It has been estimated that 4-7% of civil aviation incidents and accidents can be attributed to fatigued pilots. "In the last 16 years, fatigue has been associated with 250 fatalities in air carrier accidents." Robert Sumwalt, NTSB vice chairman, said at an FAA symposium in July.
Symptoms associated with fatigue include slower reaction times, difficulty concentrating on tasks resulting in procedural mistakes, lapses in attention, inability to anticipate events, higher toleration for risk, forgetfulness, and reduced decision-making ability. The magnitude of these effects are correlated to the circadian rhythm and length of time awake. Performance is affected the most, when there is a combination of extended wakefulness and circadian influences.
A Federal Aviation Administration (FAA) study of 55 human-factor aviation accidents from 1978 to 1999, concluded accidents increased proportionally to the amount of time the captain had been on duty. The accident proportion relative to exposure proportion rose from 0.79 (1–3 hours on duty) to 5.62 ( more than 13 hours on duty). This means that "5.62% of human factors accidents occurred to pilots who had been on duty for 13 or more hours, where only 1% of pilot duty hours occur during that time."
In another study by Wilson, Caldwell and Russell, participants were given three different tasks that simulated the pilot's environment. The tasks included reacting to warning lights, managing simulated cockpit scenarios, and conducting a simulated UAV mission. The subjects' performance was tested in a well-rested state and again after being sleep deprived. In the tasks that were not as complex, such as reacting to warning lights and responding to automated alerts, it was found that there was a significant decrease in performance during the sleep deprived stage. The reaction times to warning lights increased from 1.5 to 2.5 seconds, and the number of errors doubled in the cockpit. However, tasks that were engaging and required more concentration were found to not be significantly affected by sleep deprivation. The study concluded that "...fatigue effects can produce impaired performance. The degree of performance impairment seems to be a function of the numbers of hours awake and the 'engagement' value of the task."
One United States Air Forces study found significant discrepancies regarding how fatigue affects different individuals. It tracked the performance of ten F-117 pilots on a high-fidelity flight simulator. The subjects were sleep deprived for 38 hours and their performance was monitored over the final 24 hours. After baseline correction, the systematic individual differences varied by 50% and concluded that fatigue's effect on performance varied drastically among individuals.
The first step to understanding the critical impact fatigue can have on flight safety is to quantify it within the airline environment. An airline's management often struggles to balance rest with duty periods because it strives for maximum crew productivity. However, fatigue comes as a limitation needing increasing consideration.
A study by Reis et al. investigated the prevalence of fatigue on a group of Portuguese airline pilots. 1500 active airline pilots who had all flown within the past six months received a questionnaire. Out of the population, 456 reliable responses were received. A pretest was conducted to determine the viability of the fatigue scale adopted during the test, called Fatigue Severity Scale (FSS). The purpose of the validation survey was to set a benchmark (i.e. FSS=4) on an acceptable level of fatigue for the Portuguese culture. The scale ranged from 1 meaning no fatigue to 7 being high. Participants had one month and a half to respond to the inquiry. Results on physical fatigue found that 93% of short/medium haul pilots scored higher than 4 on the FSS while 84% of long-haul pilots scored greater than 4. Mental fatigue found short/medium haul at 96% and long haul at 92%. The Questionnaire also asked: "Do you feel so tired that you shouldn’t be at the controls?". 13% of pilots said that this never happened. 51% of all participants said it happened a few times. Limitations of the study were: fatigue levels are subjective and research did not attempt to control the number of times pilots had available to respond to the questionnaires. Overall the study establishes that pilots are subject to high levels of fatigue on the job. Levels of fatigue collected were also compared with a validation test conducted on multiple sclerosis patients in Switzerland. These patients showed average fatigue levels of 4.6 while pilots in the Portuguese study scored an average of 5.3.
High prevalence of fatigue was also revealed in a study by Jackson and Earl investigating prevalence among short haul pilots. The study consisted of a questionnaire that was posted on a website, Professional Pilot’s Rumour network (PPRUNE) and was able to obtain 162 respondents. Of the 162, all being short haul pilots, 75% were classified to have experienced severe fatigue. Based on questionnaire results, the study also demonstrated that pilots who were highly concerned about their level of fatigue during the flight often scored higher on the fatigue scale and thus were likely to experience more fatigue. Not only this, operational factors, for example a change in flights, or from flight into discretionary time often cause the pilot to experience greater fatigue.
On the other hand, research by Samen, Wegmann, and Vejvoda investigated the variation of fatigue among long-haul pilots. 50 pilots all from German airlines participated in the research. As participants, pilots were subject to physiological measures pre-departure and during flight and filled out routine logs recording their times of sleep and awakening. Pilots also completed two questionnaires. The first reflecting feelings of fatigue before and after the flight, recorded before departure, 1-hour intervals during the flight and then immediately after landing. The second questionnaire was the NASA task load index.
The second questionnaire also administered during flight, assessed different dimensions including mental, physical and temporal demand as well as performance. Key findings from the study conveyed that: outgoing flights from the home base were rated as less stressful and night flights were rated as the most stressful. The physiological measures found that microsleeps recorded by the EEGs increased progressively with flight duty. Microsleeps are recordings of alpha wave activity and they occur during wakeful relaxation often resulting in loss of attention. They are considered microsleeps if they last less than thirty seconds. Microsleep cases for pilots on outgoing flights were half compared to the number on incoming flights back to the home base showing that fatigue is more prevalent on flights returning home. Pilots are more prone to microsleeps during the cruise phase of the flight while they are more alert and less likely to experience microsleeps during the take-off, approach and landing phases of the flight. Findings also show that fatigue was greater during night flights because pilots had already been awake for more than 12 hours and would begin duty by the time they were due to go to sleep.
Pilots often have to rely on self-assessment in order to decide if they are fit to fly. The IMSAFE checklist is an example of self-assessment. Another measure that a pilot can employ to more accurately determine his level of fatigue is the Samn- Perelli Seven Point Fatigue Scale (SPS). The evaluation has a scale of 1-7, 1 described as “Fully, Alert and Wide Awake” while 7 “Completely exhausted, unable to function effectively”.
All levels in between have descriptions aiding the pilot with his decision. Another example of self-assessment is simply a visual and analogue scale. The test is represented by a line with No Fatigue and Fatigue labeled on two ends. The pilot will then draw a mark where he feels to be. Advantages of self-assessment include that they are quick and easy to administer, can be added to routine checklists and being more descriptive allow pilot to make a better decision. Disadvantages include that it is easy for the pilot to cheat and are often hard to disprove.
Between 2010 and 2012, more than 6.000 European pilots have been asked to self-assess the level of fatigue they are experiencing. These surveys revealed that well over 50% of the surveyed pilots experience fatigue as impairing their ability to perform well while on flight duty. The polls show that e.g. 92% of the pilots in Germany report they have felt too tired or unfit for duty while on flight deck at least once in the past three years. Yet, fearing disciplinary actions or stigmatization by the employer or colleagues, 70-80% of fatigued pilots would not file a fatigue report or declare to be unfit to fly. Only 20-30% will report unfit for duty or file a report under such an occurrence.
Since the 1930s, airlines have been aware of the impact of fatigue on pilot's cognitive abilities and decision making. Nowadays prevalence of fatigue draws greater attention because of boom in air travel and because the problem can be addressed with new solutions and countermeasures.