Scenarios in Pilot Fatigue

Scenarios in Pilot Fatigue

Scenarios in Pilot Fatigue


From the NTSB:

During the approximately five-hour, 25-minute night instrument flight, the pilot of a Pressurized Beech Baron elected not to stop at his planned fuel stop. Upon reaching the destination airport, weather conditions were 300 overcast and two miles visibility in drizzle, which were worse than the forecast. The pilot diverted to his planned alternate airport and attempted an ILS approach. Given the lack of a fuel stop, the pilot may have felt personal pressure to land the airplane as soon as possible. The airplane initially intercepted the localizer for the approach but did not intercept the glideslope. The airplane then proceeded left of course, above the glideslope, followed by a continued left deviation and descent below the glideslope. The tower controller asked the pilot if he was still on the localizer course and the pilot replied that he was not. The tower controller then provided heading and altitude instructions in an attempt to guide the pilot onto a missed approach. The pilot acknowledged the heading instruction but failed to turn to the assigned heading or climb to the assigned altitude. The airplane subsequently impacted a residential area about a half-mile from the runway. 

NTSB probable cause: The pilot’s failure to maintain control of the airplane during an instrument approach due to spatial disorientation.

It’s extremely rare for the NTSB to cite pilot fatigue as a contributing factor in an aircraft accident. In most cases, NTSB investigators simply don’t have the time and budget to look into the pilot’s behavior patterns in the days leading up to an accident. Five and a half hours, however, is a long time to be at the controls of an aircraft only to find yourself making an approach in LIFR. It’s quite likely the pilot’s awareness was impaired at least somewhat by fatigue. Here’s another NTSB report where investigators come this close to citing pilot fatigue as a probable cause.

The commercial pilot of an E90 King Air had filed an instrument flight rules flight plan and was departing in dark night visual meteorological conditions on a cross-country personal flight. A witness at the departure airport stated that during takeoff the airplane sounded and looked normal. The airplane lifted off about halfway down runway 24, and there was “plenty” of runway remaining for the airplane to land. The witness lost sight of the airplane and did not see the accident because the airport hangars blocked her view. 

The wreckage was located about 2,400 feet southeast of the departure end of runway 24. Examination of the accident site indicated that the airplane impacted in a nose-down attitude with a left bank of about 20 degrees. A left turn during departure was consistent with the airport’s published instrument departure procedures for obstacle avoidance, which required an immediate climbing left turn while proceeding to a navigational beacon located about 7 miles east-northeast of the airport. Examination of the wreckage did not reveal any evidence of preimpact mechanical malfunctions that would have precluded normal operation. 

The pilot had reportedly been awake for about 15 hours and was conducting the departure about the time he normally went to sleep and, therefore, may have been fatigued about the time of the event; however, given the available evidence, it was impossible to determine the role of fatigue in this event. Although the circumstances of the accident are consistent with spatial disorientation, there was insufficient evidence to determine whether it may have played a role in the sequence of events. 

NTSB probable cause: The pilot’s failure to maintain clearance from terrain after takeoff during dark night conditions. 

Occasionally NTSB reports do identify pilot fatigue as a contributing factor in an accident, such as this example.

The Cessna 414A, flown by an airline transport pilot, was approaching the destination airport after a cross-country flight in night instrument meteorological conditions. Destination weather about one minute before the accident included an overcast ceiling at 200 feet and half-mile visibility with light rain and fog. The flight received radar vectors to the final approach course for an ILS approach. The airplane’s flight path did not properly intercept and track either the localizer or the glideslope during the instrument approach. The airplane crossed the final approach fix about 360 feet below the glideslope and then maintained a descent profile below the glideslope until it leveled briefly near the minimum descent altitude. The lateral flight path from the final approach fix inbound was one or more dots to the right of the localizer centerline until the airplane was about one nautical mile from the runway threshold when it turned 90 degrees left. The turn was initiated before the airplane had reached the missed approach point; additionally, the left turn was not in accordance with the published missed approach instructions. The airplane made a series of pitch excursions as it flew away from the localizer. A simulation study determined that dual engine power was required to match the recorded flight trajectory and ground speeds, which indicated that both engines were operating throughout the approach. The simulation also indicated that the airplane likely encountered an aerodynamic stall during its course deviation. The airplane impacted the ground about 2.2 miles east-northeast of the runway threshold and about 1.75 miles east of the localizer centerline. 

The airplane impacted the ground upright and in a nose-low attitude consistent with an aerodynamic stall/spin. Wreckage examinations did not reveal any anomalies with the airplane’s flight control systems, engines, or propellers. The glideslope antenna was found disconnected from its associated cable circuit. Laboratory examination and testing determined that the glideslope antenna cable was likely inadequately connected/secured during the flight, which resulted in an unusable glideslope signal to the cockpit avionics. There was no history of recent maintenance on the glideslope antenna, and the reason for the inadequate connection could not be determined. 

Data downloaded from the airplane’s EHSI established that the device was in the ILS mode during the instrument approach phase and that it had achieved a valid localizer state on both navigation channels; however, the device never achieved a valid glideslope state on either channel during the flight. Further, a replay of the recorded EHSI data confirmed that, during the approach, the device displayed a large “X” through the glideslope scale and did not display a deviation pointer, both of which were indications of an invalid glideslope state. 

There was no evidence of cumulative sleep loss, acute sleep loss, or medical conditions that indicated poor sleep quality for the pilot. However, the accident occurred more than 2 hours after the pilot routinely went to sleep, which suggests that the pilot’s circadian system would not have been promoting alertness during the flight. Further, at the time of the accident, the pilot likely had been awake for 18 hours. Thus, the time at which the accident occurred and the extended hours of continuous wakefulness likely led to the development of fatigue. 

The presence of low cloud ceilings and the lack of glideslope guidance would have been stresses to the pilot during a critical phase of flight. This would have increased the pilot’s workload and situational stress as he flew the localizer approach, a procedure that he likely did not anticipate or plan to conduct. In addition, weight and balance calculations indicated that the airplane’s center of gravity (CG) was aft of the allowable limit, and the series of pitch excursions that began shortly after the airplane turned left and flew away from the localizer suggests that the pilot had difficulty controlling airplane pitch. This difficulty was likely due to the adverse handling characteristics associated with the aft CG. These adverse handling characteristics would have further increased the pilot’s workload and provided another distraction from maintaining control of the airplane. Therefore, it is likely that the higher workload caused by the pilot’s attempt to fly an unanticipated localizer approach at night in low ceilings and his difficulty maintaining pitch control of the airplane with an aft CG contributed to his degraded task performance in the minutes preceding the accident. 

NTSB probable cause: The pilot’s failure to maintain control of the airplane during the instrument approach in night instrument meteorological conditions, which resulted in the airplane exceeding its critical angle of attack and an aerodynamic stall/spin. Contributing to the accident were pilot fatigue, the pilot’s increased workload during the instrument approach resulting from the lack of glideslope guidance due to an inadequately connected/secured glideslope antenna cable, and the airplane being loaded aft of its balance limit. 

The Federal Aviation Administration publishes rules for flight, duty day and rest regulations applicable to air carrier operations. According to the FAA, “Fatigue threatens aviation safety because it increases the risk of pilot error that could lead to an accident. The rule recognizes the universality of factors that lead to fatigue in most individuals and regulates these factors to ensure that flight crew members in passenger operations do not accumulate dangerous amounts of fatigue.” 

The FAA does not, however, publish fatigue-mitigation regulations for most Part 91 operations. The only fatigue rule for non-91K fractional ownership aircraft that applies to light aircraft is the prohibition against flight instructors conducting more than eight hours of “dual given” in any 24-hour period. The rest of us are on our own to determine whether we’re alert enough to fly.

Pilot fatigue is a virtually unresearched and potentially major factor in general aviation accidents as well. Common symptoms of fatigue include: 

  • Measurable reduction in speed and accuracy of performance
  • Lapses in attention and vigilance
  • Delayed reactions
  • Impaired logical reasoning and decision-making, including a reduced ability to assess risk or appreciate the consequences of actions
  • Reduced situational awareness
  • Low motivation to perform optional activities 

The symptoms of fatigue are exactly the same as those of alcohol impairment. Industrial safety research concludes that after a restful, eight-hour sleep, being awake for 18 hours results in performance equivalent to that of someone with a blood alcohol content of 0.05. After being awake 23 hours, performance is the equivalent of a blood alcohol content of 0.12. Many states define “legally drunk” as a blood alcohol content of 0.08. Of course, there is no blood alcohol level permissible to serve as pilot-in-command. 

It’s not enough to get a good rest the night before a challenging trip. “Sleep debt,” the negative cumulative effect of getting less than eight full hours of sleep, applies not only the night before a flight but for several nights prior to that as well. According to research cited by the FAA, “The average person requires in excess of nine hours of sleep [in a single] night to recover from a sleep debt.”

Sleep debt and the effect of fatigue on pilot performance are likely huge as unidentified factors in aircraft accidents. Add long flights and dark
conditions, and a pilot who took off in a reasonably well-rested state may not have what it takes to safely complete a flight when presented with adverse or unusual circumstances. Many owner-pilots tend to make trips after the end of a full workday, sometimes at the end of a long work week – exposing themselves, their passengers and the people they fly over to heightened risk from fatigue.

Because the FAA’s airline pilot fatigue rules cover many scenarios for crew rest, travel across multiple time zones, and back-to-back work days, it is much more complex than we need to be concerned about in personal aviation. The underlying concept, however, is extremely relevant:

Your flight should be planned to conclude no more than 16 hours after you awoke from an uninterrupted eight hours of sleep. This includes the time to fly to any alternates. Once you are airborne, constantly evaluate your level of fatigue. If you are getting tired (yawning, find yourself missing radio calls, etc.) land right away. If your flight is delayed and you will reach the end of your 16-hour duty day, divert to land before reaching that limit.  

The National Business Aviation Association (NBAA) publishes recommended guidance for business aviation crews. NBAA recommends no more than a 14-hour duty day, including no more than 10 hours of flight time in any 24-hour period. Not explicitly stated in NBAA’s guidance, this assumes a two-person professional flight crew in turbine airplanes. Taking airline regulations and NBAA best practices into account, here’s what I suggest for single-pilot business or personal aviation operators:

Pilot fatigue is a virtually unresearched and potentially major factor in general aviation accidents.

12-hour maximum duty day. This is 12 hours “alarm clock to engine shutdown,” recognizing that while most single-pilot operators will not log multiple flight legs in a single day, they tend to fly later in the day after having expended some of their day on non-aviation but still demanding duties.

No more than three flight segments in a duty day together totaling no more than eight flight hours. 

No more than two flight segments in a duty day together totaling no more than five flight hours, if the final segment will be flown at night and/or in instrument mereological conditions. 

We need to get real about fatigue and learn to account for pilot fatigue as part of flight planning. Your “fatigue state” is as critical to the safe outcome of a flight as the airplane’s fuel state. It’s fairly easy to judge whether you feel rested enough to begin a trip, but far harder to predict how fatigued you’ll feel at the end of a flight. Although the rules and suggestions for duty day limits are somewhat arbitrary, they are the starting point for a pilot not used to active fatigue management, until he or she is more experience determining how they respond to pilot fatigue.    

About the Author

Leave a Reply