Examining Fatigue Factors in Accident Investigations

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A recent international group of scientists identified fatigue as “the largest identifiable and preventable cause of accidents in transport operations (between 15 and 20% of all accidents), surpassing that of alcohol or drug related incidents in all modes of transportation. Official statistics often underestimate this contribution.” (Rosekind, 1993). Fatigue engendered by sleep loss and circadian disruption can degrade all aspects of human capability. Significant reductions in operator performance can affect judgment and decision-making, attention, reaction time, alertness, memory, and mood (Rosekind, 1993). These degraded performance factors can increase fatigue-related risks and reduce the operational safety margin. In spite of these well-documented effects, the contributory or causal role that fatigue may play in an accident is often underestimated or potentially ignored. One reason for underestimating its contribution is that there is “no blood test for fatigue.” Thorough accident investigations will include an analysis of alcohol and drug factors. If traces of these compounds are discovered, then they are generally identified as causes to the accident. However, no simple, practical or validated “blood test” for fatigue currently exists. Therefore, to include or exclude fatigue as a cause in an accident requires the evaluation of two specific aspects of the accident. First, was there identifiable fatigue factors present at the time of the accident? Second, if fatigue factors were present, did fatigue-related performance decrements contribute to or cause the accident? This paper will outline a systematic approach to examine the role of fatigue factors in an accident investigation. First, four specific physiological factors that can create fatigue will be described, including scientific data regarding their relevance. Second, there will be discussion of how to examine whether fatigue–related performance changes played a contributory or causal role in an accident. Third, to demonstrate the application of this approach in an actual accident investigation, the crash of a DC-8 in Guantanamo Bay, Cuba will be used as a model analysis. Scientific literature indicates that there are four physiological factors that are known to underlie fatigue and are relevant to an accident investigation (Rosekind & Graber). These four fatigue factors are: sleep, continuous hours of wakefulness, circadian rhythms, and sleep disorders. Each of these physiological factors will be described and their operational relevance discussed for their potential role in an accident. On average, human adults physiologically require about 8 hours of sleep for optimal waking performance and alertness (Kryler, 1994). Sleep loss can be considered in two ways: acute and cumulative. Acute sleep loss involves the total amount of sleep obtained in a 24-hour period. An average person that obtains only 5 hours of sleep one night has an acute sleep loss of 3 hours. Sleep loss that occurs over several days builds into a cumulative sleep debt. An average person that obtains only 5 hours of sleep for 3 consecutive nights has a cumulative sleep debt of 9 hours. Recovery from a cumulative sleep debt typically involves more deep sleep and not an hour-for-hour payback of lost sleep that requires extended sleep. Generally, two nights of usual sleep, at a person’s regular bedtime, can reduce the cumulative sleep debt to 0. Calculating an individual’s acute sleep loss or cumulative sleep debt should be based on the person’s usual sleep requirement and pattern. A scientific review found that two hours of sleep loss can result in “impairment of performance and levels of alertness”` (Kryler, 1994). How long an individual operator remains awake is another physiological factor that can affect performance and alertness. The physiological complement to sleep is the subsequent number of hours of continuous wakefulness. Shift work studies examining different duty lengths (e.g., 8 vs. 10...
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