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Fatigue Manifestations (OGHFA BN)

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Article Information
Category: Human Factors Human Factors
Content source: Flight Safety Foundation Flight Safety Foundation
Operator's Guide to Human Factors in Aviation
Human Performance and Limitations
Fatigue Manifestations

Briefing Note


This Briefing Note (BN) explores some of the causes, manifestations and consequences of pilot fatigue. It also outlines the basics of fatigue management and discusses how fatigue management is important to flight safety during both long-range flight (LRF) and short-range flight (SRF).


Pilot fatigue is a concern because it can affect flight safety, efficiency, productivity and personal health. Fatigue is recognized as one of the major factors that can impair human performance and has been cited as a cause of accidents and incidents in the transport industry. It is important for you to understand and recognize the physiological and psychological signs and effects of fatigue. Once you are able to recognize the signs and symptoms of actual or impending fatigue, you can apply proven techniques to avoid its negative outcomes.

There is no universally-agreed-upon definition of what constitutes fatigue. Some describe fatigue in terms of manifestations involving muscular, sensory and mental processes that often result in negative outcomes (e.g., slowed response times, failure to detect stimuli that are discriminable, misinterpretation of sensory inputs). Others discuss fatigue in terms of its underlying causes (e.g., time on task, effort expended, sleep deprivation, circadian disruptions). Ultimately, it is important to understand both the underlying causes and the manifestations of fatigue.

For the purposes of this BN, pilot fatigue refers to decreases in alertness and feeling tired, sleepy and/or exhausted. Fatigue becomes important in aviation when efficiency is reduced or performance impaired. As a pilot, you may suffer fatigue for reasons that are different from “normal” people. The nature of your job involves north/south or east/west travel (or some combination of the two), often for great distances and times. These travel-related factors, combined with workload and scheduling issues, put you at risk for some problems caused by fatigue that most non-pilots never face.

Statistical Data

Lyman and Orlady[1] showed that fatigue was specifically implicated in 77 (3.8%) of 2,006 incidents reported by pilots to NASA’s Aviation Safety Reporting System (ASRS). When their ASRS analysis was expanded to include all factors that could be directly or indirectly linked to fatigue, incidents potentially related to fatigue increased to 426 (21.2%).

Major problems with statistics relating fatigue to accidents and incidents include the lack of a coherent definition of fatigue itself and the absence of a reliable and valid assessment tool to measure it retrospectively. Fatigue is generally difficult to investigate on a systematic basis and to code in databases. Therefore, any statistics related to fatigue and incidents/accidents are likely to be an underestimate and should be interpreted as such.

Also, the outcomes of fatigue reported by pilots, such as the reduction of alertness and lack of concentration, are not unique to sleep loss and are therefore difficult to quantify in analysing accidents or incidents. While the actual extent of pilot fatigue is difficult to measure, its main causal factors can often be assessed as part of an incident or accident investigation. For example: work schedules can be analysed to evaluate potential circadian disturbances caused by night flights or jet lag; sleep loss due to morning or night flights can be determined, and cumulative duty time can be calculated.

Table 1 shows various work schedule features that research has shown are related to circadian disturbances and/or sleep deprivation in the 7 to 15 days prior to an incident or accident.

Table 1: Causes of Circadian Disturbances and Sleep Loss in Flight Operations
Causes of Circadian Disturbances Causes of Sleep Loss
Successive night flights over 3 days or more

High number of time zones crossed (4 to 12)
associated with high layover duration
(3 local nights or more)

Two or more successive transmeridian
rotations in opposite directions

Night flight

Successive early morning flights (more than 2)

Short nights off (finishing late and starting early)

Causes of fatigue

The causes of pilot fatigue for both LRF and SRF are primarily related to sleep quality, sleep loss and the disruption of Circadian Rhythms. Pilots report that night flights and jet lag are the most important factors that generate fatigue in LRF. A common example involves two successive night flights from Paris to New York and back. This duty generally involves 48 hours with a short layover of about 22 hours. In this case, the sleep taken soon after arrival corresponds to a normal sleep period. The poor quality and quantity of this sleep, together with the long period of wakefulness before departure, increases fatigue during the nocturnal return flight.

The quality of your sleep is as important as the quantity. If you are constantly disrupted while sleeping, then the quality of your sleep will be very low, and you will feel as if you only slept for a short period of time even if you slept for many hours. Disruptions can come from environmental causes or from sleep disorders. Common environmental disturbances include noises, lighting and temperature. Sleep disorders can include sleep apnoea, restless leg syndrome or any other disorder that causes you to wake up, even for only a brief period. With most of these conditions, you may be unaware that you awakened, but you will feel the effects.

Another cause of fatigue is alcohol consumption. It is ironic that many people use alcohol as a sleep aid to help them relax and fall asleep. In fact, alcohol can have a profound negative effect on the quality of sleep leading to a fragmentation of sleep and substantial loss of rapid eye movement (REM) sleep. This fragmentation and loss of deep, restful periods of REM sleep can lead to increased fatigue even after many hours of sleeping. It is also important to understand that when you are sleep-deprived and consume alcohol, you will have even greater performance and alertness impairment than would be produced by just the loss of sleep alone.

In SRF, multi-leg flights and early wake-ups are the main causes of fatigue. Most pilots are not able to compensate for early wake-ups by falling asleep earlier. Instead, they usually lose about one hour of sleep. In addition, time constraints, high numbers of legs per day and consecutive work days increase fatigue resulting from SRF operations.

Disruption of circadian rhythms is another cause of fatigue. A circadian rhythm is a cycle in the physiological processes of living beings typically lasting approximately 24 hours. These processes include sleep, body temperature and digestion and are linked to the light-dark cycle of the earth. Traveling north/south and, particularly, east/west can wreak havoc on your natural circadian rhythm and may lead to increased fatigue and sleep problems.

Manifestations of Fatigue

Fatigue can manifest itself both physically and psychologically. Table 2 shows some of the major manifestations of fatigue.

Table 2: Physical and Mental Manifestations of Fatigue
Physical Manifestations Mental Manifestations

General feeling of tiredness

Reduction in vigilance

Growing and irresistible need to sleep

Nodding off/inadvertent napping


Slowed reaction time

Difficulties in memorizing information

Lack of concentration

Periods of inattention

Slow understanding

Tendency to forget information and actions

Bad mood

Poor decisions


Pilots often describe different manifestations of fatigue depending on whether they are talking about themselves or relating how fatigue affects other crew-members. When considering themselves, pilots cite the manifestations of fatigue caused by sleep deprivation as reductions in alertness and attention and a lack of concentration. However, when considering other cockpit crew-members, they frequently focus on mental manifestations such as increased response times and small mistakes, as well as a reduction in social communications and poor message reception. This reaction is not surprising as most people do not like to believe that their performance is degraded but will readily point out when the performance of others has declined.

Fatigue and non-technical skills

Attitudes towards interpersonal conflicts and situational awareness are the non-technical skills most affected by fatigue. When pilots are tired, they generally perform poorly when interpersonal conflicts arise. This poor performance is usually a result of an inability to remain focused on the problem or situation at hand. Often pilots will not listen actively to the ideas and opinions of others, admit mistakes or identify and resolve conflicts.

Loss of situational awareness is related to a lack of vigilance and a reduction in active monitoring, scanning and cross-checking. You may have difficulty in activities such as attending to radio calls, making accurate switch settings, making or listening to altitude call-outs and meeting crossing restrictions. If you lack the essential information normally obtained by performing these activities effectively, you will have difficulty making good judgements and decisions.

Fatigue and error

The various physical and mental manifestations of fatigue ultimately lead to pilot error. When you are tired, the probability of making errors increases particularly in the following situations:

  • Difficult flights (e.g., high-density zone crossing, unfavourable weather, technical problems)
  • During a high density of verbal exchanges (e.g., simultaneous or continuous solicitations from air traffic control, cabin crew and/or cockpit crew-members)
  • Executing actions in a limited amount of time (e.g., cockpit preparation before the flight, system monitoring under time pressure).

Strategies for Fatigue Management

The following section outlines some basic strategies and defenses you can use to manage or avoid fatigue. Following these simple steps can help you avoid costly mistakes due to the physical or mental manifestations of fatigue during both LRF and SRF.

Good sleep strategies

There are some basic yet very important strategies that you can use to improve your sleep and reduce the chance of fatigue causing you problems. Try to do each of these as part of your normal routine:

  • Sleep and wake-up around the same times every day as much as possible
  • Get into a standard pre-sleep routine by doing the same things every night before you go to sleep
  • Do not work, worry or exercise immediately before trying to go to sleep
  • Use the bedroom only for sleep
  • Do not ingest alcohol or caffeine before trying to go to sleep
  • Do not eat or drink too much within several hours of going to bed
  • If you cannot go to sleep in less than 30 minutes, get out of bed.

Fatigue management during long-range flight

Episodes of reduced alertness are often associated with LRF. Use the following preflight and in-flight recommendations for the outbound and return flights to decrease the number of episodes of reduced alertness during long-range flight:

  • During each 24-hour period, try to get as much sleep as you normally would at home during a 24-hour period
  • Use good sleep and nap management before the rotation as a function of
    (i) the timing of the flight and
    (ii) the planned in-flight rest times (when they are known and can be planned in advance, e.g., when flying as part of an augmented crew)
  • If you are about to start working, do not nap for more than 45 minutes as it may take you more time than you have available to recover from the nap and become fully awake and functional
  • For flights as part of an augmented crew, modulate rest time according to the time-frame of the flight. For example, plan a longer rest period or divide your rest into two or three periods spread out over the course of the flight if the flight schedule is unfavourable for sleep.
  • Do not eat at the same time as the other crew-members. Instead, alternate the timing of meals
  • Manage in-flight activities to reduce monotony in the cockpit and to optimize in-flight rest.

One good way to ensure that at least one pilot is not suffering from fatigue is to alternate activity and rest between the flight crew-members. This is best accomplished by alternating passive and active vigilance phases every 20 to 40 minutes with formal handovers at the end of each period. Active vigilance phases are characterized by verbal exchanges, tasks related to flight management and varied motor activities associated with mental tasks, e.g., navigation and system management. Do not eat during an active vigilance phase.

Passive vigilance phases are characterized by a lower level of direct supervision of the flight. Meals or snacks can be eaten, but preferably at the start of these phases. Activities not related to the flight are encouraged during passive vigilance phases, e.g., reading papers. During the night, or if you are sleep-deprived, it is recommended that you make use of the passive vigilance phases to take a nap of 20 to 40 minutes.

The alternation between active and passive vigilance phases is most appropriate during quiet cruise periods. Remember to announce the end of each active-passive vigilance phase to your fellow crew-member(s) so that they will be sure to know which phase they are expected to be in.

Some rules for short-haul scheduling

Your fatigue level in short-haul operations is affected by a combination of flight schedule and the number of legs you fly in a day. On the whole, morning flights induce more fatigue than afternoon flights or alternating morning and afternoon duties. Adhering to the following rules regarding duty-roster scheduling and work organization can help avoid SRF fatigue problems:

  • Limit the number of successive morning rosters (maximum two days)
  • Limit the number of successive night rosters (maximum three days)
  • Alternate morning and afternoon roster instead of using all morning rosters (e.g., two days of consecutive morning duties and two of afternoon)
  • Avoid short nights off that finish late and start early
  • Reduce the number of legs in the most difficult roster and in particular for the last day of the roster
  • Avoid short stopovers (less than 35 minutes). Short stopovers do not take into account unexpected events or the need for the aircrew to have a break

Summary of Key Points

  • Pilot fatigue is reflective of the effects of pilot work schedules and the nature of the job, which includes north/south and east/west travel. Specific factors such as night flights, jet lag and successive early wake-ups contribute to increased fatigue. For SRF, time pressure, number of legs per day and consecutive days on duty can contribute to increased fatigue. In LRF, disruption of circadian rhythms and loss of sleep are the main concerns.
  • You should be aware of the signs of fatigue, which include a reduction in alertness and attention, lack of concentration, increased response times, small mistakes, a reduction of social communications, and poor comprehension.
  • You can use simple strategies such as developing a good sleep routine, only using the bedroom for sleep, not ingesting alcohol or caffeine and not worrying or working prior to going to sleep to improve sleep and reduce fatigue.
  • You can also self-manage fatigue in the cockpit during LRF by desynchronizing activities so that one pilot rests while the other is vigilant. Organizations can reduce fatigue during SRF by good management of rosters and avoiding short stopovers.


  1. ^ Lyman, E. G., Orlady, H. W. - Fatigue and Associated Performance Decrements in Air Transport Operations. NASA Ames Research Center, Battelle Columbus Laboratories, ASRS Office. 1980

Associated OGHFA Material

Briefing Notes


Situational Examples

Additional Reading Material

  • Bourgeois-Bourgrine S., Cabon P., Gounelle C., Mollard R., Coblentz A. - Perceived Fatigue for Short- and Long-Haul Flights: A Survey of 739 Airline Pilots.- Aviation, Space, and Environmental Medicine, vol. 74, n° 10, 2003, pp. 1072-1077.
  • Bourgeois-Bourgrine S., Cabon P., Gounelle C., Mollard R., Coblentz A., Speyer J.J.- Fatigue in aircrew from short-haul flights in civil aviation: the effects of work schedules.- Human Factors and Aerospace Safety: An International Journal, vol. 3, n° 2, 2003, pp. 177-187.
  • Cabon P., Bourgeois-Bougrine S., Mollard R., Coblentz A., Speyer J.J. - Flight and Duty Time Limitations in Civil Aviation and Their Impact on Crew Fatigue: A comparative Analysis of 26 National Regulations.- Human Factors and Aerospace Safety: An International Journal, vol. 2, n° 4, 2002, pp. 379-393.
  • Cabon, P., et al. - Coping with long range flying. Recommendations for crew rest and alertness., Airbus, Nov 1995.

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