Rosekind, M. R., Weldon, K. J., Co, E. L., Miller, D. L., Gregory,
K. B., Smith, R. M., Johnson, J. M., Gander, P. H., Lebacqz, J. V. (1994, June).
Fatigue Factors in Regional Airline Operations.
Paper Presented at the
meeting of the Public Forum on Commuter Airline Safety, Atlanta, Georgia.
The modern aviation industry requires 24-hour operations. The human operator remains central to providing flight, maintenance, air traffic control, cargo, and other aviation activities. This 24-hour industry requirement poses a challenge to humans in regards to sleep and circadian physiology. Therefore, human physiological limitations and capabilities remain central to maintaining safety, productivity, and performance in aviation operations. This paper will address three areas. First, there will be a discussion of physiological mechanisms that underlie fatigue to provide a foundation for understanding human physiological considerations. Second, potential areas of regional airline operations that may interact with these physiological considerations will be suggested. Third, activities to examine these issues more fully in regional airline operations will be identified.
Human sleep and circadian physiology
A brief introduction to physiological mechanisms that underlie fatigue is presented here. For more lengthy discussions of these issues and more extensive background, readers are referred to references 1-4.
Sleep is a vital physiological function. Sleep has been historically viewed as a time when humans are shut off. However, over the past 40 years, scientific research has clearly demonstrated that sleep is a complex and active physiological process required for human survival. Just as humans need food and water, sleep is a vital physiological requirement. The human brain provides very specific signals--hunger and thirst--to indicate that vital requirements for food and water have not been met. Similarly, the human brain provides a very specific signal--sleepiness--to indicate that the vital requirement for sleep has not been met. Sleepiness is the critical physiological signal that prompts an individual to sleep. With sufficient sleep loss (acute or chronic), the human brain can spontaneously shift from wakefulness to sleep in order to meet this physiological need. The greater the physiological pressure to sleep, the more rapid and frequent can be the intrusions of sleep into wakefulness. These uncontrolled and spontaneous sleep episodes can be very short (i.e., microsleeps lasting only seconds) or longer (i.e., lasting minutes). They can occur in potentially any environment or situation, including those that would put an individual at personal risk (e.g., driving a car) (refs. 5-7).
Sleep requirements differ among individuals. Though most adults require about 8 hours of sleep, some individuals will need only 6 hours, while others need 10. An individual's sleep requirement is determined by the number of hours of sleep needed to feel wide awake, alert, and at a peak level of performance during wakefulness.
Sleepiness can degrade waking performance, vigilance, and mood. At sleep onset, an individual perceptually disengages from the external environment. This disengagement involves becoming generally unresponsive to outside information. Clearly, even a microsleep can be associated with a performance lapse when an individual is unable to respond to, or receive, external information. However, this most severe form of sleep intrusion into wakefulness is not necessary for sleepiness to decrease waking performance (refs. 8-11).
Extensive scientific literature has clearly demonstrated that sleepiness can degrade almost all aspects of human performance. For example, sleepiness can be associated with degraded decision-making skills, vigilance, psychomotor coordination, information processing, memory, and reaction time. Sleepy individuals can demonstrate decreased performance, associated with an increased effort to perform, and a general indifference to the outcome of their performance. Mood is typically worsened with decreased positive emotions and increased negative emotions.
Sleepiness associated with acute and chronic sleep loss can degrade human performance capabilities and play an insidious role in the occurrence of an operational incident or accident (ref. 12).
Sleep loss accumulates into a sleep debt. When an individual does not obtain the required sleep, over time, the loss will result in a cumulative sleep debt. An 8-hour sleeper who obtains only 6 hours (sleep deprived by 2 hours) for 5 nights in a row, accumulates a 10-hour sleep debt. This cumulative sleep debt results in physiological sleepiness and can only be reversed by sleep. Laboratory research suggests that obtaining 2 hours less sleep than required will result in a subsequent decrease in waking performance (refs. 13 & 14).
Generally, in recuperating from a sleep debt, an individual will obtain deeper sleep over two or three nights, not necessarily significantly longer sleep. Thus, to recuperate from a 10-hour sleep debt, an individual will tend to sleep more deeply, not 10 hours longer than usual.
The human circadian clock. There is a circadian clock in the human brain (as in other living organisms) that regulates physiological and behavioral functions on a 24-hour basis. In a usual 24-hour period, this clock will control sleep/wake patterns, body temperature, mood, digestion, hormone secretion, performance, and many other human functions. On a regular 24-hour schedule, the clock is programmed to have periods when these physiological and behavioral functions will peak and other times when they will be at low points. Generally, the circadian clock is programmed for wakefulness during the day and sleep during the night (ref. 15).
The circadian clock will not adjust immediately when moved to a new work/rest (i.e., wake/sleep) schedule or new environmental time zone. It can take from days to weeks for internal physiological rhythms to resynchronize to the new schedule or environment. The internal circadian rhythms may not all resynchronize at the same rate and may therefore be out of "synch" with each other for an extended period of time. The circadian disruption associated with a new work/rest (i.e., wake/sleep) schedule or new environmental time can result in difficulties such as poor sleep, increased waking sleepiness, decreased performance, worsened mood, and stomach problems.
Scientific studies have demonstrated that there are two periods of maximal sleepiness during a usual 24-hour period. One period occurs between 3 and 5 AM, and the other about 12 hours later between 3 and 5 PM. Though these are periods of maximal sleepiness, performance and alertness can be affected throughout a 12 AM to 8 AM window. Therefore, individuals working through the 3 to 5 AM window are maintaining wakefulness during a time when they are physiologically programmed for sleep. Conversely, individuals attempting to sleep during daytime hours do so when they are physiologically programmed for wakefulness. On a 24-hour basis, these individuals will combat sleepiness while working throughout the night and then combat wakefulness when attempting to sleep during the day. Overall, this can result in a cumulative sleep debt and increased physiological sleepiness during wakefulness (refs. 16 & 17).
Sleep and circadian processes are continually interacting. At any given moment, an individual's ability to sleep will be an interaction between sleep and circadian processes. The ability to fall asleep quickly and obtain a good quantity and quality of sleep will be related to prior sleep/wake history (i.e., sleep debt) and circadian time of day. Conversely, an individual's ability to maintain alertness also will be the result of the interaction between sleep and circadian processes.
Performance vulnerability can be related to three sleep and circadian factors. The scientific literature on sleep and circadian processes suggests that there are at least three core factors that determine an individual's vulnerability for a performance decrement. These three factors are: 1) cumulative sleep loss, 2) hours of continuous wakefulness, and 3) circadian time of day.
1) Sleep deficit/cumulative sleep loss. If an individual requires 8 hours of sleep to maintain peak alertness and performance, then obtaining only 6 hours (even in one 24-hour period) means the individual is operating with a sleep deficit. As previously described, sleep loss accumulates into a sleep debt. This sleep deficit/cumulative sleep debt, usually expressed in hours, is a reflection of physiological sleepiness. It will represent an individual's vulnerability for sleepiness-related performance decrements and the potential for the occurrence of spontaneous, uncontrolled sleep episodes.
2) Hours of continuous wakefulness. A usual 24-hour wake/sleep pattern might involve 16 hours of wakefulness and 8 hours of sleep. However, operational requirements often demand an extended period of wakefulness. The greater the period of wakefulness, the greater the drive for sleep and associated sleepiness.
3) Circadian time of day. This factor can play a role in two ways. First, waking performance can be decreased during a circadian window of maximal sleepiness. Second, sleep quantity and quality can be decreased when attempting sleep during a circadian window of wakefulness.
These three core factors can be involved in determining an individual's vulnerability for a performance decrement at any given time. The factors will interact uniquely in a particular situation and individual. Vulnerability is reduced when an individual is well-rested, has been awake for a limited number of the usual hours spent awake (e.g., 4 of 16), and is operating during a period of circadian wakefulness. Performance vulnerability would be increased when an individual is carrying a sleep debt, is awake for an extended number of hours beyond usual, and is operating during a circadian period of maximal sleepiness.
Fatigue Factors in Regional Airline Operations
A variety of aviation environments have been scientifically examined for the challenges they pose to human sleep and circadian physiology. Studies of short-haul, long-haul, helicopter, and overnight cargo operations have investigated the extent of fatigue, sleep loss, and circadian disruption engendered by these aviation environments (refs. 18-21). Other studies have examined potential countermeasures, such as controlled rest on the flight deck, as strategies to promote alertness and performance during operations (ref. 22). Overviews of the research, methods, and findings can be found in references 23 and 24.
Regional airline operations, like these other aviation environments, involve activities that may provide similar, or potentially unique, challenges to human sleep and circadian physiology. Regional airline operations have not been systematically studied to understand the possible role of fatigue, sleep loss, and circadian disruption in this particular operating environment. With the previous scientific information as a basis, several regional airline operational considerations will be discussed. The considerations will highlight potential areas where the operational requirements and human physiology interact and may lead to fatigue. Following this discussion, projects currently underway to systematically evaluate these considerations will be described.
Scheduled reduced rest. Under current Part 135 regulations (scheduled), it is possible that a rest period may be scheduled or reduced to a minimum of 8 hours. A rest period essentially begins when released from duty and ends at report for duty. The reduction of a rest period to 8 hours requires that a crewmember be given a subsequent rest period of at least 10 hours that must begin no later than 24 hours after the commencement of the reduced rest period. Rest can not be reduced below 8 hours under any circumstances.
This 8 hour "rest" period away from the airplane can include, at least, local transportation to and from a layover accommodation, one or two meals, an opportunity to change, a shower, attending to other physiological needs, and sleep. Straightforward arithmetic suggests that an 8 hour reduced rest period will not allow 8 hours of physiological sleep. Therefore, an 8 hour sleeper may obtain only 7 or 6 hours of actual sleep during the reduced 8 hour rest period. The circadian timing of the rest period can also affect the quantity and quality of sleep. A usual night time sleeper, attempting to sleep during the day, and restricted by an 8 hour total reduced rest period, could obtain significantly less than the suggested 7 or 6 hours of sleep. Research demonstrates that sleeping one hour less than an individual requires can affect waking alertness, while two hours of sleep loss can have a significant effect on waking performance and alertness (ref. 13).
As previously discussed, sleep loss will accumulate into a sleep debt. The requirement for a subsequent rest period of at least 10 hours can be an important factor in minimizing a cumulative sleep debt. However, it is important to consider that a flight crewmember operating with a sleep deficit, potentially through an extended duty period, does not benefit from the "subsequent" rest period during the operation.
Clearly, the 24-hour operational environment requires flexibility due to unforeseen circumstances. It is not reasonable to expect a flight crewmember to "park" an airplane at 29,000 ft because of a hard time limitation. However, scheduled reduced rest is not a response to an unforeseen operational circumstance. As a result of reduced rest, flight crewmembers could likely be operating with a sleep deficit, the extent of which would be determined by a variety of factors.
Extended or continuous duty periods. Due to unforeseen operational circumstances, it is possible to extend duty for a prolonged period. These unforeseen operational circumstances, which are out of the control of the operator, include weather, air traffic delays, equipment/maintenance delays, and other situations. While a crewmember's scheduled flight time is specifically limited, the duty period within which it falls can be extended. This can result in extended periods of continuous wakefulness beyond a usual 24-hour pattern of 16 hours wake/8 hours sleep. The duty period parallels a flight crewmember's hours of continuous wakefulness. Therefore, an important initial consideration is the continuous hours of wakefulness and then, the activities conducted within those hours that can also affect performance and alertness (e.g., workload, boredom, physical activity).
Another consideration would be circumstances when a reduced rest period is followed by an extended duty period. Performance vulnerability would increase with the combination of a sleep deficit and then operating during an extended period of continuous wakefulness. Either of these factors alone could affect waking performance and alertness; together they increase an individual's vulnerability.
Continuous duty overnights. Continuous duty overnights, also
referred to as "stand-up overnights", is a scheduling practice
used in regional airline operations. The following is an example of how
a continuous duty overnight schedule could operate (see fig. 1). Flight
crew would report at 2100, fly from 2200-2300, then "stand-up"
on duty overnight from 2315-0515, then fly 0600-0700, followed by a rest
period from 0700-1700. The flight crew would then report at 1700, fly 1800-1900,
then "stand-up" on duty 1915-0115, and then fly 0200-0300. This
example involves only one hour flight examples, a clear underestimation
of actual flight times and number of segments that might typically be flown.
With less than 8 hours of scheduled flight time the crew requires only
9 consecutive hours of rest, this example provides for 10 hours of rest
(0700-1700). This continuous duty overnight schedule could be flown 3-5
times consecutively (i.e., back-to-back).
| Report | 1 hour flt. | Cont. duty period | 1 hour flt. | |
| Period 1 | 2100 | 2200 - 2300 | 2315 - 0515 | 0600 - 0700 |
| Period 2 | 1700 | 1800 - 1900 | 1915 - 0115 | 0200 - 0300 |
This example of a continuous duty overnight schedule provides several challenges to human sleep and circadian physiology. Following the initial flight period, the crew then maintains continuous wakefulness (i.e., duty) through the nighttime circadian low (2315-0515), though without flight activity. Then after this period of continuous wakefulness, they fly in the morning (e.g., one or multiple segments). The crew then have a daytime 10-hour rest period. If the crew usually sleeps at night, the quantity and quality of sleep obtained during the 0700-1700 rest period may not be maximal. After an evening flight period, there is another window of "stand-up" duty (i.e., continuous wakefulness), followed by flight time around the circadian nighttime low.
This schedule poses challenges that include maintaining wakefulness (sometimes while flying) during physiological periods of maximal sleepiness, obtaining sleep at times physiologically programmed for wakefulness, the potential for accumulating a sleep debt within one continuous duty overnight schedule, and the potential for compounding the sleep debt through consecutive continuous duty overnight schedules. There is also the potential for unforeseen operational circumstances that could result in a reduced rest period or an extended duty period.
Other potential fatigue factors. There are a variety of other operational considerations that may also pose a physiological challenge. Reserve status is critical for airline operations and presents a range of complex issues throughout the aviation industry. Relevant to the present discussion, reserve call-out can involve a flight crewmember sleeping until 0800, and then called for duty after 15:59 hours of wakefulness. After almost 16 hours of continuous wakefulness, an extended duty period of 14 or more hours might require 30 hours or more of continuous wakefulness. Training can occur after flight duty or during nighttime hours. This could involve training activities after an extended period of continuous wakefulness or during a circadian low in performance and alertness. A commute to an assigned domicile, whether an extended surface trip or by air, can introduce another diverse set of factors that can affect preparation for flight duty. Regional airline operations represent many different activities and requirements. Flight crew workload can differ significantly based on the number of segments flown, weather, specifics of airports, baggage and ticket handling requirements, and unforeseen operational circumstances that create delays. There is a range of aircraft flown in regional operations with differing levels of automation and physical environment. These examples are not intended to be all-inclusive but an indication of the variety of other factors that may play a role in the complexity of real-world aviation operations.
Examination of Fatigue Factors in Regional Airline Operations
Extensive scientific research currently exists to help understand the physiological challenges of humans operating in 24-hour environments. Examination of human sleep and circadian physiology in response to the demands of the aviation industry has occurred in a variety of flight environments. However, regional airline operations are an arena where there is a scarcity of scientific data to determine the role of the fatigue factors presented in this paper. There is sufficient scientific data to suggest that these may be issues in regional operations. However, how the specific requirements of regional operations translate into actual sleep and circadian disruption, fatigue, performance decrements, etc. have not been systematically studied scientifically.
The National Transportation Safety Board has previously cited fatigue
as a contributory factor in a regional airline accident. In this circumstance,
the NTSB cited "the flightcrew's failure to properly manage provided
rest periods" as contributory to the accident (ref. 21). Based on
this contributory cause, the NTSB recommended that Part 135 "air carriers
provide aircrews, as part of their initial and recurrent training, information
on fatigue countermeasures relevant to the duty/rest schedules being flown
by the company." The NTSB has also initiated two significant activities
to examine a broad range of safety issues in regional airline operations.
One activity is a survey conducted with regional carriers to obtain data
on a variety of identified safety areas. Another activity is this open
forum for public discussion by all interested parties in regional airline
operations.
The NASA Ames Fatigue Countermeasures Program and the FAA have initiated
a project to systematically examine the specific issues related to fatigue
raised in this paper. A survey focused on fatigue, sleep, and circadian-related
issues will be completed by flight and management personnel at a representative
sample of regional carriers. The survey has been developed, reviewed by
a range of interested industry representatives, and is in the process of
implementation. The survey will provide extensive information on fatigue-related
issues from a large sample of flight and management personnel involved in
daily regional airline activities. The survey will be followed by a focused
field study to obtain data during actual operations. The combination of
self-report survey information and data collected during usual airline operations
will provide an important, systematic view of these fatigue factors in the
regional aviation environment. An additional resource will be a review
of NASA/FAA ASRS incident reports.
The NASA/FAA projects will provide data complementary to the diverse information obtained from the NTSB activities. Together, they will provide a unique opportunity to examine a range of issues related to regional airline operations, including those that may involve fatigue.
Regional operations have continued to grow and represent a critical component of the US aviation industry. In whatever ways are feasible, the safety margin should be maximized in every component of the industry. Aviation involves complex operational requirements to maintain its 24-hour activities. The challenge is to understand human physiological capabilities and incorporate this information where ever possible into operations. There will be no one single answer to fatigue in aviation. Consideration of personal coping strategies and countermeasures, scheduling practices, regulatory guidelines, and technology can all play a role in promoting maximal performance and alertness in flight operations. An important agenda is to maintain, and where possible, improve the safety margin, productivity, and performance.