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heavier-than-air aircraft in 1909 (DeJohn 2004), the industry has evolved from aircraft carrying a few people,
to aircraft carrying hundreds of occupants, resulting in more severe consequences from a single aircraft
accident. Such large aircraft are flown by professional pilots which is a reason for this report being focused
primarily on the professional pilot group, as mentioned above. When private pilots are involved in aircraft
crashes, the number of individuals involved is greatly reduced since the aircraft typically flown are much
smaller. Furthermore, the likelihood of causing loss of life to members of the public who are not occupants of
the aircraft (i.e. on the ground, or in other aircraft) is minimal (although it does occasionally occur).
In reality, it is rare for medical factors to be the primary cause of aircraft crashes – probably 1 per cent or less,
and for professional airline operations, well below this. It has been estimated that across the industry 3 per
1 000 aircraft accidents (15 per 1 000 fatal aircraft accidents) result from pilot incapacitation (Booze 1989),
although this does not include accidents in which medical factors may be a contributory, as opposed to
primary, cause. Because of the difficulty in proving medical causation, there may also be situations in which
a primary medical cause may have been present but which cannot be established through investigatory
processes.
Importantly, in those situations when medical causes are identified, certain causes predominate. An analysis
of fatal commercial airliner crashes in which medical factors were identified as causative, over a 20-year
period (1980-2000), ten incidents were identified. Of the ten, eight were ascribed to a psychiatric cause with
the majority (six) being related to alcohol and/or other drugs (Evans, 2007). The discussion which follows
will therefore place particular emphasis on these conditions.
3. Aims and limitations of the examination process
The primary purpose of a medical examination is often considered to be the detection of conditions with a
propensity to cause incapacitation (Evans 2006). Examples include seizures, disturbances of heart rhythm,
loss of consciousness. However this is only one aspect of the medical examination, and one with limitations.
Incapacitation can be rapid or slow, and the degree of warning will affect the consequence of incapacitation.
It must be remembered here that by far the commonest cause of in-flight incapacitation is acute gastrointestinal
upset, which is almost never predictable on routine medical examination. In considering
incapacitation, there are also differences between obvious and subtle incapacitation with the latter having the
potential for even more serious consequences due to delayed detection. A distinction may also be drawn
between passive incapacitation, in which the individual becomes unresponsive, and active incapacitation such
as in a seizure, whereby the pilot has the potential to interfere directly with the control of the aircraft.
There is a further category of in-flight incapacitation which is related not to medical factors (although are
often attributed to medical causes in reporting systems) but to exposures relating to the operational
environment, such as exposure to hypoxia, carbon monoxide or toxic fumes from combustion. These types of
incapacitation are not strongly related to individual factors, and are not predictable by medical examination.
Some degree of incapacitation risk is always present. For example, all individuals have a background risk of
seizures, which is reported as between 0.1 per cent and 1 per cent annually depending on age (Heaney, 2002).
Therefore, judgement will be required as to the acceptable level of risk. Much has been written on this
subject and many States apply a threshold of risk of no greater than 1 per cent per annum for an individual in
the multi-pilot operational environment, this being derived from a computation of acceptable risk of a
catastrophic accident, relative to risks from other causes relating to aircraft operation (Tunstall-Pedoe, 1984).
ICAO Preliminary Unedited Version — May 2010 V-1-8
The detail will not be repeated here but the essential concept is that the 1 per cent threshold was calculated to
produce a risk of catastrophic pilot incapacitation which was no greater than other catastrophic system failures
such as those of major aircraft engineering systems. It has been argued more recently that the threshold of 1
per cent could be revised (Mitchell, 2004), but the important principle is that medical examiners should have a
good understanding of the way in which aeromedical risk is assessed, and its limitations. (See Part I Chapter
3, Flight Crew Incapacitation, for further discussion of in-flight incapacitation and acceptable aeromedical
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Manual of Civil Aviation Medicine 2(148)
