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11.13.4 Human Factor Considerations


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The aviation industry is a 24-hour activity in order to meet the demands of the modern world. Flight crew are required to support this 24 hour operation. With the demands of both long and short haul operations fatigue in aviation is recognised as a serious safety concern. Fatigue and lack of sleep may not be apparent to a pilot until serious errors are made.

Pilots routinely experience fatigue throughout their aviation careers and many crewmembers consider it an occupational hazard. Commercial pressure is ever increasing and these demands with the ever present "press on itis" quickly fatigue even the most fit pilots. Sleep is a real concern and this section outlines:

  • How fatigue occurs.
  • How to help combat fatigue.
  • Sleep and sleep disorders.
The Danger of Fatigue

Fatigue is a danger to both the long haul and the short haul pilot. Because of its insidious nature an individual does not initially feel the onset of fatigue. A fatigued pilot may not be aware of the gradual and cumulative effect and consequently, may be unaware that their performance has become degraded. Because of the slow onset the pilot may not recognize the degradation of his performance.

A fatigued pilot loses the ability to self criticize and is more willing to accept inaccurate flying and poor judgment. As fatigue increases, decision making skills are lost with a slowing down of the whole thinking process. Information may have to be checked and checked again because of these problems. Reaction time is increased, irritability and mood swings easily block communication and hamper teamwork.

Apathy eventually sets in and the fatigued pilot becomes indifferent to the outcome of the flight and the operational performance.

Where a person goes without sleep for up to 24 hours the effects are similar to those of having up to 8 units of alcohol.

Vigilance Effects

Any task that requires vigilance is suspect to the effects of fatigue.

Fatigue can be described as:

  • Short Term Fatigue (Acute): The effects of normal everyday living. Acute fatigue is the tiredness a person feels after physical or mental strain. Co-ordination and alertness become dulled and performance reduced. Good rest and sleep combined with proper nutrition and exercise prevent acute fatigue.
  • Long Term Fatigue (Chronic): If sufficient recovery time is not allowed between bouts of acute fatigue then chronic fatigue may occur. The only recovery is a prolonged period of rest. During chronic fatigue performance and judgement can lower to a dangerous level.
Causes of Pilot Fatigue

Pilot fatigue is normally caused by:

  • Circadian Dysrhythmia - Jet Lag.
  • Short haul rostering - multi leg flying days.
  • Poor rostering - long standby periods, long duty periods.
  • Sleep lost because of domestic worries.

Other factors do have an effect, but the above may be taken as the main causes.

Symptoms of Pilot Fatigue

We know when we are fatigued, but can we recognize it in others.

A few symptoms are listed below:

  • Slow reaction time, both physically and mentally.
  • Errors becomes the norm.
  • Lack of self criticism.
  • Fixation on a single source of information or task.
  • Short-term memory loss.
  • Impaired judgment leading to poor Decision Making.
  • Distracted easily from the main task.
  • Inaccurate flying.
  • Lethargic.
  • Limited Situational Awareness..
  • Poor communication skills.

Coffee, concentration or will power do not get rid of fatigue. They may delay the onset but the normal result is one of worsening the effect.

Sleep and Sleep Deprivation

Chronobiology is the scientific name for the study of biorhythms. The human body follows certain biological rhythms some of which have a period of 24 hours, these are termed Circadian Rhythms (Latin: Circa - about, dies - a day). Other biorhythms, however, display different periods eg., the female menstrual cycle - 28 days, children's rest/activity cycles - 90 minutes.

One of the most studied of Circadian Rhythms, that is useful to pilots, is the sleep/wake rhythm. The body's temperature is approximated at 37°C. During a 24 hour period it cycles between 36.2°C and 36.9°. The sleep/wake cycle is bound to this change in body temperature:

  • When the temperature is rising the body is waking.
  • When the temperature is falling the body is ready to sleep.

In the diagram below the time of minimum temperature and maximum temperature are annotated on the body temperature cycle. Note that there is a dip in the temperature after the lunch time period.

This sleep/wake cycle is controlled by the body's internal clock. In America deprivation tests were carried out on an individual in a room with no time clues such as light or dark. Initially allowed a clock the individual was asked to rise at 9 am each day. After 3 days the clock was removed. Initially the person woke at 9 am. On subsequent days the waking time was delayed by one hour - on day 4 waking at 10 am, day 5 at 11 am etc.

The circadian rhythm of the body has adjusted to 25 hours, a condition known as free run. The body is contracted into a 24 hour day by the constraints of our working lives. We react to the night and day and to other time clues known as Zeitgebers (German for time giver).

The problems of the body temperature cycle and the circadian rhythms do have an affect on the pilot.

Sleep Credit/Deficit

Using a simple system of allowing +2 points for every hour asleep and -1 point for every hour awake we can show how easy it is for the body to go into sleep debt. The system is not infallible, as it does not take into account:

  • The type of sleep.
  • Whether the person is on long haul flight.

During a normal night if we assume that an individual has 8 hours sleep then when they wake they will have amassed +16 points. If that individual then stays awake for 16 hours then they will lose -16 points. Thus finishing the day with 0 points. On subsequent days the same occurs and the points score never goes below the 0 line - the individual is in sleep credit.

Now assume that the cycle is broken by a period on night shift.

Assuming a normal nights sleep:

  • The individual wakes with +16 sleep points (A).
  • Assume they stay awake for 8 hours (B) and from 1600- 1800 hrs manage to get a two-hour nap (C) before reporting for duty.
  • Duty starts at 2000 hrs through to 0600 hrs.
  • At 0600 hrs they return home but are unable to sleep because the body temperature is rising.
  • Finally they sleep at 1200 hrs (D). They sleep until 1800 hrs (E) when it is time to prepare for work
  • The cycle repeats itself and the person gets further and further into sleep deficit. This situation is termed cumulative sleep debt.

By the time the third shift is started the sleep credit is 0 before they start work. Now translate this to the pilot on a flight in the early hours of the morning with little to do but monitor the autopilot. Most pilots use coffee to stop sleep but this is only a short term measure.


The precise functions of sleep are not fully understood. Experiments have shown that sleep has a restorative function for both the body and the mind. Sleep has been investigated extensively over the last sixty years and much is known of its nature.

In experiments three main measurements are recorded:

  • Brain wave activity: EEG (electroencephalogram)
  • Eye movement: EOG (electroculogram)
  • Muscle tension: EMG (electromyogram)

The recording of these three measurements has shown scientists that the body initially goes through four linked stages of sleep termed quiet sleep. During these 4 stages there is a gradual slowing of the brain's activity as the body goes into a deeper sleep. Stages 3 and 4 of quiet sleep are known as slow-wave sleep because the EEG records little or no brain activity.

Following the quiet sleep is another stage of sleep called REM (rapid eye movement) sleep.

Also known as Paradoxical sleep. During this sleep:

  • The EEG records similar brainwaves to those recorded when someone is awake.
  • The EOG records rapid eye movements (hence REM sleep) as if searching for something.
  • The EMG records total muscle relaxation; the mind is awake but the body is asleep - hence the term paradoxical sleep.

Quiet sleep is thought to be body restorative and that REM sleep allows the brain to store what has been learned during that day and also to check and create new neural pathways. The evidence supporting this theory is that children (and adults who are in learning situations) show a higher REM sleep percentage than average.

Each full cycle of sleep takes approximately 90 minutes, successive cycles showing increasing amounts of REM sleep. In general, during a normal 8 hours sleep, the first four hours are mainly slow-wave sleep (body-restorative) and the latter four hours mainly REM sleep (mind-restorative). If the first four hours (slow-wave) or the second four hours (REM) sleep are interrupted then both are made up on the next night.

Dreams occur mainly in REM sleep but sleepwalking and nightmares occur in slow-wave sleep, hence, people remember dreams but not sleepwalking.

Alcohol affects both Quiet Sleep and REM sleep.

  • Moderate amounts affect REM sleep.
  • Large amounts affect both types because of the coma like affect of alcohol.
Sleep Disorders

The common sleep disorders include:

  • Microsleep.
  • Insomnia.
  • Sleepwalking and sleeptalking.
  • Sleep Apnoea.
  • Narcolepsy.
Sleep Loss and Microsleep

Any loss of sleep will begin a sleep debt. Fitful sleep overnight may also produce the same effect. Sleep debt is only cured by sleep.

Sleep debt and fatigue may lead to what is termed a microsleep.

Microsleeps are uncontrolled spontaneous episodes of sleep that last for a few seconds up to a few minutes. During a microsleep a person becomes detached from reality and will be unresponsive to outside influences.


Insomnia can be divided into two types:

Clinical Insomnia

Clinical Insomnia is suffered by people who are unable to sleep even in the most favourable conditions. Symptoms include:

  • Difficulty in falling asleep.
  • Difficulty in maintaining sleep.
  • Waking unrefreshed.
  • Daytime fatigue.
  • Irritability.
  • Lack of concentration.
Situational Insomnia

Aircrew can suffer situational insomnia; it is an inability to sleep due to irregular work/rest patterns. Jet lag being the most common cause.

Sleepwalking and Sleeptalking

Common in childhood and less common but present in some adults are sleepwalking (somnambulism) and sleeptalking (somniloquism). Neither are a health hazard but excessive cases need investigation.

Sleep Apnoea

A condition which affects people who snore excessively, especially those who are overweight. During sleep the snoring affects the back passages of the throat and air can be cut off from the lungs for a short period. The person effectively stops breathing. In extreme cases a person could die. The condition is treated by the use of a mask which the sufferer wears during the night. Air is passed through the mask by a compressor which ensures a positive pressure in the throat at all times.


The inability to stay awake. Sufferers have the tendency to fall asleep at any time whether they are tired or not.

Sleep Hygiene

Individuals require differing amounts of sleep. The older you are the less sleep you require. People in learning situations do require a regular sleep pattern. When studying the pressures are such that late night study or worry can disrupt the sleep pattern. But a few helpful hints are given below:

  • No strenuous exercise immediately before going to bed. This means no physical or mental exercise.
  • A high level of study activity should be avoided immediately before trying to sleep, rest for at least 30 minutes before going to bed
  • Keep the room ventilated - not too warm, not too cold.
  • Do not drink too much alcohol. Alcohol induces a coma like sleep where there is no body refreshment.
  • Try a warm milky drink - NOT COFFEE OR TEA.
  • Light reading or listening to music can help relax the mind and body.

Most people feel tired during their waking hours. Napping is a way of refreshing the body quickly and efficiently. In experiments it has been shown that a short nap can be as restorative as a longer period of sleep.


To sleep or to stay awake some pilots will resort to drugs. To stay awake the most common drug used is Caffeine; the antidote for sleepiness being a strong, black coffee. To relax and sleep alcohol is used.

  • Caffeine: The harmful and addictive effects of excessive caffeine should be noted and are well-documented.
  • Alcohol: Alcohol is a central nervous system depressant which interferes with sleep, particularly REM sleep. In small amounts alcohol does promote well-being and can relieve stress and promote relaxation. But like all drugs addiction can bring problems.
Sleeping Tablets

With normal medication, cold and flu remedies induce drowsiness because of their nervous system depressant action. Some drugs can remain in the system for hours and affect performance the next day. The half-life of a drug is an important factor that pilots must take account of. The half-life of a drug is the time it takes for a drug to decay to one half of its peak concentration. Half-life figures are not available publicly.

In order to ensure that any drug you are using is safe always consult a GP before use. Sleeping tablets have a long half-life and can affect a waking person for a few hours after rising out of bed. Newer sleeping drugs are always coming onto the market and before use a doctor should be consulted. Contrary to a lot of opinion, sleeping tablets are only meant as a short term sleeping problem fix.


Melatonin is a depressant currently being promoted in some countries as a natural hormone, which allegedly induces sleep in shift workers or elderly people. The drug is now marketed as an aid to combat Jet-Lag. Melatonin is not legal in several countries and is certainly not recommended for use by pilots. There are problems with quality control, potency and monitoring of this supplement. The biological effects and the long-term use of Melatonin are not known.

Circadian Dysrhythmia - Jet Lag

Abnormal shift work can result in a cumulative sleep debt. Longhaul pilots have the added problem of their body adjusting to new time zones. New Zeitgebers confuse the body ie new light/dark, new meal cues as time zones are crossed. For the long haul pilot it is better to be travelling westwards than eastwards. Westwards travel involves a lengthening of the day and the Circadian Rhythm is better at lengthening its cycle than shortening it. Remember that the body rhythms free-run at 25 hours.

The body's Circadian Rhythms adjust at different rates at between 1 - 1½ hours per day for every hour's difference in time zone. The result of this slow resynchronization is that you may find yourself beginning the next leg of your flight before your biorhythms have resynchronized. It is estimated that some long haul pilots spend their entire flying careers suffering from Circadian Dysrhythmia and only adjust fully when on extended periods of leave or illness.

Two methods are suggested to combat Jet Lag.

  • Method 1: Stay awake for 2 hours after landing; then rest/sleep for 4 hours; then sleep for 8 hours before reporting for duty.
  • Method 2: If the lay over is less than 24 hours then remain on the original time zone cues.

The effects of Circadian Dysrhythmia are well known and include:

  • A general lack of well-being.
  • Below par performance.
  • Stomach disorders.