a.
Flow Indication. Tappings taken from both sides of the venturi (upstream and downstream)
allow the variations in pressure to operate a flow indicator.
b.
Contents Indication. A remote oxygen contents gauge is connected to the output line of the
cylinders or LOX container. The gauge is operated by oxygen pressure but is calibrated in
quantities expressed as a fraction of FULL.
23. The switches on the regulator are:
a.
ON/OFF Lever. The ON/OFF lever is normally wire-locked in the 'ON' position.
b.
Normal/100% Lever. The Normal/100% lever allows 100% oxygen to be delivered at all
altitudes by blanking off the air entry port of the Airmix facility.
c.
Emergency/Press to Test Mask Toggle. The Emergency/Press to Test Mask Toggle when
deflected to right or left allows delivery of an additional 4 mm Hg pressure at all altitudes, thus providing
safety pressure (eg when toxic fumes are present in the cabin) or a low-pressure test of the mask seal
(mask toggle up). When pressed in, it delivers oxygen under a pressure of approximately 30 mm Hg
and so provides a high-pressure test of connections and mask seal (mask toggle down). This facility
can also be used in flight to attempt to blow debris off the mask inlet valve.
Man-mounted Pressure Demand Regulators
24. Man-mounted regulators are made possible by the miniaturization of regulator design. These
regulators function on pneumatic principles whereby the link between the demand valve and
diaphragm is pneumatic rather than mechanical.
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AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
25. Man-mounted regulators are mounted on the chest of the life preserver. There are slight
differences between regulator types in the altitudes at which the various functions are available but all
provide automatic airmix, with 100% oxygen supplied above approximately 30,000 ft and with safety
pressure operating above 15,000 ft. Pressure breathing is available to 50,000 ft. Contents and flow
indicators are remotely situated in the cockpit.
26. Man-mounted regulators require an automatic air inlet shut-off (anti-drowning) assembly in case of
immersion after ejection. This is incorporated within the air inlet and shuts automatically under a spring
load should the oxygen supply cease. Inspiration is then only possible through the mask anti-
suffocation valve and requires a greater effort than usual; thus serving to warn the user of failure
should it occur at altitude.
27. Mask seal checks and regulator failure procedures are particular to regulator types and therefore
vary. Mask seal tests are conducted by increasing delivery pressure to the mask. Regulator failure
procedures call for either a change-over to a different (emergency) regulator or selection of a separate
metered continuous flow oxygen supply.
Seat-mounted Pressure Demand Regulators
28. Seat-mounted regulators offer several advantages over regulators mounted at other sites
including:
a.
Less Susceptibility to Damage. Man-mounted regulators are vulnerable to damage during
doffing and donning and during cockpit entry and exit.
b.
Reduction in the Amount of Equipment Carried on the Man. Aircrew assemblies are
bulky and there is less space available on the man than on the seat.
c.
Larger Regulators are Possible. Since more space is available on the seat, miniaturization
is no longer of such importance and more comprehensive protection can be provided against
component failures.
d.
Duplication of Regulators. Duplication of regulators increases the flexibility and operational
capability of the system. Main or emergency oxygen supplies can be used through either of the
regulators.
e.
Fewer Regulators Required. The total number of regulators required for an aircraft fleet is
considerably less than the number required when demand regulators are issued personally to
aircrew.
29. Seat-mounted regulators have the normal 100%/Airmix facility, a press-to-test button for checking
mask fit and the delivery system for leaks, safety pressure above 15,000 ft and pressure breathing to
50,000 ft. There is also a facility for automatic closure of the air inlet in the event of oxygen supply
failure or if the supply pressure drops below a pre-determined level. Contents and flow indications are
placed remotely from the regulator at convenient places in the cockpit.
Emergency Oxygen Systems
30. A supply of emergency oxygen (EO) is available to each crew member should the main supply fail
(the EO is operated manually) or should ejection or bail-out be necessary (the EO is operated
automatically). Two principal forms of EO assembly in current service are briefly described:
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AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
a.
Continuous Flow Emergency Oxygen Assemblies. In Continuous Flow EO Assemblies,
oxygen is stored as a gas in a cylinder mounted on the ejection seat. It is connected to the user
via an oxygen flow regulator mounted on the cylinder head and a soft rubber delivery tube. Once
operated, the oxygen is supplied continuously at a rate of approximately 12 litres (NTP) per
minute initially (thereafter declining exponentially) and provides a useful duration of about 10
minutes. The flow is modified by an Inlet Warning Connector, which is fitted to the end of the
mask hose, and serves to warn the user of a disconnect in the main oxygen supply line.
b.
Demand Emergency Oxygen Assemblies. Oxygen for Demand EO Assemblies is stored
as a gas in cylinders mounted either on the ejection seat or, when for use by aircrew without such
seats, in the parachute pack. It has a contents gauge which is usually connected directly to the
cylinder, but in some cases, it is mounted elsewhere on the seat in a position where it is more
easily seen by the occupant. Once initiated by the release mechanism, the oxygen flows through
a pressure-reducing head on the top of the cylinder and thence, at a nominal pressure of 50 psi to
a regulator. In the case of aircraft which use man-mounted or seat-mounted primary regulators,
the EO passes to the seat portion of the PEC and thence to the primary regulator. This then
controls a flow to the user with its own delivery characteristics, including Safety Pressure and
Pressure Breathing. The duration of demand EO systems depends upon rate of the user; usually
its duration is of the order of 10 minutes.
Walk-around Oxygen Sets
31. Walk-around sets provide a controlled oxygen supply for aircrew whose duties may require them
to move about the aircraft during flight at cabin altitudes above 10,000 ft.
32. The least sophisticated set, the Mk 8, has a 120 litre (NTP) capacity stored at 1,800 psi. This is a
continuous flow set giving 2 litres (NTP) per minute at medium flow and 4 litres (NTP) per minute at
high flow. Medium flow is for use below 18,000 ft.
33. The more sophisticated Mk 4 set has 150 litres (NTP) capacity and a demand regulator. Oxygen
is delivered at 4 mm Hg at 30,000 ft and 11 mm Hg at 42,000 ft. There is also an emergency selection
which provides a flow at 24 mm Hg.
34. The Mk 9 set gives protection to the user in a non-respirable atmosphere. This set will provide
100% oxygen on demand, and also protects against smoke, fumes, and decompression up to an
altitude of 30,000 ft. The mask has a moulded rubber face piece with an inner mask assembly,
perspex visor, a speech transmitter in an expiratory valve and a demand regulator. A good face seal is
essential and this is provided by a cushion filled with a glycerine and water mixture.
Passenger Oxygen Systems
35.
The Ring Main System. In passenger-carrying aircraft, the primary protection against hypoxia is
cabin pressurization. The oxygen systems installed in such aircraft are designed to provide emergency
oxygen for the passengers and crew in the event of pressurization failure, or for therapeutic purposes.
Oxygen for these systems is usually stored as gas although liquid oxygen is used in some aircraft. The
high-pressure supply is reduced by valves in the normal way before passing to a ring main circuit for
passenger supply or to the pressure-demand systems usually fitted on the flight deck for crew use. A
Ring Main system is shown diagrammatically at Fig 6. During normal flight, oxygen is supplied from
the aircraft storage system to the passenger oxygen regulator. In the event of cabin pressurization
failure, and when the cabin altitude exceeds a pre-set level (usually 10,000 to 14,000 ft) the regulator
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AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
automatically raises the supply pressure to approximately 80 psi (Emergency). This increased
pressure activates a warning horn and its delivery to the ring main operates an actuator in each mask
presentation unit, causing the masks to 'drop down' in front of the passengers to a position from which
they can be applied to the face. A continuous flow of oxygen at emergency pressure emanates from
each mask, once its check valve is released, and is maintained as long as the cabin altitude remains
above 17,000 ft. When the aircraft has descended to a cabin altitude of less than 17,000 ft, the control
unit reduces the delivery pressure to Normal. Flow is maintained at a reduced level and each mask
then functions as a demand type.
6-12 Fig 6 Passenger Ring Main System
Presentation
Therapeutic Supply
Control
Ring Main
Control Valve
Supply Pressure
Pressure Gauge
Presentation
Gauge
Stowages
Quick Don
Demand
LOX Converter
Crew Masks
Regulators
Installation
OXYGEN HOSES AND PERSONAL EQUIPMENT CONNECTORS
Routeing of Oxygen Delivery Systems
36. From the oxygen source the delivery pipework is routed, via the regulator where this is panel-
mounted, onto the seat. Here the hoses may be guide-mounted directly onto the seat side, or may
pass to the seat-mounted regulator where fitted, or may plug into the seat portion of a Personal
Equipment Connector (PEC). In the first situation (panel-mounted regulator), the inlet hose then plugs
directly into the mask hose. In the second situation (seat-mounted regulator), the oxygen hose passes
to the mask hose via the man portion of a PEC. In the third situation the man portion of the PEC may
connect directly to the mask hose or, in the case of man-mounted devices, it must first pass through
the regulator to which the mask hose is directly attached. The possible routeings are summarized at
Fig 7.
37. Wide-bore oxygen hoses are only used after the regulator has stepped down the gas delivery
pressure. They are made of extruded liners of natural or vulcanized rubber, reinforced by spirally-
wound galvanized steel wire, and covered with rubberized gauze or stockinette. They are anti-kink and
incorporate various end-connectors to suit different aircraft oxygen systems.
38. The high-pressure hoses (70 psi) used in conjunction with the servo-controlled regulators are
made of narrow-bore anti-kink reinforced rubber.
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AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
6-12 Fig 7 Routes of Delivery Systems
Reg
Non-Ejection Seat
Source
P-M
Mask
Reg
Source
P-M
PEC
Mask
Reg
Ejection Seats
Source
PEC
S-M
PEC
Mask
Reg
Source
PEC
M-M
Mask
P-M = Panel-mounted S-M = Seat-mounted M-M = Man-mounted
Personal Equipment Connector (PEC)
39. A PEC is the usual means by which a user is connected to his services in an ejection seat aircraft.
It is designed to couple and uncouple these services by a single action. In addition to the main oxygen
supply the PEC provides the channel by which the emergency oxygen supply, the G-trousers supply (if
worn), the air ventilated suit supply (if worn), the filtered air supply to the aircrew respirator (if worn) and
the mic-tel are connected. On ejection, all service lines, except the emergency oxygen, are
disconnected and sealed off automatically. A PEC consists of three interlocking main parts: the
aircraft, seat, and man portions.
a.
Aircraft Portion. The aircraft portion is attached to the supply services from the airframe by anti-
kink hose and remains in the aircraft at all times. All services in this portion are provided with valves
which close automatically on disconnection, so preventing wastage of air and oxygen supplies. On
ejection, a short static line unlocks the operating lever to allow the aircraft portion to fall away.
b.
Seat Portion. The seat portion is bolted to the side of the ejection seat. Most services are
provided with inner and outer connecting valves which close when either the aircraft or the man
portion is removed. Mic-tel contacts are set beneath the surface to minimize the risk of damage
to them when the man portion is connected or disconnected. A dust cover is provided to prevent
damage to the valves and contacts when the seat is unoccupied.
c.
Man Portion. The man portion forms part of the Oxygen Mask Hose Assembly which is issued
as flying clothing to the individual. It is connected to the seat portion prior to flight. The G-trouser
and air-ventilated suit connectors are detachable should these services not be required during flight.
Dressing is also facilitated by their detachment. After flight, the man portion is disconnected
manually by use of the operating handle. On ejection, the man portion remains attached to the seat
until man-seat separation when it is unlocked automatically either by the seat mechanics or by
means of a pre-adjusted pull-off lanyard connecting the PEC to the user’s life preserver.
In aircraft, which use panel-mounted or seat-mounted regulators, the oxygen hose connected to the
man portion of the PEC is of wide bore (ie low pressure). In those aircraft which may require the use of
a pressure jerkin, the oxygen hose incorporates a chest connector for attachment to the jerkin. In
aircraft which use man-mounted regulators, the overall dimensions of the PEC are smaller, because of
the cockpit configuration, and high-pressure oxygen hose is used for connection to the regulators. In
addition, the use of high-pressure emergency oxygen in these systems has necessitated a change in
the position of various valves and connections. Service ports not required are blanked off.
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AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
P/Q Series Pressure Demand Masks and Hoses
40. Pressure demand oxygen systems require an oxygen mask which will maintain a face seal under
raised breathing pressures. The P/Q series masks, for use with panel-mounted or seat-mounted
regulators are identical except for size, the latter being smaller. Numerical suffixes (eg P2/Q2) serve to
distinguish masks used with different aircraft systems. The mask consists of a hard fibreglass
exoskeleton containing a soft silicone/non dermatitic rubber moulded face-piece with a reflected edge
which provides the self-sealing property; as pressure builds up in the mask, the seal is pressed harder
onto the face. Moulded into the bridge of the nose is a strip of malleable metal which can be shaped to
improve the fit. A typical oxygen mask of the types P and Q series is illustrated at Fig 8. The mask
incorporates several features:
a.
Chain Toggle Harness and Toggle Lever. A chain type harness is mounted on the front of the
exoskeleton. On each side, it then runs over a shaped metal bow (also mounted on the exoskeleton)
which ensures correct routeing. At each free end the chain has an oval link by which it can be attached
to the aircrew protective helmet, thus securing the mask to the wearer’s face. The chain may be further
tensioned by rotation of the mask toggle lever; under normal conditions, the toggle is said to be 'up'
(wide-ribbed extension uppermost) with the two chains bearing on the arms of the bow. When pressure
breathing is undertaken the wearer rotates the toggle downwards so tightening the chains over the bow
and clamping the mask against the face. It may also be used in this way to enhance the seal if toxic
fumes are present in the cockpit although this is strictly unnecessary provided that safety pressure is
being delivered. Post Mod 171 the chains are replaced by an anti-kinking Mask Quick Release (MQR)
adjustable wire harness.
b.
Inspiratory Valve. An inspiratory valve is mounted in the left-hand side of the mask. It is
made of soft rubber and acts as a simple non-return valve, allowing oxygen to be breathed in but
preventing expired gas from passing back down the oxygen inlet hose. A plastic mesh cover is
fitted over the valve inside the mask as an ice-guard. This prevents any accumulation of moisture
from coming into direct contact with the valve and so any ice formation does not compromise the
function of the valve. In addition, the guard encourages formation of hoar frost, through which it is
still possible to breathe, rather than solid ice.
c.
Expiratory Valve. The expiratory valve is mounted in the base of the mask to allow drainage
of any moisture collecting within the mask cavity. It is protected by a thermal insulating, flexible
rubber outlet snout. The valve plate itself is metal and is held onto a metal seating by a very light
spring which is overcome on expiration. In fact, this spring is too weak to hold the valve shut
against even the small rise in mask cavity pressure generated by safety pressure from the
regulator. It is therefore assisted by a compensating tube which feeds gas pressure from the inlet
port to a diaphragm and piston on the reverse side of the expiratory valve.
Such an arrangement is termed a compensated expiratory valve and it ensures that the valve remains
shut until expiration. However, should pressure in the inlet port be reduced for any reason, the valve in
this configuration would once again tend to open. For this reason, the valve plate above is separated
from the piston below by a second spring: this final arrangement is termed a split compensated
expiratory valve. The system of the valves is shown diagrammatically at Fig 9. Clearly, correct
functioning of a compensated expiratory valve is dependent upon the presence of a functioning
inspiratory valve since if the latter was absent or was to become wedged open by debris from within
the mask cavity, expiratory effort by the user would be transmitted back down the inlet port and along
the compensating tube to the back of the expiratory valve. Thus, the expiratory valve would be held
shut and expiration would be impossible.
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Page 13 of 22
AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
6-12 Fig 8 Typical P/Q Oxygen Mask Assembly
Exoskeleton
Chain Toggle Harness
Anti-Suffocation Aperture
Microphone
Mask Tube
Coupling
Toggle Lever
and Switch
Helmet
Connector
Expiratory
Outlet
Mask Tube
6-12 Fig 9 Valve System of a Pressure Demand Mask
Iceguard
Facepiece of Mask
Inlet
Valve
Valve Seat
Compensation
Tube
Valve Plate
Spring
Inlet Port
Piston
Diaphragm
Outlet Snout
d.
Anti-Suffocation Valve. An anti-suffocation valve is mounted in the right-hand side of those P/Q
series masks which are used with a personal hose assembly incorporating a self-sealing 'prop' valve in
the man portion of the PEC (such masks are distinguished by the additional suffix 'C'). A 'prop' valve is
a device which closes the oxygen entry of the personal hose assembly automatically when the
assembly is detached from the seat. The wearer then breathes air through the anti-suffocation valve.
Closure of the 'prop' valve prevents water entering the breathing hose should ejection be followed by
immersion. The anti-suffocation valve itself is an inward relief valve which opens when the pressure
within the mask cavity falls to 9 to 13 mm Hg below ambient pressure.
e.
Microphone and Microphone Switch. A miniature dynamic microphone and switch
assembly is mounted above the expiratory valve in the front centre of the mask. An electrical cord
assembly is attached to the microphone and connects to a pocket on the left-hand side of the
aircrew helmet.
41. The mask hose is secured at one end to the inlet connector of the mask and has at its distal end
either a Mark 7 Bayonet Connector or an Inlet Warning Connector. It is made of soft corrugated rubber
tubing to allow for maximum movement. Some types are available in both standard and longer length
versions; the latter are distinguished by the suffix 'A'. Additionally, in those aircraft from which high-speed
ejection is a possibility, the hose is strengthened by a straining cord passing through the mask tube from
the bayonet connector to a ring located in the inlet connector (the cord also reduces volume changes
within the hose and hence minimizes pressure swings at the inlet port, which might otherwise cause
difficulty in breathing out). The oxygen mask for these aircraft is further strengthened by replacing the link
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AP3456 - 6-12 - Oxygen and Aircrew Equipment Assemblies
chain with a pin-type chain harness: kinking is prevented by locating a sleeve of rubber tubing over each
chain. Such chains are being superseded by the MQR adjustable wire harness.
V/T Series Pressure Demand Masks and Hoses
42. The V/T series masks are used with miniature man-mounted regulators. They are available in large
and small sizes and are essentially the same in design and construction as the P/Q series illustrated in
Fig 8. Its features are similar to those described for the P/Q series masks except for the following:
a.
Chain Toggle Harness Assembly. Since high-speed ejection is a possibility from aircraft in
which the V mask is worn, the chain is of the bicycle pin-type (Peripin) for increased strength. A
rubber sleeve over the chain prevents kinking.
b.
Mask Quick Release (MQR) Wire Harness. Like the P/Q series, the chain assembly is also
being superseded by a MQR non-kinking, adjustable wire harness designed to withstand high-
speed ejection.
c.
Anti-Suffocation Valve. The anti-suffocation valve in V masks works in conjunction with the
anti-drowning facility incorporated in the man-mounted regulators, allowing the wearer to breathe
when the latter operates (eg on water entry following man-seat separation after ejection, or if
oxygen delivery pressure falls).
43. The V/T series mask hose is designed to attenuate regulator or cabin noise which might otherwise
be transmitted to the microphone. It is made of an inner layer of Terylene fabric and an outer layer of
silicone rubber with a layer of foam between. An integral wire coil supports the hose which is non-
extendible and incompressible. At its distal end a mask hose coupling is attached which is designed to
mate with the outlet of the regulator. Two connections must be made; one is the main breathing supply
and the other is the compensation pressure supply from the reference chamber of the regulator to the
expiratory valve. Located within the main coupling is another smaller coupling from which extends a
pipe connector. A narrow-bore silicone-rubber tube (the Compensation Tube) connects this inner
coupling with the back of the compensated expiratory valve. Thus, compensation of the expiratory
valve is accomplished, via a closed system, by the regulator rather than by direct exposure of the valve
to inlet pressure as in masks of the P/Q series, thereby reducing the risk of pressure-induced
expiratory difficulties.
44. The system employs two separate pneumatic connections between the regulator and the mask
and works well when the distance between the two is relatively short. It is colloquially called the 'Two
Tube' system. It should be noted that, in a type Vl mask, a broken or badly connected compensation
tube (so called 'two tube failure') will only be revealed by correct pre-flight checks. The V2 mask is
identical to the V1 mask except for the following:
a.
An inspiratory valve is not fitted. The presence of a compensation tube renders the need for
a non-return inspiratory valve redundant since expired gas cannot affect the compensation of
the expiratory valve by applying back pressure to it (compare with para 40c). However, this
is only the case as long as the compensating tube is intact. If it is broken or connected
wrongly, then expired gas can be applied through the leak to the back of the expiratory valve,
and so make expiration impossible. Two-tube failure in the V2 mask is therefore instantly
recognized by the user (compare with Two-tube failure in the V1 mask which may go
unnoticed in flight). In fact, the presence of an inspiratory valve in the V1 mask is
unnecessary. Its retention is a legacy of the original high altitude requirement of the mask
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which, when combined with the use of a pressure jerkin, did require such a valve to ensure
that re-breathing could not occur.
b.
A bayonet type mask hose coupling is fitted for connection to the type 417A regulator. It
incorporates a smaller coupling for the compensation tube.
45. The T1 mask is used with the type 417 miniature man-mounted regulator and is worn in
conjunction with a headset. The mask is available in a large and small size, and its design and
function are similar to those of the V2 mask described above. Thus, the normal exoskeleton and face-
piece mount a toggle harness, a compensated expiratory valve, and a microphone assembly. There is
no inspiratory valve and expiratory compensation is from the reference chamber of the regulator via a
two tube mask hose. The chain toggle harness incorporates adjusting nuts which adjust the length
and tension of the harness. Unlike the P, Q, and V masks, the chain assembly is not being replaced
by a MQR assembly. The chains are prevented from twisting whilst being tensioned by swivel links.
Faults and Corrective Drills
46. Malfunctions in the oxygen system are best understood and dealt with in the air by dividing them
into modes of presentation to the user and then providing a table or flow chart detailing the corrective
action to be taken. Such tables or charts form part of the Flight Reference Cards (FRCs) carried by
each crew member.
47. In flight, the precise cause of failure is of much less importance to the user, who may be in
considerable danger, than the need for a rapid and accurate response. Thus, any failure must be
immediately and clearly obvious either as an objective indication in the cockpit or as a subjective effect on
the user. The mode of presentation is then identified in the FRC and the required action taken.
48. Although an FRC drill makes no mention of the causes of faults, or of the reasons for the
indicated actions, these may be worked out from a knowledge of the system. The following in
particular should be noted:
a.
The first priority is to re-establish an oxygen supply (NB a rapid descent to below 10,000 ft is
not the way to combat hypoxia). The card drill always leads to operation of the Emergency
Oxygen (EO) knob if the problem is not resolved very quickly. However, even the EO will be
useless unless the hose connections are correctly made, and hence the instruction to check
connections comes before all else.
b.
Since the EO has a finite duration, the aircraft is committed to a descent to 10,000 ft cabin
altitude or below as soon as possible once the EO system has been operated.
c.
The commonest cause of a persistent black magnetic indicator (no flow) is an electrical
failure of the indicator itself, whilst that of a persistent white indicator is a leak in the system,
usually from around the facemask seal.
d.
A restriction on breathing out is an indication of inspiratory valve malfunctions: the valve is
held open by mask debris so that expired gas pressure acts on the expiratory valve from behind,
via the compensating tube, and prevents it opening.
e.
Selection of 100% oxygen is used as a diagnostic test in that normal breathing thereafter
indicates that the system is functioning, providing that all connections are intact and the mask is
sealed.
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AIRCREW EQUIPMENT ASSEMBLIES
Requirements
49. In general terms, the main aim of any clothing has always been to protect the body from the
unfavourable effects of man’s environment. Furthermore, the working conditions of a particular
occupation have sometimes led to the development of specialized clothing best suited to the rigours of
that occupation and its associated workspace. Flying clothing, or in preferred terms, Aircrew
Equipment Assemblies (AEA), has evolved in just such a way.
50. Any AEA is a collection of specialized items of clothing and equipment integrated into a functional
unit compatible with the aircrew shape and size, the cockpit workspace and the flying task.
51. The purpose of an AEA is to provide the necessary physiological support and protection required
by aircrew to combat the various factors of the aviation environment, and thus allow them to carry out
the flying task. The AEA must also provide aircrew with whatever specialized facilities are needed in
case of in-flight emergency, escape from aircraft in flight, and subsequent survival on land or in the
water. It is essential that these latter requirements of an AEA should not impede the normal flying task
unduly, nor create an unacceptable workload on aircrew. Consequently, any AEA is always a
compromise between that required to sustain normal flight, and that required to give adequate
protection during any emergency situation.
52. When aircrew have been issued with the AEA, the training aspects should not be forgotten.
Aircrew need to know, and to be instructed on, the capabilities and limitations of the various items
comprising an AEA, and to undergo practice sessions using the equipment. The AEA cannot function
properly if it is ill-fitting and not of the correct size. Therefore, it is necessary to ensure that the correct
combination of garments is worn for the aircraft type, role, and area of operation, and that the clothing
and equipment is a good fit and comfortable. Ground support personnel need to ensure that the AEA
is fully maintained in serviceable condition so that it will function as designed when required.
53. Properly authorized items and combinations of aircrew clothing and equipment for each aircraft
type in operation with the RAF and other services (both fixed wing and rotary wing) can be found in the
AEA schedules issued and updated regularly in AP 108B-0001-1.
General Clothing
54. Underwear, socks, shirts, and jersey are provided to be used in the most suitable combination for
the variety of aircraft, types, roles and flying environments.
55. Cotton underwear prevents chafing of the skin by the coarser fabrics of outer layers of clothing
and also 'wicks' away sweat from areas of excessive heat production. Socks are generally of the
Terryloop variety but specialized cold weather and immersion socks are also available. Where a shirt
is required as an extra layer between underwear and coverall, a long-sleeved fine-weave 'T' shirt with
roll neck is provided. If a substantially warmer layer is required, a long sleeved woollen pullover can be
worn in conjunction with the 'T' shirt or any combination of aircrew clothing.
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Anti-g Protection
56. Anti-g trousers are worn by aircrew operating high performance aircraft in order to reduce the
effects of positive accelerations to which they may be exposed by various flight manoeuvres. The
counter pressure applied to the abdomen and lower limbs when the bladders of the anti-g trousers are
inflated on exposure to positive acceleration helps to maintain the blood pressure in the upper part of
the body and to prevent the pooling of blood in the lower extremities. These physiological effects
compliment the various manoeuvres which increase tolerance. The use of anti-g trousers also
reduces the fatigue produced by repeated exposure to high g levels. The bladders of the anti-g
trousers are connected through a flexible hose and connector system to the outlet of the anti-g valve.
The anti-g valve automatically inflates and deflates the bladders with air or oxygen to the appropriate
pressure when positive accelerations are applied to the aircraft.
57. Anti-g trousers are provided for internal or external wear because internal wear trousers impose a
heat load which has proved to cause discomfort to some wearers especially during 'stand by' in hot
conditions. External anti-g trousers are worn outside all other clothing when summer aircrew equipment
assemblies are worn, and can be donned immediately before take-off and doffed immediately after
landing, thereby relieving the wearer of an unnecessary encumbrance when not flying.
Coveralls
58.
Coveralls, Aircrew, Mks 14 and 15. The Coverall Mk 14 is a slim fitting garment which can be
used in summer and winter. The flame resistant properties of the Nomex material used in its
manufacture are of advantage to aircrew. The Mk 14A is designated for aircrew who do not wear anti-
g trousers, and has greater leg girth and reinforced stitching to the pockets. The Mk 14B has no thigh
pockets as it is used in conjunction with external anti-g trousers. The Coverall Mk 15 is also made
from Nomex material, but is larger in girth than the Mk 14A/B, to enable it to be fitted over an inner
immersion coverall. It has pockets for equipment and personal items on the upper torso, thighs, lower
legs, and upper arms, as determined by the relevant design standard.
59.
Cold Weather Flying Suit Mk 3. The Cold Weather Flying Suit Mk 3 is a two-piece garment
designed to give protection to aircrew under medium to severe cold weather conditions. It is suitable to
use in winter land AEA combinations only and should not be used when extensive sorties over water
are undertaken. The suit comprises separate jacket and trousers made of a showerproofed gaberdine
outer and ventile inner lining. Both garments are interlined throughout with nylon mesh. The front of
the jacket is closed by a sliding fastener which, when closed, can be covered by a button-over flap.
There are two breast pockets. A large 'let-down' flap is located on the inside of the jacket. The flap,
for use under survival conditions is worn outside the trousers to give additional protection to the lumbar
and seat areas. Provision is made, inside the collar, for the stowage of a scarf which is intended for
use under survival conditions only. At the base of the collar a sliding fastener gives access to the
protective hood which, when worn (under survival conditions only), is secured across the front of the
neck by buttoned tabs. A draw cord arrangement allows the hood to be fitted close around the face if
necessary. The trousers are constructed of similar materials to those of the jacket. To facilitate
donning the lower ends of the trouser legs are gusseted and fitted with sliding fasteners.
60.
Combat Flying Suit Mk 2A. The Combat Flying Suit Mk 2A is a five-piece garment designed to
give protection to aircrew under temperate climatic conditions, and is particularly suited to 'off-base'
operations for both fixed wing and rotary wing aircraft. The suit consists of jacket, trousers, waistcoat,
rainproof jacket, and trousers. The jacket is made from a disruptive pattern gaberdine material which
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is lined only across the shoulders, upper chest and down the sleeves. The jacket is closed by a central
open-ended sliding fastener which, when closed, can be covered by a button-over flap. There are two
breast pockets and two waist pockets. A large Velcro-closed flap is located inside the lower back part
of the jacket for use under survival conditions. At the base of the collar a sliding fastener gives access
to the protective hood which when worn (survival conditions only), is secured by button down tabs and
a draw cord. The trousers are of the same material as the jacket and are loose lined from waist to mid
calf level. The lower ends of the trouser legs are gusseted and fitted with sliding fasteners. The
waistcoat is a sleeveless quilted garment closed at the front by three buttons. It is intended to be worn
under the jacket if extra thermal insulation is needed. The rainproof jacket and trousers are intended
for use on the ground only or in survival conditions.
61.
Coverall, Immersion, Mk 10/10A. The Immersion Coverall, Mk 10/10A has been designed to
provide aircrew with part of the protection needed to combat the effects of immersion in cold water
whilst at the same time minimizing the thermal stress involved in wearing a bulky garment under
normal conditions. Full protection against hypothermia can only be provided by thermal insulative
clothing worn beneath the immersion coverall since the insulation afforded by the coverall itself is low.
The principle function of the immersion coverall is to preserve the insulation afforded by the clothing
worn underneath by keeping these garments dry in the event of immersion in water. A survivor,
wearing only normal clothing and immersed in water at 5 °C for approximately 30 minutes, would have
only a 50% chance of surviving. Water temperatures around the coasts of the UK range from 5 °C in
the winter to 15 °C in the summer. For flights over the sea when water temperatures are at or below
10 °C, aircrew should wear 'winter' combination AEA comprising the Mk 10/10A coverall and the
Coverall, Inner, Knitted, Mk 1 (see para 64). The Mk 10/10A coverall is a one-piece garment
constructed from two layers of ventile fabric comprising a thick outer layer and a thinner lining. When
dry the fabric is permeable to water vapour and therefore aids body comfort. The fabric becomes
waterproof when wet. Butyl rubber waterproof seals which fit firmly against the wearer’s skin are
provided at the wrists and at the neck. The seals may be trimmed to fit the individual wearer. The
coverall is supplied with the trousers legs open so that the correct size of waterproof immersion sock
may be attached. The usual range of pockets is provided. The Mk 10A varies in having a blast
resistant collar and other changes suited to specific aircraft types.
62.
Coverall, Aircrew, Immersion, Inner, Mk 1. The Immersion Coverall, Inner, Mk 1 is a one-piece
garment which is designed to be worn under the Mk 15 aircrew coverall. It is fully cut and shaped at
the knees, seat, and arms. It is made from cotton ventile fabric which has the ability to allow body
vapours to permeate through the suit under normal conditions. Upon immersion in water, the fibres
expand to close the fabric pores and the fabric becomes waterproof.
63.
Coverall, Immersion, Winchman, Mk 2. The helicopter winchman’s Immersion Coverall, Mk 2 is
similar in principle and design to the Immersion Coverall, Mk 10. The suit is a one-piece garment
made from heavy-duty nylon/terylene fabric proofed with neoprene. It is traffic yellow in colour. The
front entry sliding fastener, wrist, and neck seals are similar to those fitted to the Immersion Coverall,
Mk 10. The garment is intended for use with the aircrew rubber Immersion Boot, Mk 3/4, the correct
size of boot being fitted for the individual wearer. There is a pencil pocket on the upper left sleeve and
envelope pockets attached to each lower leg.
64.
Coverall, Inner, Knitted, Mk 1. The Coverall, Inner, Knitted, Mk 1 is a one-piece garment knitted
from 100% wool, worn under an immersion coverall, to provide the necessary thermal insulative layer
in event of cold sea immersion.
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65.
Quick-don Immersion Coveralls. In some aircraft which operate regularly over the sea, it may be
impractical for the crew or passengers to wear the normal type of immersion coverall - they require a
garment which gives an adequate degree of protection and can be donned quickly in an emergency. It
should be easy to don by individuals who are unfamiliar with it or may be suffering from minor injuries.
a.
Coverall, Aircrew, Immersion, Quick-don, Mk 1. The requirements above led to the adoption
by the RAF and RN of the Mk 1, Quick-don, Immersion Coverall. This coverall is a simple, red
coloured, one-piece garment constructed with an integral hood and overboots. It is of a universal sizing
and stowed in a valise. It is recommended that aircrew adopt and practise a donning method suitable
to their crew station and having due regard for the conditions likely to prevail in an emergency. Should
circumstances dictate that passengers use this coverall, they should, if possible, be supervised and
assisted during donning. This garment is being replaced by the Coverall, Passenger, Immersion, Mk 1
(see next sub-para), which will be used by aircrew and passengers alike.
b.
Coverall, Passenger, Immersion, Mk 1. The Coverall, Passenger, Immersion, Mk 1 is
designed to meet the requirement for easy donning by wearers unused to complex aircrew
equipment. The coverall is a simple dayglow coloured, one-piece garment constructed with an
integral hood, overboots and protective mitts (see Fig 10). Rubber seals are fitted at the neck and
wrist apertures. It is available in small, medium, and large sizes. The large size is also available
as a Mk 1G in NATO Green. The coverall is stored in a valise in the aircraft.
6-12 Fig 10 Coverall, Passenger, Immersion, Mk 1
Aircrew Body Armour
66. Body armour is provided to protect aircrew members of helicopters and other slow, low flying
aircraft operating in forward combat areas. There are two types of armour in use; contoured front and
back panels of specially processed fibreglass, and a torso plate made from a sandwich construction of
aluminium oxide tiles mounted on a backing of glass fibre reinforced plastic. The fibrous nature of the
materials used allows fragments to be absorbed and reduces ricochets. The torso plate is held in
position by a support jerkin.
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Pressure Garments
67.
Pressure Jerkin Mk 6. Above 40,000 ft, pressure breathing with 100% oxygen is required to prevent
hypoxia. The magnitude of the pressure breathing required above 50,000 ft is such that counter pressure
must be applied to the trunk and lower limbs. The Pressure Jerkin Mk 6 is a sleeveless garment which
covers the trunk and upper thighs. It has an internal bladder which, when inflated by oxygen, provides this
necessary counter pressure (the anti-g trousers can be used to apply the counter pressure to the lower limbs
during pressure breathing). The jerkin connector contains a valve which isolates the jerkin from the main
breathing line during normal breathing and at all altitudes where pressure breathing is not required. The
valve opens quickly and fully during pressure breathing to allow rapid inflation of the jerkin.
Aircrew Lifepreservers
68. The main purpose of a lifepreserver is to provide sufficient additional buoyancy so distributed that
the survivor will achieve a satisfactory flotation attitude with the airway clear of the water under all
circumstances ie landing face down in the water irrespective of the clothing assembly worn, and in the
event of injury. The buoyancy stole of the current lifepreservers is constructed of a strong butyl fabric
bladder inflated by a carbon dioxide cylinder and operating head. The assembly is arranged as a
horseshoe collar and attached to a waistcoat which also contains pockets for the stowage of survival
and location aids and lifting beckets for the attachment of a Grabbit hook.
69.
Lifepreserver Design Requirements. A disadvantage of using carbon dioxide for filling the stole
is that the rate of inflation is markedly slowed at low temperatures as a proportion of the gas
condenses as snow and only slowly re-evaporates and fills the stole. It can take 30 to 60 seconds to
inflate the stole at a sea temperature of 5 °C. The ideal flotation attitude is only achieved when
wearing lightweight minimum bulk clothing assemblies. Some assemblies ie inner coverall and
immersion coverall, trap air so that inherent buoyancy leads to adverse flotation attitudes with minimal
self-righting. It is therefore important that aircrew should take positive action to expel the trapped air
from within the immersion coverall as soon as possible after water entry. All lifepreservers are fitted
with a Personal Locator Beacon and a selection of other survival and location aids depending on
aircraft role and the amount of space available. Lifepreservers are designed to be suitable for
particular types of aircraft, and although there are numerous small differences across the range of
lifepreservers they all perform the same task and are all of the same basic design.
Helmets
70.
The Mechanisms of Head Injury. In general terms, the mechanisms of head injury can be
summarized as being due to:
a.
Direct impact (soft tissue and bony injury).
b.
Linear acceleration (concussion).
c.
Angular acceleration (concussion).
In the absence of head impact, forces transmitted through the neck may cause fractures to the base of
the skull, or concussion by initiating high angular accelerations of the head (see Volume 6, Chapter 14).
71.
Protective Helmets. Ideally, protection against these effects can be afforded by the provision of a
hard, rigid shell around the head to minimize direct impact damage, and a means of increasing the
distance through which the head travels after impact before stopping, thereby reducing the accelerative
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forces involved. RAF aircrew protective helmets employ a frangible fibreglass shell which breaks up on
impact, dissipating some of the energy. The impact load is transmitted to the head and distributed over a
wide area by means of a webbing suspension harness which provides an initial air gap of about one inch
to maximize the stopping distance. Energy is absorbed by the shell inelastically each time a glass fibre
ruptures, or is pulled out of the resin matrix. Peripherally, energy is absorbed by crushable foams. Use of
this technique implies a compromise with the requirement for a strong rigid shell.
72.
Helmet Functional Requirements. The aircrew helmet must also serve several secondary
functions:
a.
Intercommunication facility.
b.
Noise attenuation.
c.
Oxygen mask suspension mechanism.
d.
Eye protection against birdstrike, solar glare, and air blast.
e.
Mounting platform for vision enhancement devices.
The end design of a protective helmet for aircrew use must inevitably be a compromise between the
extent of the protection provided against impact, the overall weight (to allow good head and neck
mobility), size (not too unwieldy) and noise attenuation. With the development of aircraft able to
perform repeated high-g air combat manoeuvres, has come the requirement to reduce the weight of
the standard RAF aircrew helmet/mask combination.
Boots
73. The standard pattern flying boot provides aircrew with a rugged item of footwear suitable for use in
flight and in any survival situation. The boot consists of black leather uppers which are lined and
bonded to a tough composite sole. The uppers are extended high on the ankle and are fitted with a
foam-padded rim for comfort. Fastening is by eyelet and laces. The underside of the sole is moulded
into a non-skid, anti-FOD pattern. The leather is proofed to provide the maximum degree of protection
in a land survival situation. A lightweight version is also available.
Gloves
74. The general-purpose aircrew glove is the cape leather glove. The glove is constructed from
strong, supple close fitting leather which provides protection against abrasion and from fire without
detriment to tactility and dexterity. There is a water resistant version of similar construction but having
slightly thicker leather and a coating of waterproof solution on the inside. Wrist seals are provided to
complete the waterproof integrity.
75. Helicopter winchmen are provided with gloves constructed of stout rough leather with metallic
reinforcement to the index finger and thumb in order to withstand chafing from the moving wire strop of
the helicopter winch.
Summary
76. Although individual items have been described in this chapter it should be remembered that each
AEA is designed as a carefully integrated functional system which ensures that the appropriate degree
of protection is afforded to aircrew and is, at the same time, fully compatible with the aircrewman’s
ability to perform the flying task with a minimum of restriction. Aircrew should always wear the
recommended AEA as defined by the relevant operating authority.
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CHAPTER 13 - AIRCREW HEALTH
Introduction
1.
This chapter will briefly cover the function of the RAF Medical Services and also give some
general advice on health care.
The RAF Medical Branch
2.
The RAF Medical Branch was originally formed in the early days of aviation because of medical
problems which were encountered as a result of flying. It was decided that these problems could best be
tackled by doctors who were part of the same organisation as the aircrew and whose first duty would be to
look after the health and effectiveness of flying personnel. Today, the Medical Officer’s (MO) first duty
remains the medical care of aircrew. Over the years the MOs role has expanded to not only treat the
problems encountered as a result of flying, but also to ensure that aircrew are medically fit to be able to fly
safely.
Self-medication
3.
In general, aircrew should not take any pills or potions from chemists, supermarkets, herbalists,
etc. The main reasons for this are:
a.
If aircrew feel sufficiently unwell to want to take a drug of any kind, they should almost
certainly not be flying.
b.
Many drugs are dangerous to take when flying; they can impair performance and increase
susceptibility to both hypoxia and disorientation. Particular culprits in this respect are headache
remedies, cold 'cures', drugs for hay fever, and drugs for motion sickness.
Medical Care from Civilian Doctors
4.
All RAF Medical Officers undertake training and qualify as a Military Aircrew Medical Examiner. It
is unreasonable to expect civilian doctors to be aware of the special factors which have to be taken
into account when treating aircrew. Therefore, if civilian doctors have to be consulted, for whatever
reason, they must be made aware that the patient is military aircrew. Furthermore, the RAF Medical
Officer must be informed of any such treatment, particularly if medication was prescribed.
Annual Medicals
5.
Aircrew are required to undergo periodic medical examinations throughout their careers to ensure
that they are fit to maintain their flying category. These medical examinations occur annually in the
subject’s birth month and will involve blood tests on the following occasions:
a.
On entry.
b.
Age 25 and 30.
c.
Two-yearly from age 32 to age 40.
d.
Annually, after the age of 40.
The annual medical is an exercise in preventative medicine, giving the Medical Officer a chance to
pick up potential problems at an early stage, when they can often be easily resolved. It also gives
individuals a chance to raise any medical topics which may concern them.
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Exercise and Physical Fitness
6.
There are two main reasons for getting and staying physically fit. The first reason is fitness for
the job. A physically fit person will be less prone to many of the hazards of flying. Remember that,
when in a survival situation, the living and the dead can be separated, not only by their knowledge or
skills, but also by their physical fitness. The second reason for being fit is that it produces a benefit in
terms of general health and well-being. A fit body is an efficient body, and a fit person uses less
energy to perform the same job than an unfit one. Fit people, therefore, have more energy left over to
enjoy recreational pursuits, feel less tired at the end of the day and can lead much fuller lives.
Cardiovascular disease is much less common in people who take regular exercise. Doctors are often
asked what is the best way of keeping fit. Unfortunately, there is no easy or quick way; the only way is
to take regular exercise. It does not matter what form the exercise takes, as long as it causes a
moderate rise in the pulse and current recommendations are for 30 minutes, five times a week.
Cardiovascular Disease
7.
In England and Wales, cardiovascular disease (CVD) is one of the biggest causes of death and
disability, for both men and women, accounting for over 150,000 deaths annually. Sitting under the
CVD umbrella is Heart Disease which usually takes the form of deposits on the walls of the arteries,
which supply oxygenated blood to the heart muscle. These deposits increase with age in most people
in Western Europe and North America, and eventually result in inadequate oxygen supplies to the
heart muscle. The effects resulting from this form of heart disease may include limitation of activity
due to chest pain on exertion, and death due to 'heart attack'. Doctors are often asked about how to
reduce the risk of suffering from CVD. There are a number of risk factors that are recognised as being
important and these are:
a.
Family History. The fact that CVD runs in some families is well known. Unfortunately, this
is as a result of our genetic make-up and nothing can currently be done to alter this risk factor.
However, the altered metabolic activity which leads to disease symptoms can often be treated,
lessening the likelihood of having serious symptoms.
b.
Smoking. The death rate from CVD in smokers is roughly double that of non-smokers.
Smoking is also a potent risk factor for other diseases such as lung cancer, bronchitis and
emphysema, stroke, high blood pressure, peptic ulcers and stomach and bladder cancers. On
stopping smoking, however, the risk of developing these diseases gradually reduces to almost
the same level as in people who have never smoked.
c.
High Blood Pressure. Studies have shown that even mildly elevated blood pressure is a
risk factor for heart disease. It is, therefore, vitally important that blood pressure is measured
regularly.
d.
High Cholesterol. Population studies have shown a good correlation between average
blood cholesterol and the incidence of heart disease in Western Europe and North America.
Lowering blood cholesterol by diet and/or drugs can result in significant reduction in the chances
of an individual developing symptomatic CVD.
e.
Obesity. Obesity in itself is a highly significant risk factor. It is almost invariably associated
with raised blood fats, and other risk factors such as high blood pressure, which greatly increase
the risk. Obesity is also highly correlated with the development of adult onset diabetes Type II;
this is another major risk factor for CVD.
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f.
Diet. There is suggestive evidence that a diet with a higher proportion of polyunsaturated
fats than saturated fats may reduce risk of CVD. The aim should be to reduce the proportion of
total calorific intake derived from fat of whatever source.
8.
The risk factors discussed in the previous paragraph were determined from population studies
and should not be rigidly applied to individuals. However, they are cumulative, and it is important to
work at reducing those that it is possible to alter. Measures which constitute a healthy lifestyle can be
taken to reduce the risk of CVD include:
a.
Body weight control.
b.
A sensible diet, low in saturated fats (avoid animal fat).
c.
Regular exercise.
d.
Stop smoking.
RAF annual medicals include both blood pressure measurements and periodic blood cholesterol tests.
Diabetes
9.
Diabetes is a condition where the amount of glucose in your blood is too high because the body
cannot break it down for use as fuel. The hormone called insulin is responsible for breaking down
glucose (sugar) and insulin is produced by the pancreas. Diabetes develops when the pancreas does
not produce any insulin at all (Type 1) or when the insulin produced does not work properly (Type 2).
The result is a build up of sugar in the blood stream which damages the lining of blood vessels in the
brain, heart, eyes, kidneys and other organs. Diabetes can therefore lead to a stroke, heart attack,
blindness and kidney failure. There are two main types of diabetes:
a.
Type 1: This happens when there is no insulin to break down sugar. It typically occurs in
younger persons. The risk of developing this type of diabetes is usually related to one’s genes
and ethnicity, which are factors that cannot be controlled. Type 1 diabetics will usually require
insulin injections.
b.
Type 2: This happens when the insulin produced by the pancreas does not work properly. It
typically occurs in persons over the age of 40. The risk of developing this type of diabetes
increases with a lack of regular exercise, weight gain and a poor diet amongst other factors such
as genetics. Up to 80 per cent of cases of Type 2 diabetes can be delayed or prevented by
making simple changes in one’s lifestyle including adopting a healthy diet, undertaking regular
exercise and controlling one’s weight. Medication may become necessary if lifestyle changes fail
to take effect.
Alcohol
10. Alcohol has been used and abused by people since time immemorial. It is a central nervous
system depressant and produces its pleasurable effects by interfering with some of the inhibitory
mechanisms in the brain, as well as reducing feelings of anxiety. In sensible amounts, it probably
does more good than harm. Chronic alcohol abuse, however, is a major cause of death and disability
in the UK. Even small amounts have been shown to impair judgement and increase reaction time and
make aviators more prone to spatial disorientation. How much alcohol is safe? It must be
remembered that different individuals react differently to drinking alcohol. The actual rate of uptake
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AP3456 - 6-13 - Aircrew Health
and elimination by an individual will depend on many factors, for example, the proportion of fat, body
size and gender. The figures given in the following sub-paras give some guidance with regard to
alcohol uptake and elimination from the body.
a.
The current recommended safe alcohol consumption levels are 21 units per week for males,
and 14 units per week for females. Where one unit of alcohol equals 10 ml of ethanol, the alcohol
content of a drink can vary significantly, particularly in the case of beer.
b.
One unit of alcohol raises the blood alcohol concentration by approximately 15 mg per
100 ml. Six units therefore raises blood alcohol level by 90 mg per 100 ml, which is over the legal
limit for driving. This does not imply that by drinking fewer than 6 units of alcohol that an
individual will be under the legal limit for driving. As has been emphasised above, different
individuals react differently to alcohol and some individuals will be above the drink drive limit
having ingested considerably fewer than 6 units. Current advice is that no alcohol should be
taken before driving.
c.
Blood alcohol concentration can fall at a rate of approximately 10 mg per 100 ml per hour,
and therefore, it may take up to nine hours to eliminate six units of alcohol from the body. Again,
these figures will vary with the individual.
11. A new law, introduced on 1 Nov 2013, permits the power to test for alcohol and drugs, when a
commanding officer of a person subject to service law has reasonable cause to believe that that
person’s ability to perform safety-critical duty is impaired because of alcohol or drugs. Full details can
be found in 2013 DIN 01-212 and further guidance in chapter 6 of JSP 835 (Alcohol and Substance
Misuse and Testing). A safety-critical duty is statutorily defined as one where the performance of duty
while impaired, through drugs or alcohol, would result in a risk of death, serious injury, serious
damage to property or serious environmental harm. Guidance on what is considered to be a safety-
critical duty is contained within the DIN and JSP. Within the RAF the most notable prescribed duties
are aircrew, Remotely Piloted Air Systems (RPAS) operators, air traffic controllers (this includes any
person controlling the direction of flight of an aircraft such as aerospace battle managers and forward
air controllers), aircraft maintenance technicians and their supervisors, flight authorising officers, live
armed personnel and drivers. This list is not exhaustive and there is provision for a CO to designate a
duty as safety-critical as described in the DIN.
Alcohol Limits for Safety-critical Duties
12. The alcohol limits for prescribed safety-critical duties have been set at two levels; Higher and
Lower alcohol levels.
a.
Higher Alcohol Levels.
The majority of safety-critical duties fall into the higher alcohol
limit for testing of breath, blood and urine. The higher limits are:
Blood
-
80 mgs of alcohol in 100 mls (England & Wales)
50 mgs of alcohol in 100 mls (Scotland)
b.
Lower Alcohol Levels. Some safety-critical duties require a heightened speed of reaction in
an emergency situation and therefore are subject to a lower alcohol limit. The lower limits are:
Blood
-
20 mgs of alcohol in 100 mls
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AP3456 - 6-13 - Aircrew Health
The higher level is the same as the current UK road drink/drive limit. The lower level is the same as
that stated in the Railways and Transport Safety Act 2003, which has applied to civilian pilots (and
others performing an ‘aviation function’) in the UK for a number of years. All personnel involved in
safety-critical duties, including supporting flying operations, should ensure that they are not suffering
the effects or after effects of alcohol when reporting for duty. Current advice is that personnel should
not consume alcohol within 24 hours of their flying duties.
HIV, AIDS and other Sexually Transmitted Diseases
13. In 1981, in the USA, there was an outbreak of a rare type of pneumonia in apparently healthy
homosexual men. Investigation of this outbreak lead to the recognition of the Acquired Immune
Deficiency Syndrome (AIDS), an apparently new disease. Two years later, the infectious agent
causing this disease was identified as a previously unknown virus, which was subsequently named the
Human Immuno-Deficiency Virus (HIV). Infection with HIV leads to damage of the immune system,
rendering the individual susceptible to a wide variety of infections. It can also lead to the development
of various cancers. The HIV virus also attacks cells in the central nervous system causing dementia.
The most common way of getting HIV in the UK is by anal or vaginal sex without a condom. 95% of
those diagnosed with HIV in the UK in 2013 acquired HIV as a result of sexual contact. HIV may also
be acquired by inoculation via a contaminated needle, injecting instrument, unscreened blood or blood
products through direct exposure of mucous membranes or an open wound to infected bodily fluids; or
by a human bite that breaks the skin. There is a risk of transmission from mother to baby during
pregnancy, birth or breastfeeding.
The initial infection with HIV is usually symptomless and is followed by an incubation period during
which the patient appears normal. The incubation period is very variable but can range from 5 to 7
years. The diagnosis of HIV infection is confirmed by a blood test, which detects antibodies to the
virus. There is no cure for this disease and no vaccine available to provide protection from infection. .
Prevention of transmission of HIV infection, as well as several other sexually transmitted diseases,
depends entirely on those at risk modifying their behaviour. The adoption of safe sex practices is very
much an individual matter. Discussion of specific measures is not appropriate in this document;
information is available from a variety of sources (including the internet), but these matters should
ideally be discussed with a doctor, or other medical professionals.
14. The RAF’s policy towards AIDS is briefly explained by the following points, which should answer
most questions:
a.
HIV infection may be compatible with Service employment.
b.
HIV infection may be compatible with flying duties in a restricted capacity, due to the effects
of the virus on the brain.
c.
The RAF does not currently test for HIV routinely. No HIV testing is carried out on blood
tests done at annual aircrew medicals.
15. In recent years, there has been a significant increase in other sexually transmitted diseases,
specifically gonorrhoea and syphilis. The important point is that safe sex practices can protect a
person from a multiplicity of sexually transmitted diseases.
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AP3456 - 6-13 - Aircrew Health
Travel Advice
16. Aircrew can expect to travel widely during their careers. Fast jet aircrew are now deployed to
many corners of the globe, while transport crews regularly route through remote areas. In addition,
leisure travel to exotic locations is now easily available and more affordable.
17. Some countries, realising the economic importance of travel, may devote more importance to the
Ministry of Tourism than to the Ministry of Health and not spend adequate money on public health
measures. It is, therefore, imperative to be aware of the measures that can be taken to reduce the
risk of contracting disease while abroad.
18.
General Advice. Only 5% of travel illness can be prevented by immunisation. However, many
problems can be avoided by observing a strict personal hygiene routine and taking a few basic
precautions. These measures should be observed in
all parts of the world. They are:
a.
Never drink tap water unless it is declared fit for drinking by a reliable authority, preferably
military Public Health representatives. Remember that even cleaning teeth in contaminated water
can be enough to cause illness.
b.
Avoid ice in drinks, unless you are certain that the ice is made from treated water.
c.
Peel all fruit and vegetables. The skin may have been contaminated by someone with a
communicable disease. In addition, the use of human fertilizer is widespread, and contributes to
the spread of infectious diseases.
d.
Avoid salad stuffs and other raw foods which may have been washed in contaminated water.
A major outbreak of food poisoning occurred in 1992 after the passengers on a 216 Sqn Tristar,
en-route to the Falkland Islands, had to divert to Dakar where they ate salad which had been
washed in contaminated water. Nearly 150 people suffered a particularly vicious episode of
prolonged diarrhoea.
e.
Avoid dehydration. Increase fluid intake considerably in hot climates. Remember that thirst
is not an adequate indicator of hydration. A light-yellow urine colour, as opposed to dark yellow
or orange, is more reliable.
f.
Beware of the sun. Remember sunburn can be a debilitating illness, and that heat stroke
can be fatal.
g.
Wear appropriate clothing. Long-sleeved shirts and long trousers are a must in malaria
zones. Exposed skin should be protected with insect repellents; the head and neck must be
protected from direct exposure to the sun.
h.
When planning a journey, account must be taken of the entire trip, not just the final
destination. A stopover may occur in an area where certain diseases are prevalent and
appropriate precautions therefore need to be addressed.
i.
Any illness developing on return from foreign travel must be reported to the MO, with full
details of locations and dates. The onset of malaria, for example, may take place up to a year
after exposure.
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AP3456 - 6-13 - Aircrew Health
Common Travel-acquired Diseases
19.
Malaria. The WHO estimates that in 2010 there were 219 million cases of malaria resulting in
660,000 deaths. Malaria is presently endemic in a broad band around the equator, in areas of the
Americas, many parts of Asia, and much of Africa; in Sub-Saharan Africa, 85–90% of malaria fatalities
occur. Every year, 125 million international travelers visit these countries, and more than 30,000
contract the disease. Deaths in Britain from the disease average seven to ten per year, with over 1000
cases being reported. As resistance to chemoprophylactic drugs increases, simple measures to avoid
being bitten by the carrier mosquitoes take on added importance.
a.
Measures to reduce the risk of contracting malaria are:
(1) Be aware of the risk. Find out if the countries to be travelled to, or through, are
malarious areas. Local medical authorities should be able to provide this information. If not,
Service infectious disease consultants are available to advise them.
(2) Sleep in properly screened rooms and use a knockdown insecticide spray to kill any
mosquitoes that may have entered the room during the day.
(3) Use mosquito nets round the bed at night, checking that there are no holes and tucking
the edges under the mattress before nightfall; protection may be enhanced by impregnating
the netting with an insecticide such as DEET every 6 months.
(4) Use an electric mat to vaporise insecticide overnight or burn mosquito coils.
(5) Wear long-sleeved clothing and long trousers when out of doors after sunset.
(6) Use insect repellent on exposed skin and spray it onto garments.
b.
Drug prophylaxis against malaria is very important. The drugs to be taken vary according to
the area of the world concerned, as well as the mode of travel. Some prophylactic medications
prescribed for passengers are totally inappropriate for aircrew because of unwanted side effects;
therefore, military medical authorities should always be part of deployment planning. Most
medications must be started before entering the malaria’s area and need to be continued for four
weeks after return. Failure to adhere to this is one of the main reasons why travellers contract
the disease; it is vital that all travellers comply with the instructions given.
20.
Typhoid. Typhoid fever is acquired mainly through food or drink that has been contaminated with the
excreta of a human case or carrier. It is, therefore, predominantly a disease of countries with poor
sanitation and poor standards of personal and food hygiene. Outbreaks of infection have been caused by
corned beef (Aberdeen 1964), water supplies (Zermatt 1963), and shellfish contaminated by infected water
or sewage. Over 80% of the infections reported in the British Isles have been acquired abroad, principally
in the Indian sub-continent. Typhoid can be prevented by observing good personal hygiene practices and
by adhering to the general principles of eating and drinking given earlier. Typhoid vaccine is available, both
in injectable and oral form, a course giving protection for three years.
21.
Yellow Fever. Yellow fever is an acute viral infection occurring in tropical Africa and South
America. During epidemics, the fatality rate can reach 50%. The disease is spread from infected to
susceptible persons by the bite of the 'Aedes Aegypti' mosquito, a mosquito which lives and breeds in
close association with man. Immunisation against yellow fever, documented by a valid International
Certificate of Vaccination, is compulsory for entry into some countries. Requirements must be
checked before travel. Yellow fever can be prevented by the administration of a vaccine that confers
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AP3456 - 6-13 - Aircrew Health
immunity in nearly 100% of recipients; immunity persists for at least ten years and maybe for life. The
RAF Medical Centre may be able to administer the vaccine.
22.
Hepatitis A. Hepatitis A is an infection of the liver, transmitted by the faecal-oral route. Person
to person spread is the most common method of transmission, although contaminated food or drink
may sometimes be involved. Hepatitis A is rarely fatal, but it can incapacitate a person for up to four
weeks. It is, therefore, of great significance to deployed military forces. The risk of contracting
Hepatitis A can be minimised by paying scrupulous attention to personal, food, and water hygiene.
Two other methods of protection are available:
a.
A vaccine against the disease has been developed, giving protection for up to ten years,
after a full course.
b.
For shorter-term protection, human immunoglobulin, given by injection, is effective.
23.
Acute Gastroenteritis/Dysentery. Ten million people die each year from gastroenteritis. Most
infectious agents are contracted through poor food hygiene and can therefore be avoided by paying
particular attention to food and drink hygiene. Gastroenteritis is the commonest cause of pilot
incapacitation whilst on duty. A study of over 5000 airline pilots discovered that 29% had at least one
episode of incapacitation due to uncontrolled bowel actions. Gastroenteritis is, therefore, a threat to
flight safety as well as an inconvenience that could ruin your holiday.
24.
Traveller’s Diarrhoea. 25% to 50% of travellers visiting underdeveloped countries develop
diarrhoea; the highest incidences being in Asia, Africa, Central and South America. The reason is that
visitors have a low resistance to the local bacteria and viruses. The commonest cause is an organism
called 'E. coli'. Patients usually suffer from abdominal cramps, diarrhoea and wind, which start on the
third day and last for up to four days. A doctor should be consulted if:
a.
There is general physical illness, with a fever.
b.
There is blood or pus in bowel movements.
c.
Symptoms persist.
The usual treatment is oral fluids, but some cases may need antibiotics.
25.
Hepatitis B. Hepatitis B is caused by a virus transmitted by bodily fluids, such as blood and
semen. The disease is common in Asia and Africa. Individuals with the infection may develop chronic
infection or become an asymptomatic carrier. This increases their risk of developing chronic
active hepatitis, cirrhosis and hepatocellular carcinoma. Aircrew are at low risk and are, therefore, not
currently vaccinated against Hepatitis B.
26.
Summary. Avoiding the infectious illnesses listed, as well as many other insect-borne and food-
borne illnesses is, in the vast majority of cases, within an individual’s control. Personal hygiene, eg
hand washing, and being certain that food and water are from approved sources, are critical measures
an individual should take. The many insect-borne diseases, such as malaria, dengue, yellow fever,
leishmaniasis, and so on, are largely preventable by the use of skin and clothing insect repellents, bed
nets, and appropriate prophylactic medications. Individuals need to take ownership of their own good
health when away from home and behave responsibly and conscientiously regarding preventive
measures.
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AP3456 - 6-14 - Principles of Head Protection
CHAPTER 14 - PRINCIPLES OF HEAD PROTECTION
Introduction
1.
Aircrew do not like wearing heavy helmets. Helmets are cumbersome and are often
uncomfortable. The additional mass of helmets, especially if the centre of gravity is displaced
forwards, may interfere with head movement under conditions of sustained G, as well as increase
fatigue. It is, therefore, essential for aircrew to appreciate the importance of head protection. It must
also be stressed that an aircrew helmet has many functions, and that the helmet components which
contribute to head protection, such as the helmet shell and energy attenuating liners, comprise only a
fraction of the total mass of the helmet (see Volume 6 Chapter 12).
2.
Head impact need not be a feature of whole body impact acceleration (ie the crash situation) since the
provision of an effective restraint harness, together with adequate head clearance and attention to other
features of crashworthiness, should preclude it. Nevertheless, experience shows that, even in the cockpit,
a protective helmet must be regarded as a highly effective last line of defence for that most critical part of
the human anatomy, the brain. This is particularly true in helicopters, where the absence of ejection seats
means that the aircrew must crash with, rather than separate from, their aircraft. Apart from crashes,
potential causes of head impacts in the aviation environment include through-canopy ejection, battle
damage, bird strike, windblast protection, parachute landing, and the activities of helicopter winchmen.
3.
In the study of brain injury, it is essential to understand potential injury mechanisms and to be
familiar with the threshold levels at which irreversible brain damage may occur. It should be stressed
that, even a fully recoverable injury can prove fatal if it prevents escape from a burning or sinking aircraft.
Mechanics of Head Injury
4.
Current head protection is based on the premise that brain damage may result from any of the
four injury mechanisms, which may be summarized as:
a.
Local deformation of the skull, with or without fracture.
b.
Injuries penetrating the skull.
c.
Excessive linear acceleration.
d.
Excessive angular acceleration.
5.
Skull Injury. When the human head is subjected to a heavy blow, much of the energy of impact
is absorbed by the skull bones which disintegrate in a characteristic way. The skull is, however,
flexible enough, under certain conditions of impact, to be dented transiently by up to 10 mm,
underlying brain damage then being produced in the absence of a fracture. The breaking strength of
bone and soft tissue depends very much on the site of impact; 30 G for the nose, 40 G for the jaw, 100
G for the front teeth and 200 G for the forehead.
6.
Concussion. Concussion is a transient state of instantaneous onset of loss of consciousness
and occurs without much evidence of structural head injury. Even relatively minor head injuries can
cause concussion. Only rarely are there any lasting effects, although a period of memory loss may
occur. The mechanisms of concussion are complex, but it is thought that linear and rotational
accelerations of the head are major factors. Experiments have shown that the risk of cerebral
concussion depends on the time for which a given acceleration is applied. The longer the duration of
the acceleration and/or the greater the value of G experienced, the greater the risk of concussion.
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AP3456 - 6-14 - Principles of Head Protection
This is shown graphically in Fig 1. It is now generally accepted that the human brain can withstand
linear crash impact forces of the order of 300 G to 400 G without skull bone fracture or concussion,
provided that there is no local deformation of the skull.
6-14 Fig 1 Tolerance of the Human Brain to Impact Acceleration
240
220
200
180
160
140
)
Risk of
(G
n
120
Concussion
tio
ra
le
e
100
c
c
A
80
60
40
Safe Region
20
0
5
10
15
20
25
30
35
40
45
Time (ms)
7.
Membrane Injury. The idea that excessive rotational acceleration is a major factor in accidental
brain injury is supported by the observation that it is very difficult to produce experimental concussion
if the head is prevented from undergoing any rotation (ie if the forces applied are purely linear). An
effective boxer’s punch is an off-axis blow to the jaw, producing a high angular acceleration. With
such a blow, the inertia of the brain causes it to twist relative to the skull, with resulting stretching and
tearing of blood vessels and excessive shearing strains in the superficial grey matter.
Head Protection
8.
The problem of preventing head injury on impact may be approached in a number of ways, all of
which are equally important.
9.
Provision of Adequate Restraint Systems. Restraint harnesses can do much to prevent
contact of the head with surrounding structures. However, even with acceptable harness restraint,
there may be multi-directional flailing of head, arms, legs and, to a lesser extent, the torso within the
restraint harness during crash impact. In addition, parts of the aircraft structure may intrude into the
aircrews’ space in spite of adequate restraint systems.
10.
Adequate Space Surrounding the Occupant. The provision of adequate space in the cockpit within
the occupant’s immediate environment, helps to reduce the injury associated with flailing of the head and
contact with surrounding structures during abrupt deceleration. Cockpit space is usually at a premium,
especially in combat aircraft, and it is not always possible to site structural parts of the aircraft at a sufficient
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AP3456 - 6-14 - Principles of Head Protection
distance to prevent the occupant from striking them. Typical hazards in the cockpit area are window and
door frames, instrument consoles, control columns, displays, seat backs, avionics boxes and panels.
11.
Treatment of Surfaces. Where it is not possible to design the cockpit in such a way that the
occupant’s head is prevented from striking surrounding objects, it is often possible to treat surfaces in order
to minimize injury. Dangerous surfaces or projections within the cabin may be constructed from deformable
materials which allow for a measure of energy absorption when the head strikes them. Although many
aircraft cockpits are treated in this manner, the structures and equipment used are far from ideal.
Helmets
12. Helmets give protection against injury through a number of mechanisms. These are:
a.
Resisting Penetration. In order to resist penetration, the shell of the helmet must be strong
and have limited flexibility.
b.
Spreading the Impact Load. In order to spread the impact load, not only must the shell be
strong, but it must also be separated from the skull by an appropriate distance so that some
flexion or distortion of the helmet shell becomes acceptable. When flexion or distortion occurs,
the load has to be transmitted to a large area of the skull by a suitable suspension system.
c.
Increasing the Stopping Distance of the Head after Impact. By providing a finite
stopping distance, a protective helmet can reduce the peak acceleration imposed in a given
impact. This can be achieved either by using a suspension harness which provides an air gap of
about 25 mm or, for even better impact attenuation, placing a layer of permanently deformable
foam beneath the shell (this foam crushes on impact to about 40% of its initial thickness). The
following example illustrates the benefit of an increased stopping distance:
Assume that a human head, weighing 5 kg and travelling at a velocity of 10 m/s, strikes a solid
wall. The frontal bone fractures and is depressed to a depth of 20 mm (0.02 m).
Since v2 = 2as, where v = velocity, a = acceleration and s = stopping distance,
v2
then, a =
.
2s
The average deceleration of the head is therefore:
102
a =
= 2,500 m/s2 = 255 G .
2 × 0.02
If the head had been protected by a helmet with 25 mm of crushable foam, giving another 10 mm
of stopping distance, then the average deceleration of the head would be reduced to:
102
a =
= ,
1 666 m/s 2 = 169 G
2 × 0.03
This example emphasizes the profound importance of stopping distance on the forces in a given
impact. It is important to realize that suspension systems only spread impact forces over a large
area, providing some impact attenuation, whereas increasing the stopping distance with energy-
absorbing material (permanently crushable foam) actually absorbs energy and prevents it from
reaching the head. If the foam in a helmet is crushed, even partially, during an accident or
otherwise, it will be rendered less effective and should be replaced.
13. If a helmeted head strikes a surface at an acute angle, then the head may either slide or roll
along the surface, depending on friction at the contact area. If the helmet shell is made smooth and
external protuberances reduced to a minimum, then the tendency to slide is increased and rotational
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AP3456 - 6-14 - Principles of Head Protection
acceleration is reduced. For the same reason, any essential projections should be fared or designed
to break away at a non-injurious force level.
14. No dedicated helmet standard for civilian aviation use was forthcoming until the 1990s (British
Standards Institution 1996). Unlike earlier specifications, which defined helmets in terms of their
materials, dimensions and production, the later performance standards defined helmets largely in
terms of their function, i.e. instead of describing the helmets, the standards defined how to test the
helmets. The standards served two immediate purposes: tools for the evaluation of existing helmet
designs and guides for the development of new headgear.
15. In the UK, some of the military aircrew helmets still in use are based on design standards of
motorcycle helmets. In the past, there was no specific standard for military aircrew helmets. The Mk4
helmet used in rotary-wing aircraft and in fixed-wing aircraft for certain roles is tested against BS 2495,
while the Mk10 or ALPHA helmet is used in aircraft fitted with ejection seats and is tested to BS 6658.
Both standards were developed for the evaluation of motorcycle helmets; with the introduction of a
new European standard, EN regulation 22, these earlier motorcycle standards have been superseded.
EN regulation 22 is the culmination of many years of analysis of motorcycle helmet impact data, with
the standard better reflecting the threat seen in motorcycle accidents. As a result, it made its use in
the procurement of aircrew helmets less tenable and made the development of helmet test standards
specifically for aircrew helmets all the more important. In the UK, a helmet standard has been
developed specifically for aircrew helmets and as with all helmet design standards it originally covered
three major aspects: resistance to penetration, shock absorption and retention.
16. The standard was developed from the findings of research programmes that included
assessment of existing equipment and the cockpit environment, detailed review of aircrew accident
statistics including impact events and injury outcomes, impact test methodology, and evaluation
techniques for damaged helmets. Two types of helmet are specified: Type E for use in aircraft fitted
with ejection seats and Type S for aircraft fitted with static seats.
17. Since its introduction as a Defence Standard in 2004 it has undergone a further review and in
2014 revised impact test standard requirements were developed. The test for shock absorption
involves fitting the test helmet on an instrumented head form and dropping it in guided freefall on to
either a flat or a hemispherical anvil. The head form and its supporting carriage have a combined
mass of 5 kg. Each impact is followed by a second impact at the same site but at half the energy; on
no occasion must the acceleration of the head form exceed 300 G. Helmets may be impacted at any
point over the shell.
18. To evaluate a helmet’s resistance to penetration, a test helmet is mounted on a rigid head form
and struck by a conical striker with a 0.5 mm radius tip. The striker weighs 1.8 kg, and the striker is
dropped in guided freefall from the required height dictated by the particular test standard. A test
failure occurs by detecting penetration by transient electrical contact between the tip of the striker and
a soft metal insert at the top of the head form. However, recent reviews of UK and US accident
damaged helmets have demonstrated that the penetration impacts during accidents are very unlikely.
The penetration test and the hemi-spherical anvil impact test drive the design of helmet shells to be
stiffer than would otherwise need to be to meet flat anvil impact requirements; a consequence of this,
together with the provision that the hemi-spherical anvil test is retained, has resulted in the
penetrations test requirement for MAHIS to be removed.
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AP3456 - 6-14 - Principles of Head Protection
19. Helmet retention is of great importance, especially in view of the large windblast forces
encountered in high-speed ejections. Good retention is achieved by care in fitting and by a correctly
tensioned chin strap and mask. The helmet offers no protection if it comes off during an accident and it
is, therefore, essential to always ensure the straps are properly adjusted and fastened.
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AP3456 - 6-15 - Crash Dynamics
CHAPTER 15 - CRASH DYNAMICS
Introduction
1.
By far the commonest cause of injury in aircraft accidents is the very abrupt deceleration that
occurs when an aircraft strikes the ground or water. The kinetic energy of the aircraft is so great that
anything but a well-executed crash landing or ditching results in the application of damaging forces to
the machine and its occupants. These forces are quite variable. They depend on the type of aircraft,
the all-up weight and the speed and angle at which the aircraft hits the terrain. Much information has
been obtained by experiments, in which different types of aircraft were deliberately crashed at various
speeds and angles of impact; measurements of acceleration were taken in different sites within the
aircraft fuselage. Fig 1 illustrates typical recordings of the acceleration profile measured in the
longitudinal direction at the cockpit floor of a jet fighter aircraft deliberately crashed at various angles
of impact and at an impact speed of 97 kts. The pattern of longitudinal deceleration measured at the
cockpit floor is very irregular; the profile produced, and the magnitude of the acceleration, vary with the
impact angle.
6-15 Fig 1 Jet Fighter Crash Acceleration Profiles
150
125
)
100
Impact speed 97 kt
(g
n
75
tio
ra
50
le
e
c
c
25
A
0
-25
0
0.1
0.2
0.3
0.4
0
0.1
0.2
0.3
0
0.1
0.2
0.3
Time (s)
Time (s)
Time (s)
18°
22°
27°
2.
The deceleration measured at any point in the fuselage is usually defined in terms of magnitude,
duration and direction of application. Magnitude is measured in units of 'G', duration in fractions of a
second and direction as longitudinal, vertical or lateral. Experimentally, abrupt decelerations may be
considered as 'impulses', similar in shape to that shown diagrammatically in Fig 2.
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AP3456 - 6-15 - Crash Dynamics
6-15 Fig 2 An Acceleration Impulse
Peak
n
tio
ra
Average
le
e
c
c
A
Time
3.
Measurements of the accelerations produced at various sites in the fuselage during experimental
crashes of aircraft are usually presented in the form shown in Table 1. This example is taken from an
experimental helicopter crash in which the measurements of deceleration were made on the cockpit
floor, close to the seats.
Table 1 Typical Experimental Crash Results
Direction of
Acceleration (G)
Pulse
Acceleration
Peak
Mean
(seconds)
Longitudinal
30
15
0.104
Vertical
48
24
0.054
Lateral
16
8
0.097
4.
The actual profile of deceleration forces obtained during an aircraft crash is governed by the
diminishing momentum of the aircraft as the terrain resists its forward motion by friction, or collisions
with objects on the ground. If the structures of a crashing aircraft are crushed or deformed
progressively, then much of the kinetic energy of the crash is absorbed and the overall deceleration
profile is relatively smooth. If the parts of the structure of the crashing aircraft (eg engines, rigid
members, keel structure) plough into the ground during forward motion, then the momentum of the
aircraft is reduced more rapidly in peaks of abrupt deceleration of high magnitude which appear on the
recorded deceleration profile. Very high peak values of deceleration occur when the crashing aircraft
strikes solid objects (such as rocks, posts, or buildings) during its travel across the ground.
5.
When an aircraft ditches, the forces are dependent on:
a.
The longitudinal and horizontal velocities relative to the water.
b.
The sea state.
c.
The site of the aircraft impact relative to the wave front.
Little attenuation can be expected from fuselage deformation, as impact with the water tends to
produce a very uniform load distribution across the lower surface of the fuselage.
Structural Damage Causing Injury
6.
Data obtained from experimental crashes has revealed that certain types of airframe damage are
likely to result in injuries to the occupants of the aircraft.
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AP3456 - 6-15 - Crash Dynamics
a.
Longitudinal Loads on Cockpit Structures. During a crash in soft earth, nose structures
scoop up earth as the aircraft slides in contact with the terrain surface. The scooped earth is
accelerated quickly to the same velocity as the aircraft. This produces momentarily high forces,
which must be supported by the forward bulkhead of the aircraft. The cockpit structure may
collapse, causing injury to the legs of the occupants, who may then be unable to move. The high
levels of acceleration generated in the aircraft structures are also transmitted to the seated
occupants. Occasionally, a combination of crushing of the nose section and friction between
aircraft structures and the terrain causes the forward structures to be pulled beneath the rest of
the aircraft. In such circumstances, very high longitudinal acceleration forces are generated,
often causing the cockpit floor to rupture.
b.
Vertical (Crushing) Loads on the Fuselage. Collapse of the fuselage shell, due to high vertical
loads, often occurs in accidents where the aircraft hits the ground with a high sink rate. It also occurs
in roll-over accidents. Collapse of the shell of the aircraft is often aggravated by large masses
positioned above the cockpit, such as engines, rotors, or high wings. Crush injuries are common.
c.
Transverse (Bending) Loads on the Fuselage. Rupture or collapse of the aircraft
protective shell often occurs due to severe bending loads, where rapid changes in pitch or yaw
develop. This occurs in crashes where the aircraft impacts the ground at a moderate to high
impact angle. Rupture of the protective shell exposes the occupants to injury through direct
contact with the impact surface, ragged metal edges, etc. Miscellaneous equipment may strike
the occupants after the aircraft breaks up.
d.
Deformation (Buckling) of Floor Structure. Break-up of the floor structure is a common
sequel in a variety of aircraft crashes. Since much of the aircraft equipment is mounted on the
floor (including, directly or indirectly, the seats), accidents of this type may result in serious
multiple injuries to the occupants.
e.
Landing Gear Penetration of the Fuselage. Where the landing gear is forced upwards
through the floor, the occupants may be injured by direct trauma, or through fire caused by
rupture of fluid lines and containers.
7.
From this summary of crash damage mechanisms, it is clear that much can be done to improve the
chances of survival of aircrew during crashes if certain measures are adopted in the design and
construction of aircraft. The mnemonic '
CREEP' is important in this respect, and is explained as follows:
a.
Container. The airframe should be such that the structures surrounding the occupant
remain reasonably intact and provide a protective shell.
b.
Restraint. The purpose of the restraint system is to hold the occupant in the workspace
during violent manoeuvres of the aircraft, thus providing protection from the effect of sudden
deceleration during impact.
c.
Environment. The structures, especially those in the immediate vicinity of the occupants,
should crush and deform in a controlled and predictable manner, so that the forces of
acceleration acting on the occupants are absorbed and minimized. These structures should
deform without fracture, and materials should crush, twist or buckle without rupture.
d.
Energy Absorption. Aircraft seats should be designed in such a way that they assist in the
absorption and distribution of high energy loads before they reach the occupant. Associated
restraint systems should provide good coupling of the occupant to the seat to give good 'ride
down' characteristics.
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AP3456 - 6-15 - Crash Dynamics
e.
Post Crash Factors. Attention must be paid to the events that happen after a crash. These
include fire prevention, escape, and survival.
Restraint Systems
8.
The purpose of the restraint system is to hold the person in the workspace during violent
manoeuvres of the aircraft, and so protect them from the effects of sudden deceleration during impact.
9.
The qualities that a harness restraint system should possess are:
a.
Comfort. The harness should be comfortable and capable of being adjusted over the
required size range.
b.
Efficiency. The harness must protect the wearer from injury in the presence of multi-
directional forces during impact. It should be designed to provide maximum distribution of these
forces and should not itself cause injuries. Ideally, it should be capable of being readily adjusted
so that little or no relative movement can take place between the wearer and the sitting platform.
c.
Ease of Use. The restraint system must be easy to put on and release and should be as
simple as possible. A single-point release mechanism is desirable. The operating loads of the
harness release mechanism should be between 66 and 177 Newtons (15 to 40 lb force). This is
high enough to avoid inadvertent release, yet low enough to allow single-handed operation. Two
separate sequenced actions for release are also desirable, in order to prevent inadvertent release.
d.
Minimum Restriction. The restraint system must give the user sufficient freedom to
operate all of the aircraft controls, and to carry out normal flight tasks.
Types of Harness Restraint
10.
Lap Belt. The lap belt is one of the simplest types of restraint system and is very easy to use. A
typical lap belt harness is shown in Fig 3. It requires only two anchorage points, either on the aircraft
seat or on the floor, and it causes minimum restriction to the user. However, there are major
disadvantages. Firstly, the upper torso is not restrained and will jack-knife on impacts with –Gx or +Gz
components. Unless the arc described by the head is free of obstruction, or unless a braced position
has been adopted, serious or incapacitating injury may result. Secondly, if the lap belt rises up from
the pelvis to lie across the front of the abdomen, serious abdominal and lumbar spinal injuries may
occur. However, as can be seen from Figs 4 and 5, simple belts can be designed to produce better
restraint. The retention of lap belts in public transport aircraft owes much to their simplicity of use and
their utility in providing restraint under conditions of turbulence. Provided that a proper braced position
is adopted, they are probably also appropriate as a restraint device with current aircraft seating
systems, which are stressed to withstand only low levels of impact.
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AP3456 - 6-15 - Crash Dynamics
6-15 Fig 3 A Typical Lap Belt
11.
Diagonal Belt. The diagonal belt has the advantage of simplicity but, in a crash, is probably less
efficient than a lap belt restraint alone. Since the pelvis is not restrained, the seat occupant tends to
rotate out of the harness. It can cause a lethal neck 'whip' action when subject to lateral forces. It can
also produce internal chest injuries during severe impact.
12.
Diagonal and Lap Combined Harness ('Three-point Harness'). The combined harness is the
most widely used type, probably as a result of its widespread adoption in cars. With careful design, it
gives good restraint for all except lateral accelerations. It is important that the harness should be
properly adjusted, and that the seat cushions should be reasonably stiff, since, otherwise, the lap belt
component may rise off the pelvis during an impact, giving rise to abdominal injuries similar to those
encountered with the lap belt alone. Correctly fitted, the harness will give acceptable restraint at
accelerations of about –30Gx.
13.
Double Lap and Shoulder Harness ('Four-point Harness'). The double lap and shoulder
harness is a satisfactory assembly and provides better restraint than the combined diagonal and lap
harness described in the previous paragraph. One typical form of the double lap and shoulder
harness is illustrated in Fig 4.
6-15 Fig 4 A Double Lap and Shoulder Harness
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AP3456 - 6-15 - Crash Dynamics
14.
Double Lap and Shoulder Harness with a Negative G Strap ('Five-point Harness'). The simple
four-point harness is much improved by the addition of a negative G strap (see Fig 5). This strap, also
known as the 'lap-belt tie-down strap' or 'harness stabilizing strap', rises from the seat in the mid-line
between the legs, to join the harness at a central quick-release point. It prevents distortion of the
harness by forces imposed on the torso. It is extremely effective during aerobatics and aircraft
manoeuvres that involve negative G, vertical vibration in high-speed low-level flight, and under crash
impact.
6-15 Fig 5 A 'Five-point' Harness
The advantages of a five-point harness may be summarized as:
a.
During Aerobatic Manoeuvres. Where there is negative G, it is essential that aircrew
should be restrained in all three axes, so that:
(1) All aircraft controls remain within reach.
(2) Protective helmets do not strike the cockpit canopy.
(3) The view of cockpit instruments and weapons systems (such as sighting systems) is
maintained.
It is also important that the aircrew feel secure. The negative G strap ensures good pelvic
restraint under these conditions and prevents the quick-release point of the harness moving away
from the seat, thus restraining the shoulders and preventing excessive extension of the trunk.
b.
During Vertical Vibration. Vertical vibration may occur during high-speed, low-level flight in
fixed-wing aircraft and in helicopters flying in turbulence. It may cause such a degree of
movement of the pilot in the seat that control of the aircraft is jeopardized. A five-point harness
provides acceptable restraint.
c.
During Crash Impact. In a forward crash impact (–Gx), the negative G strap is of particular
importance. As the torso of the aircraft occupant decelerates, tension is placed on both the lap
and shoulder straps. The tension applied to the shoulder straps causes elevation of the central
point of a simple four-point harness, and this increases the angle at which the lap strap intersects
the seat platform. This, in turn, allows the pelvis to rotate underneath the lap strap (known as
'submarining'), so that the lap straps slide upwards off the pelvis and on to the soft tissues of the
abdomen. The spine is allowed to flex and the tolerance of the occupant to the vertical
acceleration, which often follows initial horizontal impact, is greatly reduced. The addition of a
negative G strap to the four-point harness prevents rise of the centre point of the harness on
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AP3456 - 6-15 - Crash Dynamics
crash impact, and maintains the correct angle for the lap straps, so that the broadest part of the
pelvis bears the major part of the decelerative load.
d.
Personal Survival Pack Retention. The five-point harness prevents any movement or
displacement of the personal survival pack, which often forms the sitting platform in the ejection
seat of many fixed-wing military aircraft. Under negative G, the sitting platform is effectively kept
in place by the well-restrained occupant, without resort to locks and releases on the survival pack
itself. Ejection injury to the back is also much less likely if the pilot is firmly anchored to the seat
so that the minimum relative movement occurs.
Other Forms of Protection
15.
Rearward Facing Seats. Rearward facing seats offer an attractive means of improving
passenger restraint in –Gx impacts. It is obviously essential that such seats should incorporate an
integral headrest, and should be adequately stressed, but, with these provisos, rearward facing seats
undoubtedly offer the best impact protection. There has, however, been considerable resistance to
their adoption on the grounds of:
a.
Presumed passenger dislike.
b.
Cost.
c.
Weight.
d.
Lack of comfort on take-off and landing.
A superficial review of survivable airline accidents over recent years, suggests that the widespread
adoption of rearward facing seats might have resulted in the saving of a few lives. It is unlikely that
rearward facing seats will be adopted in the foreseeable future.
16.
Energy Attenuating Seating. Aircraft structures and seats can be designed to collapse
progressively when exposed to high impact forces. By this means, the loads applied to the seat
occupant can be limited. Simple calculation will show that the greatest contribution to load limiting is
achieved by the careful design of seating systems. Most practical designs rely on the plastic
deformation of metal to achieve energy attenuation, and seats designed in this way can be retrofitted
to existing helicopters using either floor or ceiling mountings. It is difficult to accommodate more than
⅓ of a metre of movement (stroke), although calculation shows that this is a barely acceptable
distance. Assuming a velocity change of 13.1 m/s, and an ideal attenuating material, the peak G can
be calculated as follows:
V2
G = 2gS
where
G is the peak acceleration in G units,
V is the initial velocity in metres per second,
S is the stopping distance in metres,
g is the acceleration due to gravity (9.81 m/s2).
13.12
Therefore, G =
= 26.5
2 × 9.81× 0.33
It should be noted, however, that simple attenuating devices work at a constant force, rather than a
constant acceleration. Since Force = Mass × Acceleration, variation in the weight of the seat occupant
(including equipment) will cause the lightweight occupant to experience a higher acceleration, and
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AP3456 - 6-15 - Crash Dynamics
therefore use less of the available stroke. Conversely, the heavier occupant will experience a lower
acceleration and would require a larger stroke, resulting in the system 'bottoming'. To reduce this
problem, provision has to be made for modifying the force required to operate the energy attenuator
according to the boarding weight of the occupant.
17.
Escape from Aircraft. Although it could be possible to provide all aircraft with the capability to
absorb the energy of impacts it becomes more problematic in fighter aircraft. Combat aircraft are likely
to impact the ground at high velocities; the structural strength and energy absorption necessary to
permit the aircrew to survive would result in an extremely heavy aircraft, with an inevitable decrease in
agility and aircraft performance. For aircrew to survive and escape from a disabled aircraft with limited
crashworthiness, they have to parachute from the aircraft before ground impact. Initially simple bailout
was the only method used, but as aircraft speeds increased this became increasingly dangerous and
hence, ejection seats were developed.
18. The need to escape from an aircraft may arise on the ground or during flight. The means for
escape must be available at all times and must take account of the forces that may be operating on
the aircraft. Most high-performance military aircraft have assisted escape systems, which use
mechanical and explosive power for aircrew to leave the aircraft. Assisted escape systems must have
sufficient thrust to eject the occupant clear of the aircraft structure at all speeds and provide sufficient
ground clearance to enable full deployment and inflation of the main parachute before ground impact.
After initiation of the ejection sequence the system should be fully automatic, relieving the occupant of
any action, other than preparing for the parachute landing, and it should restrain the occupant
sufficiently and modulate any forces of the body, so that the risk of injury is minimized. Modern
ejection systems enable either aircrew of a twin-seat aircraft to initiate the ejection despite the other
crew member being totally unprepared for ejection. Thus, the system has to pre-position the aircrew in
the ejection seat by a harness retraction system while canopy jettison or canopy fragmentation
devices are clearing the ejection path. Ejection systems have now been developed which can
automatically eject the aircrew with the decision to eject being made by onboard aircraft computers
outside the control of the pilot.
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AP3456 - 6-16 - Royal Air Force Aircrew Conditioning Programme (ACP)
CHAPTER 16 - ROYAL AIR FORCE AIRCREW CONDITIONING PROGRAMME
(ACP)
Introduction
1.
The Aircrew Conditioning Programme (ACP) is a preventative strategy designed to enhance pilot
performance through reducing fatigue and strain injuries, with a particular emphasis on the neck. The
main aims are to engender a culture of career-long neck and upper quadrant maintenance, maintain a
neutral cervical spine position under load, reduce compensation strategies during loading and
strengthen the muscles involved in the anti-G straining manoeuvre (AGSM). Aircrew receive a period
of specialist instruction in all the exercises which will enable them to continue their individualised
conditioning programmes independently.
ACP Delivery
2.
The ACP is to be delivered to aircrew that hold a full medical flying category and with no current
injury. It should be delivered to all aircrew within the Flying Training (FT) pipeline, regardless of phase
of training or aircraft type and has been designed to become more role and platform specific as
students move through the FT pipeline. Minimum standards of each element are recommended for
each stage of flying training. The ACP should also be delivered to qualified aircrew on Front Line
Units. The ACP is delivered by PTIs holding the ACP Instructors Course (ACP IC) competence from
within station Physical Education Flights (PEd Flts). The RAF Physiotherapist (Aviation Specialist
Physiotherapist - ASP) is responsible for the specialist assessment of the neck and to deliver
overarching direction, guidance and governance for the ACP. A presentation is delivered to the
aircrew at the start of each stage of their flying training which provides an overview of the ACP and the
reasons behind completing it. This is delivered by the ASP.
ACP Assessment
3.
Assessment (Table 1) is conducted by an ASP and an ACP IC qualified PTI and occurs at the start
of each stage of the FT pipeline. It is recommended that all aircrew undertake an annual ACP evaluation
with the PTI and ASP. This will be used to inform aircrew how best to maintain optimal physical flying
performance and is aimed to engender a culture of independent responsibility for flying fitness.
Table 1 ACP Assessment
Method of
Element
Description
Measurement
Neck range of CROM device
Active and in neutral spinal alignment, sat in a chair. Measure
motion
flexion, extension, lateral flexion (left and right) and rotation (left and
right).
Outcome measure – maximum active range of motion in each
direction.
Neck strength Lafayette
Maximal voluntary isometric contraction (MVIC) 3 x 5 seconds with
(isometric
manual
load 30 seconds recovery between each contraction. Sat in a chair in
muscle
cell (push load neutral spinal alignment with hands crossed across chest.
strength)
cell)
Measured in flexion, extension, lateral flexion (left and right),
anterolateral flexion (left and right) and posterolateral flexion (left and
right) directions.
Outcome measure – maximum load for each direction.
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AP3456 - 6-16 - Royal Air Force Aircrew Conditioning Programme (ACP)
6-15 Fig 1 Directions of Neck Strength Measured
6-15 Fig 2 Sample Radar Graph Showing Isometric Neck Strength
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AP3456 - 6-16 - Royal Air Force Aircrew Conditioning Programme (ACP)
(Table 1 Continued)
Neck
Tape measure
At level of C5/C6, with neck in neutral spinal alignment.
circumference
Outcome measure – neck circumference in cms.
Neck length
Tape measure
From top of C7 spinous process to base of occiput, with neck in
neutral spinal alignment.
Outcome measure – neck length in cms.
Whole
body Functional
Comprised of 7 specific movement patterns that require a balance of
flexibility and Movement
mobility and stability. An inability to perform these movements would
movement
Screen (FMS)
indicate poor biomechanics and could lead to injury.
control
Outcome measure – maximum score achieved by the participant.
Anaerobic
Running-
Adapted from the Wingate Anaerobic Test protocol as a tool to
capacity
based
assess repeated sprint ability and power, and from original RAST.
Anaerobic
Consists of six times 20m maximal effort discontinuous sprints, with
Sprint
Test 10 second turnaround between each sprint. Fatigue index and power
(RAST)
output calculated from sprint times.
Outcome measure – fatigue index for each participant.
Strength
1
Repetition Muscular strength and endurance assessment of most important
Maximum
muscles for anti-G straining manoeuvre.
Outcome measure – maximum weight (in kg) of a single repetition of
double leg press (leg muscles), bar bell flat bench press (chest
muscles), timed plank to failure (sub-maximal endurance measure of
core/abdominal muscles – lying on front with forearms on the
ground, keeping elbows under shoulders and feet together. Raise
the body upward off the floor and hold this position with the body in a
straight line).
ACP Components
4.
The ACP consists of four main elements:
a.
Whole body flexibility and mobility – involves exercises in specific movement patterns
that require a balance of mobility and stability.
b.
Cardiovascular fitness – focusing on anaerobic capacity, training sessions involve a
combination of weighted whole-body exercises and high intensity cardiovascular exercises.
c.
Stabilisation and motor control exercises – for the neck, shoulder girdle and lower back.
The aim is to maintain a neutral cervical spine position under load in all positions and develop
rotational core control in a seated position.
d.
Strengthening exercises – of the neck, back, abdominal and leg muscles, incorporating
isometric neck loading in a spinal neutral position, upper quadrant and Olympic type lifting
techniques.
ACP Exercises
5.
Exercise sessions last 1-2 hours, delivered twice a week and are described in greater detail in
Table 2. Sessions should be supervised by an ACP IC qualified PTI and should be delivered during
the mandated (under QR) PEd sessions during both the ground-school and flying phase of flying
training. These supervised sessions should last for 12 weeks.
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AP3456 - 6-16 - Royal Air Force Aircrew Conditioning Programme (ACP)
Table 2 ACP Exercises
Element
Equipment
Description
Flexibility
and Foam roller
General stretching exercises. Foam rolling to main muscle
mobility
groups.
Neck
Pressure
All exercises are performed isometrically in a neutral spine
strengthening
biofeedback
position. Four levels of exercise which become progressively
cuff,
elastic more difficult.
exercise band,
head
harness Initially exercises are performed to activate segmental
with
weighted stabilisers, then global stabilisers followed by global movers.
pulley
Low loads are used throughout (1-5 kg) and weights are
increased for the upper body movements.
Core
stability Exercise
ball, Stability exercises for the trunk, shoulder and neck.
(includes
bosu
ball, Exercises progress from maintaining a neutral posture in all
scapular
elastic exercise positions, to static rotation control in all positions, to dynamic
control)
band,
hand rotation control on a stable base, then on an unstable base.
held
weights
and
weighted
pulleys.
Strength
Olympic
Exercises include whole body compound movements, Olympic
training
weights,
hand type exercises which includes squats, deadlifts, bench press,
held
weights, bent over row and push press.
kettle bells and
power bags.
Exercises progress from initial technique instruction, developing
technique competency, then progression of weight whilst
maintaining technique.
Cardiovascular
Running,
CII All exercise sessions are anaerobic and interval-based
training
rower, weights.
sessions.
(anaerobic
based)
ACP Efficacy
6.
Aircrew complete a Customer Satisfaction Survey on completion of the supervised sessions and
after six months of supervised ACP. This is used as an audit tool and demonstrates some anecdotal
evidence to support the ACP:
a.
Rotary wing aircrew have reported reduced fatigue and pain after 2 hours of NVG flying.
b.
Fast jet aircrew have reported an improved ability to cope with air combat manoeuvre
sorties and less neck pain during flying training as a result of participation in the ACP.
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Document Outline