the human component in
general aviation accidents
David Bryman, D.O Senior Aviation Medical Examiner FAA, Transport Canada,
pilots are familiar with the term “Human Factors”. We have heard this
term many times in reference to NTSB reports when discussing the cause of
an aircraft accident.
Although the flying machine may fail us occasionally, it is the human
component that is the cause of aviation accidents more than 70% of the
time. I’m not referring to aeromedical factors which involve a medical
condition that leads to a sudden pilot incapacitation, such as stroke,
heart attack, seizures, and so on. Fortunately, these account for less
than 3% of accidents.
The human factors I’m referring to include more common conditions, many
of which are “self-induced”, such as: Fatigue, hypoglycemia, illness,
noise, hypoxia, dehydration, vibration, visual illusions, jet lag,
disorientation, alcohol, smoking, and self medication (to name only a
few). These conditions make the pilot function less efficiently, causing
the dreaded “link” in the accident chain of events. I would like to
review just a few of the more common human factors that we see involved
in accidents in a little more detail.
Fatigue is one of the most common complaints that a physician hears about
in the medical office. It has been mentioned as a factor in so many
aviation accidents that the airlines have developed “counter fatigue”
programs for their pilots. Fatigue’s presentation, like hypoxemia, is
usually insidious. Symptoms are not immediately recognizable, so the
pilot must self-monitor.
Some of the symptoms of fatigue include: increased reaction time,
channelized focus, fixation, short-term memory loss, impaired judgment,
and poor decisions. The pilot may be easily distracted, have decreased
visual perception, and overall sloppy flying. Fatigue’s onset may be
acute or chronic. Acute onset is typically benign and related to a recent
event. Chronic fatigue is more serious and is associated with an
increased risk of infections and increased vulnerability to adverse
effects of stress. When fatigue is chronic, the pilot may experience a
personality change, be irritable, and be short-tempered with the flight
crew. Chronic fatigue from lack of productive sleep can ultimately lead
Fatigue leads to a pronounced impairment of performance, decreased
motivation, decreased alertness, and a tendency toward “micro sleep” with
no awareness that you were asleep. Small errors and mistakes are
Many factors influence a pilot’s ability to cope with fatigue’s effects.
One of the most logical factors is age. It would not be surprising to
learn that a 25-year-old pilot will tolerate fatigue differently than a
58-year-old pilot. Heading west is easier and less fatiguing than heading
east, as you are traveling with the sun. Also, it should be noted that a
change in one’s circadian rhythm causes the greatest chance for pilot
error between 4:00am-6:00am on the pilot’s home rhythm.
Diet may have an effect on fatigue as well. For example, a diet high in
carbohydrate may have a sedative effect, whereas a high protein meal
would have the opposite effect. If the goal is to stay awake, the pilot
may do better with the high protein bar rather than the high carbohydrate
Another human factor that affects performance and flight safety has to do
with alcohol and tobacco use. Most pilots understand the adverse effects
of hypoxia; however few pilots realize the association between hypoxia
and smoking or alcohol use prior to flight. It has been shown that one
ounce of alcohol translates into an added 2,000 feet of altitude if
consumed prior to flying. This is known as the physiologic altitude, the
altitude that the pilot “feels” like he is at.
Alcohol interferes with the ability of the cells in the body to use
delivered oxygen. . In regards to alcohol and aviation accidents, a
recent study has shown that pilots with DWI convictions were about 3.5
times more likely than pilots without convictions to have alcohol-related
general aviation accidents. (DWI Convictions Linked to a Higher Risk of
Alcohol-Related Aircraft Accidents Authors: K.L. McFadden Reference:
Human Factors, 2002, Vol. 44, pp. 522-529). The “8hr bottle to throttle”
is inadequate as far as the human physiology is concerned. The presence
of any alcohol in the body may be detrimental to the pilot’s performance.
In regards to nicotine, if a pilot were to smoke 3 cigarettes quickly or
20 cigarettes in the prior 24 hrs, their physiologic altitude would make
it feel as if they were 3-5000ft higher altitude then they are. This is
due to the adverse effect of carbon monoxide in the ability of hemoglobin
to carry oxygen to the cells. The adverse effects of smoking have been
well documented over the years. However, what about the pilot that is
quitting tobacco use? Could there be any risk to flight safety, as far as
symptoms of nicotine withdrawal?
A recent published study revealed that “abrupt cessation of smoking may
be detrimental to flight safety and the smoking withdrawal syndrome may
influence flying parameters”.
(According to G. Giannakoulas, A. Katramadous, N. Melas, I.
Diamantopoulos, and E. Chimonas in an article published by Aviation,
Space, and Environ. Medicine 2003, Vol. 74, pp. 247-251 Acute Effects of
Nicotine Withdrawal Syndrome in Pilots during Flight)
The above study revealed that “the most frequent symptoms reported during
nicotine deprivation were nervousness, craving for tobacco,
tension-anxiety, fatigue, difficulty in concentration, decrease in
alertness, disorders of fine adjustments, prolonged reaction times,
anger-irritability, drowsiness, increase in appetite, and impairment of
judgment. Systolic BP and heart rate tended to decrease and diastolic BP
tended to rise during withdrawal, although the differences were not
statistically significant. Finally, all tests recorded an impairment of
cognitive functions during abstinence”. It is a great idea to quit
smoking, but one must be aware of the symptoms of nicotine withdrawal and
seek medical help if needed (caution- many medications to help with these
symptoms are not FAA approved)
When it comes to medication, or more importantly “self-medication”, one
must consider that all medication have an effect (desired) and a side
effect (usually un-desired). For example taking diphenhydramine
(Benadryl) for allergies. This chemical is a very common ingredient found
in a variety of allergy and cold meds. The use by a pilot is not
permitted prior to flying as the adverse effects include significant
drowsiness, dry mouth and dizziness.
There are numerous over the counter medications that would render the
pilot un- fit for flight. Occasionally, medications are studied under
driving conditions, and very rarely have been tested in the flight
environment. When it comes to taking any medication as a pilot, the best
advice would be to discuss it with your AME.
Even though the safety record of general aviation is improving over
recent years the same accidents seem to occur year after year. These
accidents include pilots that fly perfectly good airplanes into terrain (CFIT)
due to lost situational awareness and of course pilots that succumb to
disorientation when flying VFR into IMC. Unfortunately, poor decision
making skills still exist when it comes to scud running, flying in poor
weather and in reduced visibility.
If we are to decrease accidents in general aviation, it is absolutely
necessary to better educate pilots in the physical limitations of the
human component. General aviation safety may improve if we ask ourselves
a few questions prior to flight. Is the airplane airworthy? Am I properly
trained and current for this flight? Am I physically fit to fly today?