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Pilot Fatigue
Source:
Aerospace Medical Association
By Dr. Samuel Strauss
Fatigue and flight
operations
Fatigue is a threat to
aviation safety because of the impairments in alertness and performance
it creates. "Fatigue" is defined as "a non-pathologic state resulting in
a decreased ability to maintain function or workload due to mental or
physical stress." The term used to describe a range of experiences from
sleepy, or tired, to exhausted. There are two major physiological
phenomena that have been demonstrated to create fatigue: sleep loss and
circadian rhythm disruption. Fatigue is a normal response to many
conditions common to flight operations because of sleep loss, shift
work, and long duty cycles. It has significant physiological and
performance consequences because it is essential that all flight crew
members remain alert and contribute to flight safety by their actions,
observations and communications. The only effective treatment for
fatigue is adequate sleep (1).
In a National
Transportation Safety Board (NTSB) safety study of US major carrier
accidents involving flight crew from 1978 to 1990, one finding directly
addressed the concern about fatigue. It stated: "Half the captains for
whom data were available had been awake for more than 12 hours prior to
their accidents. Half the first officers had been awake for more than 11
hours. Crews comprising captains and first officers whose time since
awake was above the median for their crew position made more errors
overall, and significantly more procedural and tactical decision errors
(2)."
An example of fatigue
as a probable cause of a US commercial aircraft accident occurred on
August 18th, 1993 in Guantanamo Bay, Cuba involving a DC-8.
The airplane was destroyed by impact forces and post-accident fire, and
the three flight crewmembers sustained serious injuries. Visual
meteorological conditions prevailed, and an instrument flight rules plan
had been filed. The following is the NTSB summary report:
The airplane
collided with terrain aprx 1/4 mi from the approach end of the runway
after the captain lost control of the airplane. Flightcrew had
experienced a disruption of circadian rhythms and sleep loss; had been
on duty about 18 hrs and had flown aprx 9 hrs. Capt did not recognize
deteriorating flightpath and airspeed conditions due to preoccupation
with locating strobe light on ground. Strobe light, used as visual
reference during approach, inoperative; crew not advised. Repeated
callouts by the flight engineer stating slow airspeed conditions went
unheeded by the capt. Capt initiated turn from base leg to final at
airspeed below calculated vref of 147 kts, and less than 1,000 ft from
the shoreline, and he allowed bank angles in excess of 50 deg to
develope. Stall warning stick shaker activated 7 secs prior to impact, 5
secs before airplane reached stall speed. No evidence to indicate capt
attempted to take proper corrective action at the onset of stick shaker.
Operator's management structure and philosophy were insufficient to
maintain vigilant oversight and control of the rapidly expanding airline
operation.
Probable Cause
The impaired
judgement, decision-making, and flying abilities of the captain and
flightcrew due to the effects of fatigue; the captain's failure to
properly assess the conditions for landing and maintaining vigilant
situational awareness of the airplane while maneuvering onto final
approach; his failure to prevent the loss of airspeed and avoid a stall
while in the steep bank turn; and his failure to execute immediate
action to recover from a stall. Additional factors contributing to the
cause were the inadequacy of the flight and duty time regulations
applied to 14 cfr, part 121, supplemental air carrier, international
operations, and the circumstances that resulted in the extended
flight/duty hours and fatigue of the flightcrew members. Also
contributing were the inadequate crew resource management training and
the inadequate training and guidance by the airline, to the flightcrew
for operations at special airports, such as guantanamo bay; and the
navy's failure to provide a system that would assure that the local
tower controller was aware of the inoperative strobe light so as to
provide the flightcrew with such information.
(NTSB REPORT AAR-94/04,
ADOPTED 5/10/94)
When the sleep patterns
of this flight crew were analyzed it was found that the entire flight
crew suffered from cumulative sleep loss. They worked under an extended
period of continuous wakefulness, and slept at times opposite to their
normal circadian sleep patterns. The accident occurred in the afternoon,
at the time of their maximum physiological sleepiness (2).
SLEEP AND SLEEP LOSS
Sleep is a vital
physiological function. Like food and water, sleep is necessary for
survival. Sleepiness results when sleep loss occurs. Like hunger and
thirst, sleepiness is the brain's signal that sleep is needed. "Sleep
loss" describes the phenomenon of getting less sleep than is needed for
maximal waking performance and alertness. If an individual normally
needs 8 hours of sleep to feel completely alert, and gets only 6 hours
of sleep, 2 hours of sleep loss has been incurred. Sleep loss over
successive days accumulates into a "sleep debt." If the individual
needing 8 hours of sleep gets only 6 hours a night for 4 nights in a
row, an 8 hours sleep debt has been accumulated. The negative effects of
one night of sleep loss are compounded by subsequent sleep loss. Sleep
loss and the resultant sleepiness can degrade most aspects of human
performance. In the laboratory, it has been demonstrated that losing as
little as 2 hours of sleep can negatively affect alertness and
performance. Performance effects include: degraded judgment, situation
awareness, decision-making, and memory; slowed reaction time; lack of
concentration; fixation; and worsened mood. Other effects are decreased
work efficiency, degraded crew coordination, reduced motivation,
decreased vigilance, and increased variability of work performance. The
brain is programmed for two periods of maximal sleepiness every 24 hours
from about 3 - 5 am and 3 - 5 pm (3).
SYMPTOMS AND EFFECTS OF FATIGUE
Conditions which
contribute to fatigue include the time since awake, the amount of time
doing the task, sleep debt, and circadian rhythm disruption. As fatigue
progresses it is responsible for increased errors of omission, followed
by errors of commission, and microsleep. "Microsleep" is characterized
by involuntary sleep lapses lasting from a few seconds to a few minutes
(3). For obvious reasons, errors or "short absences" can have
significant hazardous consequences in the aviation environment.
Many of the unique
characteristics of the flight deck environment make pilots particularly
susceptible to fatigue. Contributing aircraft environmental factors
include movement restriction, variable air flow, low barometric pressure
and humidity, noise, and vibration. In commercial aircraft, hands on
flying has been mostly replaced by greater demands on the flight crew to
perform vigilant monitoring of multiple flight systems. Research has
demonstrated that monotonous vigilance tasks decreased alertness by 80%
in one hour (4). This phenomenon is often referred to as "boredom
fatigue."
Fatigue and sleepiness
may be less evident to a pilot due to stimuli such as noise, physical
activity, caffeine, nicotine, thirst, hunger, excitement, and
interesting conversation. Sleep-deprived pilots may not notice
sleepiness or other fatigue symptoms during preflight and departure
flight operations. However once underway and established on altitude and
heading, sleepiness and other fatigue symptoms tend to manifest
themselves.
When extreme, fatigue
can cause uncontrolled and involuntary shutdown of the brain. That is,
regardless of motivation, professionalism, or training, an individual
who is extremely sleepy can lapse into sleep at any time, despite the
potential consequences of inattention. Transportation incidents and
accidents, such as the one cited above, provide dramatic examples of
this fact.
CIRCADIAN RHYTHMS
"Circadian rhythms" are
physiological and behavioral processes, such as sleep/wake, digestion,
hormone secretion, and activity, that oscillate on a 25 hour basis. Each
rhythm has a peak and a low point during every day/night cycle. Time
cues, called "zeitgebers," keep the circadian "clock" set to the
appropriate time of day. Common zeitgebers include daylight, meals and
work/rest schedules. If the circadian clock is moved to a different
schedule, for example when crossing time zones or changing from a day
work shift to a night shift, the resulting "sleep phase shift" requires
a certain amount of time to adjust to the new schedule. This amount of
time depends on the number of hours the schedule is shifted, and the
direction of the shift. During this transition, the circadian rhythm
disruption or "jet lag" can produce effects similar to those of sleep
loss.
Transmeridian flights
in excess of three time zones can result in significant circadian rhythm
disruption. When flying in a westerly direction the pilot’s day is
lengthened. When flying east, against the direction of the sun, the
pilot’s day is shortened. Thus the physiological time and local time can
vary by several hours. Symptoms of jet lag are usually worse when flying
from west to east as the day is artificially shortened. It takes about
one day for every time zone crossed to recover from jet lag. When
circadian disruption and sleep loss occur together, the adverse effects
of each are compounded (3).
CREW
REST AND FLYING DUTIES
Scheduling of adequate
crew rest needs to take several important factors into consideration.
These include time since awake, time on task, type of tasks, extensions
of normal duty periods, and cumulative duty times (3).
The "time since awake"
is the starting point for fatigue to build. This can be prolonged prior
to flying due to the effects of jet lag, early awakening due to
disturbances in the sleep environment, the extra time needed to get up
check out of a hotel and travel to the airport for flight check in, and
delays in getting started preflight procedures including for mechanical
problems or weather delays. "Time on task" is the time required to
preflight and fly. This is the time from check-in to block-in plus
fifteen minutes on the last flight of the day. The "type of tasks"
depend on the crew position, type of aircraft, and the nature of the
flights. Extensions of normal duty periods can occur from events which
prolong the flight longer than scheduled. Such events include delays for
en route weather, rerouting due to traffic or, more rarely, diversions.
Research on duty period duration suggests that duty periods greater than
twelve hours are associated with a higher risk of errors. In determining
maximum limits for extended duty periods, consideration needs to be
given to all factors which contribute to fatigue including the numbers
of legs in the day’s flight plan, whether jet lag is a factor in the
crew duty day, and the time since awake. "Cumulative duty times" are
most fatiguing when there are consecutive flying days with minimal or
near minimal crew rest periods. This can result in sleep debt which
requires additional time to overcome (3).
A brief review of US
Federal Aviation Administration (FAA) flight time and rest rules for
scheduled domestic commercial carriers (US Code Title 14, part 121.471)
are as follows:
The flight crew duty
day starts with check-in, and is considered concluded at block-in plus
15 minutes for that day’s final flight. Rest periods are times when the
crewmember is not scheduled for flying duty. These are not periods of
restful sleep. Adequate restful sleep, however, must be achievable
during these rest periods. In addition to FAA regulations, company rules
and practices also influence crew scheduling and rest issues. Company
contracts with pilots, scheduling practices for bids and reserve, and
productivity demands all play a part in the balance between work
requirements and crew rest.
RESTFUL SLEEP REQUIREMENTS
There is considerable
variability in individual sleep needs. Some individuals do well with 6
hours sleep per night, yet others need 9 or 10 hours sleep. However,
most adults require 8 hours of restful sleep to stay out of sleep debt.
With aging there is usually a significant decline in habitual daily
sleep due to increased nighttime awakenings. Therefore, in older
individuals decreased quality of nighttime sleep can result in increased
daytime fatigue, sleepiness, dozing and napping (5) (6). Napping seems
to compensate for the loss of quality sleep during nighttime hours, but
the need for a mid-day nap may not be compatible with flight duty
demands on short haul flights (3). Research has demonstrated that
pre-planned cockpit rest has improved in-flight sustained attention and
psychomotor response speed (7). Some international airlines have created
policies allowing pilots to nap during long haul flights at times of low
workloads. Thus far, the US Federal Aviation Regulations have not made
reference to planned in-flight crew rest.
Complete recovery from
significant sleep debt may not occur after a single sleep period.
Usually 2 nights of recovery are required. Eight to 10 hours of recovery
sleep per sleep period may be required for most people to achieve
effective levels of alertness and performance (8). Obtaining the
required sleep time under layover conditions depends on the length of
the off duty rest period. Off duty time must be adequate to allow for at
least 8 hours of restful sleep per night in order to recover from sleep
debt, and therefore the potentially hazardous effects of flying while
fatigued.
CONCLUSION AND RECOMENDATIONS
Pilot fatigue has been
shown to be a hazard in commercial flight operations. Many factors
contribute to fatigue in the commercial aviation environment. Circadian
rhythm disruption, prolonged work schedules, inadequate crew rest, and
inadequate restful sleep contribute to the potential for pilot fatigue.
When the regulations regarding "rest" are compared to identified
requirements for "restful sleep," one can see that adequate restorative
rest may not occur. Reviews of federal research activities, hours of
service/rest regulations, and airline company scheduling policies are
needed to correct existing systemic problems. Enhanced pilot training is
also needed to prevent fatigue, and to recognize it when it occurs so
that effective countermeasures can be employed (1). Doing so will help
insure that pilots fly adequately rested and alert thereby improving
flying safety.
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