Fatigue is the general term used to describe physical and/or mental weariness which extends beyond normal tiredness.
Physical fatigue concerns the inability to exert force with ones muscles to the degree that would be expected. It may be an overall tiredness of the whole body, or be confined to particular muscle groups. Physical fatigue most commonly results from physical exercise or loss of sleep. Physical fatigue often leads to mental fatigue.
Mental fatigue, which may include sleepiness, concerns a general decrease of attention and ability to perform complex, or even quite simple tasks with customary efficiency. Mental fatigue often results from loss or interruption of the normal sleep pattern and is therefore of great concern to pilots and ATCOs, who are frequently required to work early in the morning or at night.
Sleep patterns are naturally associated with the body's circadian rhythms. Shift patterns and transit across time zones can interrupt circadian rhythms so that, for example, it may be difficult for flight crew or pilots on duty in the early hours of the morning or flight crew operating long-haul routes through multiple time zones to achieve satisfactory rest prior to commencing duty.
It is important to note that people are not the best evaluators of their own alertness state. They are often sleepier than they report.
There are three types of fatigue: transient, cumulative, and circadian:
- Transient fatigue is acute fatigue brought on by extreme sleep restriction or extended hours awake within 1 or 2 days.
- Cumulative fatigue is fatigue brought on by repeated mild sleep restriction or extended hours awake across a series of days.
- Circadian fatigue refers to the reduced performance during nighttime hours, particularly during an individual’s “window of circadian low” (WOCL) (typically between 2:00 a.m. and 05:59 a.m.).
Researches show that the accumulation of "sleep debt", e.g. by having an hour less of sleep for several consecutive days needs a series of days with more-than-usual sleep for a person to fully recover from cumulative fatigue.
Fatigue usually results in impaired standards of operation with increased likeliness of error. For example:
- Increased reaction time;
- Reduced attentiveness;
- Impaired memory; and,
- Withdrawn mood.
In a pilot, fatigue may manifest itself by:
- Inaccurate flying;
- Missed radio calls;
- Symptoms of equipment malfunctions being missed;
- Routine tasks being performed inaccurately or even forgotten; and, in extreme cases,
- Falling asleep - either a short "micro-sleep" or for a longer period.
In an air traffic controller, fatigue may result in:
Fatigue is also a signficant risk among aviation maintenance personnel due to nighttime work, rotating shift work,and long, unregulated duty times, among other factors.
- Circadian adaptation, i.e. adjustment of the body internal clock (e.g. due to the shift pattern, jet lag, etc.)
- Length of previous rest period;
- Time on duty;
- Time awake prior to duty (duties that start in the evening are more likely to cause fatigue than those beginning at e.g. 8 a.m.)
- Sleep/nap opportunities (during the duty but also at layover destinations)
- Physical conditions (temperature, airlessness, noise, comfort, etc.);
- Workload (high or low);
- Emotional stress (in family life or at work);
- Lifestyle (including sleeping, eating, drinking and smoking habits) and fitness; and,
- Ensure that work schedules, including consecutive shift-working patterns, are constructed so as to have the least possible impact on off duty - and, if applicable, on duty rest.
- Seek to provide optimum working conditions;
- Use Crew Resource Management (CRM) or Team Resource Management (TRM) training to promote awareness to fatigue and sleep issues.
- Establish a Fatigue Risk Management System (FRMS), either as a part of the Safety Management System (SMS) or as a standalone system. An effective FRMS is data-driven and routinely collects and analyzes information and reports related to crew alertness as well as operational flight performance data. Computer models can be used to predict average performance capability from sleep/wake history and normal circadian rhythms.
Pilots and ATCOs
Adopt personal strategies which are likely to decrease the effects of fatigue such as the following:
- Planning activities, meals, rest and sleep patterns during off-duty periods;
- Making the most of permitted rest breaks, including naps;
- Advising colleagues if one detects feeling drowsy;
- Alerting colleagues if they appear to be becoming drowsy.
Accidents & Incidents
Events in the SKYbrary database which include fatigue as a contributory factor:
On 8 June 2022, a Boeing 757-200 making a night visual approach to Tulsa inadvertently landed on runway 18R instead of 18L as pre-briefed and cleared. ATC did not intervene and neither pilot realised the error until the Captain realised that having intentionally landed long because the turn off was at the end of the much longer 18R there was less runway ahead than he had expected. Although both pilots reported not being fatigued, it was concluded that lack of recognition of their error suggested otherwise and probably facilitated plan continuation bias aided by inability to efficiently integrate available information.
On 17 January 2018, two Airbus A320s both inbound to Surabaya at night were vectored to the same waypoint to hold, one at FL210 and the other at FL200 but the one initially given FL210 was then re-cleared to also descend to FL200. The two aircraft subsequently received and followed coordinated TCAS RAs which restored prescribed separation. The Investigation found that before the conflict, the experienced controller involved had made several transmissions to aircraft other than the intended ones and noted that the usually-available ATC conflict alerting system had been temporarily out of service without any consequent mitigations in place.
On 4 March 2019, an Embraer 145 attempting to land off an ILS approach at Presque Isle in procedure-minima weather conditions flew an unsuccessful first approach and a second in similar conditions which ended in a crash landing abeam the intended landing runway substantially damaging the aircraft. The accident was attributed to the crew decision to continue below the applicable minima without acquiring the required visual reference and noted that the ILS localiser had not been aligned with the runway extended centreline and that a recent crew report of this fault had not been promptly passed to the same Operator.
On 6 January 2018, a Boeing 737-900 and an Airbus A320 both inbound to Surabaya with similar estimated arrival times were cleared to hold at the same waypoint at FL100 and FL110 respectively but separation was lost when the A320 continued below FL110. Proximity was limited to 1.9nm laterally and 600 feet vertically following correct responses to coordinated TCAS RAs. The Investigation found that all clearances / readbacks had been correct but that the A320 crew had set FL100 instead of their FL110 clearance and attributed this to diminished performance due to the passive distraction of one of the pilots.
On 29 October 2019, an Airbus A321 was descending towards its destination, Kaohsiung, when the First Officer suddenly lost consciousness without warning. The Captain declared a MAYDAY and with cabin crew assistance, he was secured clear of the flight controls and given oxygen which appeared beneficial. He was then removed to the passenger cabin where a doctor recommended continuing oxygen treatment. On arrival, he had fully regained consciousness. Medical examination and tests both on arrival and subsequently were unable to identify a cause although a context of cumulative fatigue was considered likely after three consecutive nights of inadequate sleep.
On 28 November 2020, a Boeing 737-300F taxiing for an early morning departure at Singapore Changi crossed an illuminated red stop bar in daylight and entered the active runway triggering an alert which enabled the controller to instruct the aircraft to immediately exit the runway and allow another aircraft already on approach to land. The Investigation found that the flight was the final one of a sequence of six carried out largely overnight as an extended duty predicated on an augmented crew. The context for the crew error was identified as a poorly managed operator subject to insufficient regulatory oversight.
On 15 August 2016, the cognitive condition of an Airbus A320 Captain deteriorated en-route to Riga and he assigned all flight tasks to the First Officer. When his condition deteriorated further, an off duty company First Officer travelling as a passenger was invited to occupy the flight deck supernumerary crew seat to assist. Once descent had commenced, the Captain and assisting First Officer swapped seats and the flight was thereafter completed without any further significant event. The Investigation concluded that the Captain’s serious physical and mental exhaustion had been the result of the combined effect of chronic fatigue and stress.
On 7 November 2018, a Boeing 747-400F overran wet landing runway 14 at Halifax at night and was sufficiently damaged as a result of exceeding the available RESA to render it a hull loss. The Investigation attributed the overrun to a combination of factors including use of un-factored landing distance, momentary mishandling of the thrust levers just after touchdown, a pilot-caused lateral deviation diverting attention from deceleration, inadequate braking and late recognition of an approach tailwind component. Poor NOTAM presentation of runway availability also led the crew to believe that the longer and more suitable runway 25 was not available.
On 23 February 2018, an Embraer 195LR and an Airbus A320 on SIDs departing Brussels lost separation after the 195 was given a radar heading to resolve a perceived third aircraft conflict which led to loss of separation between the two departing aircraft. STCA and coordinated TCAS RA activations followed but only one TCAS RA was followed and the estimated minimum separation was 400 feet vertically when 1.36 nm apart. The Investigation found that conflict followed an error by an OJTI-supervised trainee controller receiving extended revalidation training despite gaining his licence and having almost 10 years similar experience in Latvia.
On 28 April 2017, a Boeing 777-300 made a 3.2g manual landing at Hong Kong, which was not assessed as such by the crew and only discovered during routine flight data analysis, during a Final Line Check flight for a trainee Captain. The Investigation noted that the landing technique used was one of the reasons the Check was failed. The trainee had been an experienced 737 Captain with the operator who had returned from 777 type conversion training with another airline and was required to undertake line training to validate his command status in accordance with local requirements.
On 16 August 2008, an AMC Airlines Boeing 737-800 inadvertently began a night take off from an intersection on runway 27L at Paris CDG which left insufficient take off distance available before the end of the temporarily restricted runway length. It collided with and damaged obstructions related to construction works in progress on the closed section of the runway but sustained only minor damage and completed the intended flight to Luxor. The context for the flight crew error was identified as inadequate support from the Operator and inadequate airport risk assessment for operations with a reduced runway length.
On 16 October 2012, a Brit Air Bombardier CRJ 700 landed long on a wet runway at Lorient and overran the runway. The aircraft sustained significant damage but none of the occupants were injured. The Investigation attributed the accident to poor decision making by the crew whilst showing signs of complacency and fatigue and failing to maintain a sterile flight deck or go around when the approach became unstable. A context of deficiencies at the airport and at the Operator was also detailed and it was concluded that aquaplaning had occurred.
On 31 July 2008, the crew of an HS125-800 attempted to reject a landing at Owatonna MN after a prior deployment of the lift dumping system but their aircraft overran the runway then briefly became airborne before crashing. The aircraft was destroyed and all 8 occupants were killed. The Investigation attributed the accident to poor crew judgement and general cockpit indiscipline in the presence of some fatigue and also considered that it was partly consequent upon the absence of any regulatory requirement for either pilot CRM training or operator SOP specification for the type of small aircraft operation being undertaken.
On 6 July 2013, an Asiana Boeing 777-200 descended below the visual glidepath on short finals at San Francisco after the pilots failed to notice that their actions had reduced thrust to idle. Upon late recognition that the aircraft was too low and slow, they were unable to recover before the aircraft hit the sea wall and the tail detached. Control was lost and the fuselage eventually hit the ground. A few occupants were ejected at impact but most managed to evacuate subsequently and before fire took hold. The Probable Cause of the accident was determined to be the mismanagement of the aircraft by the pilots.
On 20 August 2011, a RAF Aerobatic Team Hawk failed to complete a formation break to land near Bournemouth and the aircraft flew into the ground, destroying the aircraft and killing the pilot. The subsequent Inquiry concluded that the pilot had become semi conscious as the result of the sudden onset of G-induced impairment characterised as A-LOC. It was found that the manoeuvre as flown was not radically different to usual and that the context for the accident was to be found in a range of organisational failures in risk management.
- Fighting Pilot Fatigue, video by Boeing’s Fatigue Risk Management team in partnership with Delta airlines to portray the effects of fatigue on pilots. It describes technologies in the flight deck that can monitor and prevent fatigue-related events.
- Operator's Manual: Human Factors in Aviation Maintenance, FAA, 2014.
- Safety Behaviours: Human Factors Resource Guide for Engineers, CASA (Australia), 2013. Chapter 5 describes the causes and impacts of fatigue, and strategies to manage it (for maintenance personnel).
- Human Performance and Fatigue Research for Controllers, Gawron et al., 2011.
- FAA Fact Sheet – Sleep Apnea in Aviation, Feb 2015.
- Coping with long range flying. Recommendations for crew rest and alertness., Airbus, Cabon, P., et al., Nov 1995.
- Fatigue and Alertness Management in Aviation
- Being Prepared for the Outbound Flight - Checklist
- Being Prepared for the Return Flight in Eastward Rotations - Checklist
- Being Prepared for the Return Flight in North and South Rotations - Checklist
- Being Prepared for the Return Flight in Westward Rotations - Checklist
- CASA Fatigue Risk Management System Handbook, 2013
- CANSO Fatigue Management Expert Group