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Fatigue

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Category: Aeromedical Aeromedical
Content source: SKYbrary About SKYbrary
Content control: SKYbrary About SKYbrary

Description

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.

Fatigue Types

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.

Hazards

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.

Typical Scenarios

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 ATCO, fatigue may result in:

  • Slow reaction to changing situation;
  • Failure to notice an impending confliction;
  • Forgetfulness.

Contributory 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,
  • Health.

Solutions

Employers:

  • 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;
  • 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.

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Related Articles

Accidents & Incidents

Events in the SKYbrary database which include fatigue as a contributory factor:

  • A320 / B789 / A343, San Francisco CA USA, 2017 (On 7 July 2017 the crew of an Airbus A320, cleared for an approach and landing on runway 28R at San Francisco in night VMC, lined up for the visual approach for which it had been cleared on the occupied parallel taxiway instead of the runway extended centreline and only commenced a go-around at the very last minute, having descended to about 60 feet agl whilst flying over two of the four aircraft on the taxiway. The Investigation determined that the sole direct cause of the event was the poor performance of the A320 flight crew.)
  • AT72, Karup Denmark, 2016 (On 25 January 2016, an ATR 72-200 crew departing from and very familiar with Karup aligned their aircraft with the runway edge lights instead of the lit runway centreline and began take-off, only realising their error when they collided with part of the arrester wire installation at the side of the runway after which the take-off was rejected. The Investigation attributed the error primarily to the failure of the pilots to give sufficient priority to ensuring adequate positional awareness and given the familiarity of both pilots with the aerodrome noted that complacency had probably been a contributor factor.)
  • H25B, vicinity Akron OH USA, 2015 (On 10 November 2015, the crew of an HS 125 lost control of their aircraft during an unstabilised non-precision approach to Akron when descent was continued below Minimum Descent Altitude without the prescribed visual reference. The airspeed decayed significantly below minimum safe so that a low level aerodynamic stall resulted from which recovery was not achieved. All nine occupants died when it hit an apartment block but nobody on the ground was injured. The Investigation faulted crew flight management and its context - a dysfunctional Operator and inadequate FAA oversight of both its pilot training programme and flight operations.)
  • B733, vicinity Kosrae Micronesia, 2015 (On 12 June 2015, a Boeing 737-300 crew forgot to set QNH before commencing a night non-precision approach to Kosrae which was then flown using an over-reading altimeter. EGPWS Alerts occurred due to this mis-setting but were initially assessed as false. The third of these occurred when the eventual go-around was initially misflown and descent to within 200 feet of the sea occurred before climbing. The Investigation noted failure to action the approach checklist, the absence of ATC support and the step-down profile promulgated for the NDB/DME procedure flown as well as the potential effect of fatigue on the Captain.)
  • FA20, vicinity Kish Island Iran, 2014 (On 3 March 2014, a Dassault Falcon 20 engaged in navigation aid calibration for the Regulator was flown into the sea near Kish Island in dark night conditions. The Investigation concluded that the available evidence indicated that the aircraft had been inadvertently flown into the sea as the consequence of the crew experiencing somatogravic illusion. It was also noted that the absence of a functioning radio altimeter and pilot fatigue attributable to the long duty period was likely to have exacerbated the pilots' vulnerability to this illusion.)
  • A306, vicinity Birmingham AL USA, 2013 (On 14 August 2013, a UPS Airbus A300-600 crashed short of the runway at Birmingham Alabama on a night IMC non-precision approach after the crew failed to go around at 1000ft aal when unstabilised and then continued descent below MDA until terrain impact. The Investigation attributed the accident to the individually poor performance of both pilots, to performance deficiencies previously-exhibited in recurrent training by the Captain and to the First Officer's failure to call in fatigued and unfit to fly after mis-managing her off duty time. A Video was produced by NTSB to further highlight human factors aspects.)
  • B772, San Francisco CA USA, 2013 (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.)
  • A319, London Heathrow UK, 2013 (On 24 May 2013 the fan cowl doors on both engines of an Airbus A319 detached as it took off from London Heathrow. Their un-latched status after a routine maintenance input had gone undetected. Extensive structural and system damage resulted and a fire which could not be extinguished until the aircraft was back on the ground began in one engine. Many previously-recorded cases of fan cowl door loss were noted but none involving such significant collateral damage. Safety Recommendations were made on aircraft type certification in general, A320-family aircraft modification, maintenance fatigue risk management and aircrew procedures and training.)
  • A319, Mumbai India, 2013 (On 12 April 2013, an Airbus A319 landed without clearance on a runway temporarily closed for routine inspection after failing to check in with TWR following acceptance of the corresponding frequency change. Two vehicles on the runway saw the aircraft approaching on short final and successfully vacated. The Investigation concluded that the communication failure was attributable entirely to the Check Captain who was in command of the flight involved and was acting as 'Pilot Monitoring'. It was considered that the error was probably attributable to the effects of operating through the early hours during which human alertness is usually reduced.)
  • Vehicles / B722, Hamilton ON Canada, 2013 (On 19 March 2013 a Boeing 727 freighter was cleared to take off on a runway occupied by two snow clearance vehicles. The subsequent cancellation of the take off clearance was not received but a successful high speed rejected take off was accomplished on sight of the vehicles before their position was reached. The Investigation attributed the occurrence to the controller's failure to 'notice' the runway blocked indicator on his display and to his non-standard use of R/T communications. The late sighting of the vehicles by the aircraft crew was due to the elevated runway mid section.)
  • A321, Hurghada Egypt, 2013 (On 28 February 2013, the initial night landing attempt of a Ural Airlines Airbus A321 at Hurghada was mishandled in benign conditions resulting in a tail strike due to over-rotation. The Investigation noted that a stabilised approach had been flown by the First Officer but found that the prescribed recovery from the effects of a misjudged touchdown had not then been followed. It was also concluded that communication between the two pilots had been poor and that the aircraft commander's monitoring role had been ineffective. The possibility of the effects of fatigue was noted.)
  • B738, en-route, south south west of Brisbane Australia, 2013 (On 25 February 2013, a Boeing 737-800 about to commence descent from FL390 began to climb. By the time the crew recognised the cause and began to correct the deviation - their unintended selection of a inappropriate mode - the cleared level had been exceeded by 900 feet. During the recovery, a deviation from track occurred because the crew believed the autopilot had been re-engaged when it had not. The Investigation noted the failure to detect either error until flight path deviation occurred and attributed this to non-compliance with various operator procedures related to checking and confirmation of crew actions.)

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