Fatigue is a ubiquitous risk in all modes of transportation. In operational environments, cognitive performance degrades with sleep loss, often referred to as a fatigue effect, which can be measured through a simple sustained attention task, the psychomotor vigilance task (7). In most cases, the effects of fatigue on performance are due to inadequate sleep and/or functioning at a nonoptimal circadian phase, but such effects can also occur as a function of prolonged work hours (time on task) and inadequate recovery times. The effects of inadequate sleep (rest) and recovery on cognitive performance are primarily manifested as increasing variability in cognitive speed and accuracy, resulting in increasingly unreliable behavioral performance. When partial or total sleep deprivation produces increased performance variability, this is hypothesized to reflect state instability, defined as moment-to-moment shifts in the relationship between neurobiological systems mediating wake maintenance and sleep initiation (7). Sleep-initiating mechanisms repeatedly interfere with wakefulness, making cognitive performance both increasingly variable and dependent on compensatory measures, such as motivation, which are unable to override increased sleep pressure without consequences (7).

Sleep deprivation induces a wide range of effects on cognitive performance. Although cognitive performance usually becomes progressively worse with extensions of time-on-task, performance on even very brief cognitive tasks that require speed of cognitive throughput, working memory, and other aspects of attention are sensitive to sleep deprivation. Moreover, divergent and decision-making skills involving the prefrontal cortex are adversely affected by sleep loss (7). These include skills critical in operational environments such as risk assessment, assimilation of changing information, updating strategies based on new information, lateral thinking, innovation, maintaining interest in outcomes, insight, communication, and temporal memory
skills (7). In addition, fatigue and deficits in neurocognitive performance due to sleep loss compromise many working memory and executive attention functions important in operational environments. These include, but are not limited to assessment of the scope of a problem due to changing or distracting information, remembering the temporal order of information, maintaining focus on relevant cues and flexible thinking, avoiding inappropriate risks, gaining insight into performance deficits, avoiding perseveration on ineffective thoughts and actions, and making behavioral modifications based on new information (7).

Time zone transit, prolonged work hours, and work environments with irregular schedules contribute to performance decrements and fatigue, and therefore pose risks to safe operations. Both aviation and space environments impose such demands on their respective flight crew.

Operator fatigue associated with jet lag is a major concern in aviation, particularly with travel across multiple time zones. Flight crews consequently experience disrupted circadian rhythms and sleep loss. Studies have documented episodes of fatigue and the occurrence of uncontrolled sleep periods (microsleeps) in pilots (8). Flight crewmembers remain at their destination only for a short period, and therefore do not have the opportunity to adjust physiologically to the new time zone and/or altered work schedule before embarking on another assignment, thereby compounding their risk for fatigue.

Notably, although remaining at the new destination after crossing time zones for several days is beneficial, it does not ensure rapid phase shifting (or realignment) of the sleep-wake cycle and circadian system to the new time zone and light-dark cycle. Usually, pilots and flight crew arrive at their new destination with an accumulated sleep debt (i.e., elevated homeostatic sleep drive), because of extended wake duration incurred during air travel. As a result, the first night of sleep in the new time zone will occur without incident—even if it is abbreviated due to a wakeup signal from the endogenous circadian clock. However, on subsequent nights, most individuals will find it more difficult to obtain consolidated sleep because of circadian rhythm disruption. As a result, an individual’s sleep is not maximally restorative across consecutive nights, which leads to increased difficulty in maintaining alertness during the daytime. Such cumulative effects are incapacitating, often taking more than a week to fully dissipate through complete circadian reentrainment to the new time zone.

The magnitude of jet lag effects is also partly dependent on the direction of travel (8). Normally, eastward travel is more difficult for physiological adjustment than westward travel because it imposes a phase advance on the circadian clock, whereas westward transit imposes a phase delay. Because the human endogenous clock is longer than 24 hours, lengthening a day is easier to adjust physiologically and behaviorally than shortening a day by the same amount of time (8). However, adjustment to either eastward or westward phase shifts often requires at least a 24-hour period for each time zone crossed (e.g., transiting six time zones can require 5-7 days), assuming proper daily exposure to the new light-dark cycle. Regardless of the direction of phase shift imposed on flight crews, if there is inadequate time to adjust physiologically to the new time zone, cumulative sleep debt will develop across days and waking performance deficits will manifest—even if crews report no such deficits (9). These facts and guidelines need to be considered when designing and implementing flight schedules for aviation crews. Current Federal Aviation Regulations (FAR) limit aviation crew scheduling to no more than 14 hours in a day (with 10 consecutive hours of rest immediately proceeding) (10), 34 hours in any 7 consecutive days (11), and 120 hours in any calendar month (11). Beyond aviation pilots, Air Traffic Control Specialists (ATCS) who work on the ground and experience the demands of 24-hour operations also report fatigue (12). Many ATCS work counterclockwise by rapidly rotating shift schedules. Such schedules not only result in difficulties in sleeping but also can produce fatigue, exhaustion, sleepiness, and symptoms of gastrointestinal distress typically found in shift workers (12). The high levels of alertness required over extended periods of time makes ATCS vulnerable to the neurobehavioral and work performance consequences of circadian disruption and sleep loss, both of which can compromise air safety.

As is true for flight crew, astronauts can also experience performance decrements and fatigue in space. Space operations couple high-level sustained neurobehavioral performance demands with time constraints, and thereby necessitate precise scheduling of sleep opportunities to best preserve optimal performance and reduce fatigue (13). Astronaut sleep is restricted in space flight, averaging only 5 to 6.5 hr/d, as a result of endogenous sleep disturbances (e.g., motion sickness, circadian rhythm desynchrony, etc.), and environmental sleep disruptions (e.g., noise, movement around the sleeper, etc.), as well as sleep curtailment due to work demands (13). Such restriction is of concern, as ground-based experiments indicate that cognitive performance deficits and fatigue progressively worsen (i.e., accumulate) over consecutive days when sleep is restricted to amounts comparable to those experienced repeatedly by astronauts (9,13).

Scheduled sleep time during space flight is primarily determined by the mission’s operational needs. Sleep is regulated by a complex neurobiology involving homeostatic and circadian mechanisms that interact to determine sleep timing, duration, and structure (7). Therefore, total sleep time during space flight is usually less than scheduled time in bed. Reduced sleep efficiency, the percentage of time actually spent asleep in space flight, produces fatigue and it can therefore pose a serious risk to performance of critical mission activities. Consequently, sleep-wake schedules that optimize the recovery benefits of sleep, including reducing fatigue, while minimizing required sleep time need to be identified (13). To this end, the current practice in military
aviation and some space flight operations of using sleeping medication to ensure that sleep is obtained each day needs to be evaluated for its effectiveness in maintaining performance without creating sedating carryover effects. More research to develop other effective countermeasures for fatigue in space is currently underway, including investigating the use of timed light and the use of timed naps in space (13).

Fatigue, sleepiness, and performance decrements, including increased reaction times, memory difficulties, cognitive slowing, and attentional lapses, result from acute and chronic sleep loss and circadian displacement of sleep-wake schedules. As such, these are common occurrences in aviation and space environments, which utilize 24-hour work situations. Neurobehavioral and neurobiological research has demonstrated that waking neurocognitive functions depend on stable alertness resulting from adequate daily recovery sleep. Therefore, understanding and mitigating the risks imposed by physiologically based variations in fatigue and alertness in the operational workplace are essential for developing appropriate countermeasures and ensuring safety of flight and space crew.

Human error is a very broad topic the boundaries of which are not well defined. For example, no clear boundary exists between degradation of quality of performance of diverse tasks—as for example occurs under fatigue or stress—and discrete failures of action, such as forgetting to set wing flaps before attempting to takeoff.

James Reason’s book Human Error provides an excellent broad overview of the types of error that occur in everyday and workplace settings, the cognitive processes underlying vulnerability to error, and the ways in which organizational factors contribute to or mitigate against the propagation of errors into accidents (14). In aviation, and probably in most domains of skilled performance, most accidents are attributed to human error. An article in 1996 stated that estimates in the literature indicate that somewhere between 70% and 80% of all aviation accidents can be attributed, at least in part, to human error (15).

When professionals make errors while performing tasks not considered terribly difficult, both ordinary citizens and investigating organizations tend to assume these errors to represent some sort of deficiency on the part of the professional who made the error—that person must have lacked skill, was not conscientious, or failed to be vigilant (16). In some cases, those assumptions may be correct, but human factors research reveals that these cases are the exception rather than the rule. Both correct and erroneous performances are the product of the interaction of events, task demands, characteristics of the individual, and organizational factors with the inherent characteristics and limitations of human cognitive processes.

Some level of error is inevitable in the skilled performance of real-world tasks, such as flying airplanes, operating in emergency rooms, or driving automobiles. For example, Helmreich et al. have found that airline flight crews make one or more errors in approximately two third of flights observed (17). (These figures are probably undercounts because observers cannot detect all errors.) Because the performance of even the most skilled of experts is fallible, some may argue that automation should replace humans whenever possible. However, this perspective ignores the realities of the human tasks and the situations in which they are performed.

Computers can perform some tasks much faster and more reliably than humans, and those tasks should be assigned to computers. Nevertheless, human judgment is required for situations involving novelty, value judgments, or incomplete or ambiguous information. One would not want the decision of whether to divert a scheduled flight because of a sick passenger to be made by a computer! However, the very reasons that these tasks require human judgment make it unlikely for that judgment to be always correct.

Most errors made by professionals do not result in accidents, either because the errors do not have serious consequences or because the errors are caught and mitigated. Airline operations, for example, have extensive systems of defense to prevent or catch errors (e.g., the use of checklists and having the two pilots in the cockpit cross-check each other’s actions). When accidents do occur it is typically because several factors interact to undermine defenses which the organizations have erected to catch errors. This interaction is partly random, which makes it hard to prevent, as the number of factors that might interact is very large.

Because space does not permit a full review of the large research literature addressing the issues described earlier, we will briefly summarize a recent study that illustrates the nature of the errors of skilled experts, the causes of those errors, and possible countermeasures (16). The authors of this study reviewed the entire set of 19 major accidents in U.S. airlines occurring over a 10-year period, and in which the National Transportation Safety Board (NTSB) found flight crew error to be causal. The study examined the actions of each crew as the flight evolved, asking why might any highly experienced crew, in the situation of the accident crew, and knowing only what the accident crew knew at each moment, be vulnerable to doing things in much the same way as the accident crew.

The study found that almost all the events leading to these accidents clustered around five themes, defined as much by the situations confronting the pilots as by the errors they made:

The study also identified a range of cross-cutting factors that contribute to the vulnerability of pilots in making the sorts of errors described earlier. In many accidents, several of these cross-cutting factors were working simultaneously. Six of these factors are as follows:

This study has implications for understanding pilot error and for preventing future accidents. Foremost is that errors and accidents are best thought of as vulnerabilities and failures in the overall system rather than as deficiency on the part of pilots who met with accidents. Terms such as complacency and lack of situation awareness are labels, not explanations for why errors occur. Errors occur on almost all flights, but in the vast majority of flights, these errors are prevented from causing accidents. In aviation, a large number of safeguards have been erected to detect and mitigate errors, especially crew resource management (CRM), discussed later in this chapter. The latest version of CRM emphasizes threat and error management (TEM). The methods airlines use to achieve an extraordinarily high level of safety can be adapted to other fields, such as medicine and nuclear and chemical plant operations.

From a systems perspective, the best way to reduce vulnerability to errors and accidents is to design the overall operating system for resilience to equipment failures, unexpected events, uncertainty, and human error. Equipment, procedures, and training must be designed to match human operating characteristics, rather than expecting humans to adapt to the quirks of the equipment they must operate. Pilots can be trained to recognize situations in which they are vulnerable to error and can be provided techniques
for reducing vulnerability to specific forms of errors. All organizations—not just airlines—should periodically conduct systematic reviews of their operating procedures and revise procedures that are conducive to error.

Finally, organizations should recognize that efficiency and production throughput are pressures that often compete against safety. Organizations must recognize this conflict and take responsibility for establishing policies, procedures, and reward structures that truly support, giving safety the highest priority.

“Traditional Anthropometric measurements of bone and other tissue, though of scientific and practical value, are not functional; their applicability is limited to those rather standard conditions which exist when they are taken and they may not be transferred to other postures. Postural and kinematic problems may only be solved by a functional system of measurements.”

W.T. Dempster—from the Space Requirements of the Seated Operator, 1955 (19)

Anthropometrics is the measurement of human bodily characteristics (20). Everyone has seen long lists of anthropometric dimensions, usually presented as tables of percentile values. Ranges of joint motion, reach envelopes, strength profiles, and a great deal of other information on human capabilities and variation are presented in this manner as well. These data are commonly used in specifications for equipment design and to describe population variability. In reality, data of these types are of limited use. Current methods in anthropometry and biomechanics are more along the lines of Dempster’s vision. When designing and testing items of equipment, a systematic approach is used which includes the following:

Designing aircraft cockpits to accommodate the wide range of body sizes existing in the U.S. population has always been a difficult problem for crew station engineers. The approach taken in the design of military aircraft has been to truncate the range of body sizes allowed in flight training, and then to develop standards and specifications to ensure that most of the remaining pilot sizes are accommodated. Accommodation in this instance is defined as the ability to perform the following:

Each of these areas is directly affected by the body size of the pilot. Assignment of individuals to aircraft in which they are poorly accommodated puts them at increased risk for mishap. Although currently only a few accident investigations have reported body size as the sole cause of the mishap, there have been many mishaps where body size may have been a contributing factor. Methods for correcting this problem by predicting pilot fit and performance in the United States Air Force (USAF) aircraft based on anthropometric data were developed by the military in the 1990s. These methods, discussed in the subsequent text, can be applied to a variety of design applications where fitting the human operator into a system is a major concern.