the effects of alcohol on pilot
performance and safety
Dr Dougal Watson
The piloting of an
aircraft is a complex task. It requires the interpretation of a variety of
sensory information, the cognitive evaluation of this information, and the
performance of various motor tasks in response to the perceived situation.
The basic faculties required to be able to successfully pilot an aircraft
include adequate and unimpaired senses of vision and hearing, sufficient
intelligence and judgement, suitable personality, and motor skills. The
motor skills necessary include adequate power, dexterity, and coordination
to manipulate aircraft controls, sufficient power and coordination of
speech for radio based communication, and the strength and agility to
allow entrance-to and egress-from an aircraft. Substantial training is
then required to turn these basic attributes into the ability to
successfully and safely pilot an aircraft.
The demands of flying an aircraft are greater than those of driving a car.
The pilot is exposed to additional factors such as the hypoxia of
increasing altitude, high noise levels, the requirement for radio
communication with the outside world, higher accelerations during aircraft
manoeuvring, and visual-vestibular illusions with the potential for loss
of three dimensional orientation. Even quite low levels of
alcohol can act to impair the human faculties required to fly in a safe
and effective manner.
The ingestion of alcohol influences virtually every system in the human
body in some way or another. The most readily apparent effects of alcohol
are usually a result of its effect on our central nervous system. The
metabolism of all other body systems is altered. Included is the
gastro-intestinal tract, the liver and pancreas, muscles, the blood, the
heart, endocrine organs, the immune system, the respiratory system, fluid
and electrolyte balance, and possibly even the incidence of cancer. The
effect of alcohol most pertinent to aviation is its impairment of a
variety of central nervous system functions.
Association with fatal aircraft accidents.
During the last four decades the relationship between alcohol ingestion
and fatal aircraft accidents has been reviewed intensively (5). A high
incidence of the presence of alcohol in the blood of pilots involved in
fatal general aviation accidents has been demonstrated.
In the United States the percentage of pilots with elevated blood alcohol
levels involved in fatal general aviation accidents during the early 1960s
was approximately 43%. This proportion had fallen somewhat, but remained
between 15% and 20% during the 1970s. The association between elevated
blood alcohol levels and fatal, general aviation, aircraft accidents has
tended to remain at the 10% - 30% level in recent times. This relationship
does not appear to persist when military or professional commercial
aviation accidents are investigated. A variety of factors, primarily the
effect of putrefaction on measured blood alcohol levels, have caused some
to argue that these postmortem studies do not accurately reflect the true
incidence of alcohol ingestion by aviators. No similar Australian data has
been seen by the author.
A statistical correlation between elevated blood alcohol levels and fatal
civil general aviation accidents does not necessarily infer a causal
relationship. Despite technical discrepancies in some of the studies,
especially concerning the issue of putrefaction, the balance of the data
does suggest a strong correlation. The considerable amount of data
available and the consistency and trends of the results would tend to
support the postulate that alcohol has a causative role in many of these
Impairment of flight.
The statistical correlation between elevated blood alcohol levels and
fatal civil general aviation accidents has prompted attempts at
identifying pilot impairment during flight, simulated flight, and a
variety of flight related tasks.
In-flight evaluation of pilots with blood alcohol levels of 0%, 0.04%,
0.08%, and 0.12% has suggested that even quite low blood concentrations of
alcohol cause significant performance decrements in flight. This study
concluded that “blood alcohol concentrations of 0.04% are associated with
substantial and highly significant increases in the number and potential
seriousness of procedural errors committed by both inexperienced and
highly experienced pilots”. Other studies performed using aircraft flight
simulators support the relationship between the blood alcohol level and
the number of aviation procedural errors.
due to the ingestion of alcohol depend, in part, on the blood alcohol
levels produced and on the ability requirements of the task. This
observation has been supported by studies on the effects of alcohol on the
performance of aviation related tasks. These tasks have been shown to be
impaired by blood alcohol concentrations of 0.025%, 0.04%, 0.08%, 0.1%,
Impairment of higher cortical functions.
The safe and successful piloting of an aircraft requires sound functioning
of the higher cortical faculties responsible for planning, judgement,
cognition, calculation, attention, vigilance, sequencing, and memory. All
of these faculties are impaired in some manner by the acute ingestion of
alcohol. Acute intoxication produced by increasing concentrations of
alcohol in the blood produces impairment of psychological functions such
as perception, discrimination, association, and voluntary response.
Psychomotor and spatial orientation capabilities of pilots are impaired by
0.1% blood alcohol levels. Complex task performance and reaction times
have been shown to be impaired by blood alcohol levels in excess of 0.04%
and 0.08% respectively. Blood alcohol levels as low as 0.027% cause a
decrease in visual tracking performance during whole body motion and in
non-moving individuals. Alcohol disrupts the laying down of memory with a
likely subsequent reduction in aviation safety.
Reaction times to different stimuli have been shown to be increased by
alcohol ingestion. The monitoring and decision components of reaction time
tasks are also impaired by alcohol levels of 0.09%.
All of these functions play an important role in the safe piloting of
aircraft. Impairment of any of these functions will be detrimental to
The effects of alcohol on visual and visual-vestibular function.
Vision is the prime sensory modality used during aviation. The sense of
vision is required for spatial orientation and navigation during both
‘visual’ and ‘meteorological’ flight conditions as well as the monitoring
and adjustment of aircraft performance. In the absence of adequate visual
stimulus control of an aircraft is typically lost within 60 seconds. Any
impairment of the sense of vision, therefore, has the potential of
adversely influencing flight performance and flying safety.
The speed of the eyes, in pursuing a target, is reduced by alcohol.
Similarly the speed of the eye’s saccadic motion, their latency times, and
reaction times are impaired by blood alcohol concentrations in excess of
0.04%. Double vision and dilatation of the pupils, resulting in blurred
vision, can also result from alcohol intoxication. Blood alcohol levels of
0.05% and above have been shown to slow the ability of the eyes to
accommodate or adjust their focus.
During the angular accelerations of flight there occur reflex rapid,
oscillatory eye movements called nystagmus which tend to impair the view
of objects within the aircraft. This can result in blurring of vision of
instruments and a subsequent impairment of performance. Usually a pilot is
able to suppress this nystagmus by deliberately fixating on an instrument.
Alcohol ingestion, and low light levels, impair the ability to suppress
this nystagmus. Impairment occurs at blood alcohol levels as low as 0.02%.
One study concludes “that serious problems may even be encountered by the
pilot who drinks lightly and who considers flying, especially at night”.
Another condition, called Positional Alcohol Nystagmus (PAN), also results
from alcohol ingestion and also threatens flight safety. Positional
Alcohol Nystagmus results in rapid, oscillatory eye movements when the
head is placed in specific positions in the absence of angular
acceleration. This condition may result in impairment of vision as well as
spatial disorientation and has been measured 34 hours after alcohol
ingestion, long after there is no measurable alcohol in the blood.
Positional Alcoholic Nystagmus has also been reported 48 hours after
alcohol intake during long duration radial acceleration. Positional
Alcohol Nystagmus has been proposed as the cause of some aviation
accidents where there are no detectable blood alcohol levels.
Alcohol has little direct effect on visual acuity and the information
concerning its effects on colour vision is conflicting.
The maintenance of correct spatial orientation is an important requirement
during flight. Maintaining orientation depends primarily on vision but the
vestibular apparatus and the somatic sensory organs also contribute. The
loss of spatial orientation, called spatial disorientation, can lead to
loss of control of the aircraft and an accident will result unless control
The function of the vestibular apparatus, and its interaction with the
eyes in maintaining correct posture and balance is impaired by alcohol
levels greater than 0.04%. High doses of alcohol retard the suppression of
post-rotatory nystagmus, an important consideration in turning aircraft.
Positional Alcoholic Nystagmus, mentioned above, may play a role in
Impairment, by alcohol or any other agent, of the visual system and the
intimately related vestibular system would cause some degree of pilot
incapacitation, and could lead to spatial disorientation and an aircraft
accident. Alcohol could adversely affect flight safety in this manner.
Impairment of motor skills.
Frequent, coordinated, motor actions are required during routine flying
operations. Usually only slight to moderate forces need be applied but
during aircraft emergency procedures significant physical force may need
to be applied. Fine dextrous movements are also required in the operation
of aircraft radio communication and navigation equipment.
While alcohol has little effect on muscular strength it impairs the
coordination of motor functions. Basic motor coordination tasks such as
standing still, hand steadiness, walking, especially with the eyes closed,
and a variety of sensorimotor tracking/pointing tasks are all impaired by
Impaired coordination during sensorimotor actions could lead to reduced
pilot performance and a reduction in flight safety.
Effects in conjunction with altitude hypoxia.
The hypoxia produced by aviation altitude exposure will subtly or potently
impair pilot performance. The degree of hypoxic impairment varies with the
altitude exposure. The issue of whether alcohol and altitude act to impair
performance in a purely additive manner or whether there is a synergistic
effect is also of concern in the practice of aviation medicine.
It has been traditionally considered that alcohol and altitude hypoxia had
a synergistic effect on performance impairment. A number of mechanisms for
these effects were proposed. Recent work has tended to discount this
hypothesis. It now seems likely that there is little, if any, synergistic
decrease in performance due to alcohol and altitude at less than 12,500
feet and that the rate of alcohol absorption from the gastro-intestinal
tract is not increased by such altitudes.
While alcohol and altitude hypoxia both impair pilot performance it has
not been conclusively shown that their interaction is anything more than
additive in nature.
Alcohol induced hypoglycaemia.
Hypoglycaemia is the state of a lower than normal blood sugar level. When
the blood sugar level is lower than normal performance may be impaired due
to insufficient sugar for the central nervous system to function. Low
blood sugar is not compatible with the safe piloting of an aircraft.
Alcohol ingestion results in a lowering of the blood sugar levels which,
in turn, has lead to at least one fatal aircraft accident.
Performance impairment due to alcohol induced hypoglycaemia is likely to
contribute to a reduction in flying safety.
Tolerance to positive radial acceleration.
In performing a balanced turn in an aircraft the pilot is exposed to a
centrifugal force due to the radial acceleration. This acceleration
results in an increase in his weight. High levels of this acceleration can
result in impairment of vision and even unconsciousness as blood is unable
to reach the eyes and brain. High levels of acceleration are not routinely
experienced in general aviation but may result during aerobatics, steep
turns or emergency manoeuvres, or uncontrolled spiral flight.
Ingestion of alcohol reduces the tolerance of this acceleration. A
‘moderate’ dose will reduce the threshold by 0.1 - 0.4 G (‘G’ = Unit of
Gravitational Acceleration) and will intensify the severity of the
symptoms produced by a given level of acceleration.
Interference with speech based communication.
Speech based communication is an essential component of most types of
flight. A pilot relies on radio communication for traffic and procedural
information, weather and safety warnings, navigational assistance, and
emergency procedures. Any impairment in a pilot’s ability to speak or
understand the spoken word may have a direct effect on aviation safety.
Alcohol ingestion causes alterations of speech including ‘thick, slurred
speech’, ‘difficulty in speech’, ‘repetitive speech’, ‘low, raspy speech’,
and ‘slow, mumbled, and incoherent’ speech. Memory for words, fluency in
their use, and quality of word associations are also impaired by alcohol.
Pilot radio communication has been shown to be impaired by alcohol. This
impairment was found to be greater in older pilots than younger pilots.
Alcohol, in impairing various aspects of speech, will interfere with the
efficiency of aircraft radio communication and intercommunication. This
has the potential for reducing operational safety.
Risk taking behaviour.
The euphoria induced by alcohol as well as the impairment of judgement may
cause a pilot to undertake manoeuvres that he would not undertake while
sober. Attempting such manoeuvres while psychomotor performance is
impaired may lead to an aircraft accident.
Post Alcohol Impairment has been defined as ‘performance impairment after
alcohol is no longer detectable’. This condition is the equivalent of the
lay term of ‘hangover’.
Post Alcohol Impairment has been observed 14 hours after alcohol ingestion
(to 0.08% and 0.1% blood alcohol concentration) in simulated flight tests.
There are other studies that failed to demonstrate any hangover related
There is conflicting evidence and opinion concerning whether or not there
exists any consistent hangover related performance deficit that could
adversely affect aviation safety.
Flying is a complex task requiring continuous and coordinated sensory,
cognitive, and motor functioning by the pilot. Alcohol impairs most
aspects of the flying task. Some flight related skills are adversely
affected by blood alcohol levels as low as 0.025% while aircraft flight
and simulator flight is clearly impaired by levels of 0.04%. Higher blood
alcohol levels result in correspondingly more profound impairment of
flying skills and reduction in flying safety.
The opinion of this author is mirrored by a concluding statement from one
of the studies previously cited - “Any concentration of ethanol in the
living pilot is unacceptable and can contribute to aircraft accidents”