Crew Factors in Flight Operations II:
Psychophysiological Responses to Short-Haul Air Transport Operations

1. OPERATIONAL OVERVIEW

This report is the second in a series on the physiological and psychological effects of flight operations on flight crews, and on the operational significance of these effects. This overview presents a comprehensive review and interpretation of the major findings. The supporting scientific analyses are described in detail in the rest of the text.

To document the psychophysiological effects of flying commercial short-haul air transport operations, 74 pilots from two airlines were monitored before, during, and after 3-day or 4-day trip patterns. All flights took place on the East Coast of the United States and data were collected throughout the year. Eighty-five percent of the pilots who had been awarded the trips selected for study agreed to participate. The population studied was experienced (average age 41.3 yr, average airline experience 14.6 yr) and averaged 68.6 hr of flying per month in all categories of aviation.

Subjects wore a portable biomedical monitor which recorded core-body temperature, heart rate, and wrist activity every 2 min. They also rated their fatigue and mood every 2 hr while awake, and recorded sleep episodes, naps, showers, exercise, duty times, food and fluid intake, voidings, cigarettes, medications, and medical symptoms in a daily logbook. A background questionaire was administered which included basic demographic information, sleep and life-style habits, and four personality inventories. A cockpit observer accompanied the crews on the flight deck and kept a detailed log of operational events.

The trips studied were selected to provide information on the upper range of fatigue experienced by pilots in predominantly daytime and evening operations. Common features were early report times and long duty days with multiple flight segments (average 5.5 per day). Daily duty durations averaged 10.6 hr which included, on average, 4.5 hr of flight time. One third of all duty periods studied were longer than 12 hr. The mean rest-period duration, as defined by the pilots in their daily logs, was 12.5 hr. The mean rest-period duration calculated from the last wheels-on of one duty day to the first wheels-off of the next duty day was significantly longer (14.0 hr). Overnight layovers after successive duty days occurred progressively earlier across most trips.

On trip nights, subjects reported taking about 12 min longer to fall asleep, sleeping about 1.2 hr less, and waking about 1.4 hr earlier than on pretrip nights. They also rated their sleep on trips as lighter and poorer overall, and reported significantly more awakenings. In contrast, in the laboratory, sleep restriction results in more rapid sleep onset and more consolidated sleep (refs. 1-4). The longer sleep latencies and more frequent awakenings reported by pilots on trips may reflect the commonly reported need to "spin down" after coming off duty and the disruptive effects of sleeping in unfamiliar environments. The fact that sleep during trips was reported not only as shorter but also as more disturbed, suggests that the effects of this sleep restriction on subsequent daytime sleepiness, performance, and mood may be greater than those reported in laboratory studies with similar levels of sleep restriction.

The effects of duty demands on subjective fatigue and mood are most clearly seen in the comparisons of ratings made pretrip, during flight segments, during layovers, and posttrip. During layovers, fatigue and negative affect were rated as highest and positive affect and activation as lowest. Positive affect was rated as highest during flight segments, even though fatigue ratings were higher than for either pretrip or posttrip. Posttrip recovery was indicated by return of fatigue levels to baseline, the lowest negative affect ratings, and the highest levels of activation. Significant time-of-day variations were found in fatigue, negative affect, and activation. Fatigue and negative affect were low in the first three ratings after awakening, and rose thereafter to reach their highest daily values in the final rating before sleep. As expected, activation showed the opposite time-of-day variation. No significant relationships were found between the timing, duration, or flight hours in a duty period and the fatigue and mood during layovers. This may well have been because of the high levels of individual variability in these ratings.

The use of tobacco did not change on trip days relative to pretrip and posttrip days. However, significantly more caffeine and alcohol were consumed on trips. Additional caffeine consumption occurred primarily in the early morning, associated with the earlier wake-up times on trips, and also around the time of the mid-afternoon peak in physiological sleepiness. The urge to fall asleep at this peak time would increase progressively with the accumulating sleep debt across trip days. The additional alcohol consumption may be assumed to have occurred after coming off duty and before going to sleep. The common practice of using alcohol to relax before sleep is not recommended. Although alcohol may facilitate falling asleep, it has well-documented disruptive effects on sleep which can adversely affect subsequent waking alertness and performance. There were no significant changes in the use of medications, or in the number of reports of medical symptoms between trip days and pretrip or posttrip days. Similarly, the number of exercise sessions reported was no different on trip days than on pretrip or posttrip days.

The number and timing of meals on trip days was not significantly different from pretrip or posttrip days. However, more snacks were eaten, and they were eaten earlier, on trip days. This suggests that meals on trip days may have been smaller or less filling than meals on pretrip or posttrip days.

Heart rates during takeoff, descent, and landing were compared with values during mid-cruise for 72 pilots during 589 flight segments. Increases in heart rate were greater during descent and landing for the pilot flying. The difference between flying and not flying during descent was greater for first officers than for captains. Heart-rate increases were greater during takeoff and descent under instrument flight conditions than under visual flight conditions. On the basis of similar findings, (ref. 5) proposed that the number of segments flown per day should be regulated.

A number of ways of reducing fatigue during short-haul air-transport operations are suggested by this study. First, since daily duty durations were more than twice as long as daily flight durations, and since about one third of all duty periods were longer than 12 hr, it would seem reasonable to limit duty hours in addition to, flight hours in short-haul operations. There may also be some advantage to defining the rest period more precisely, since significant variability is possible within the present system of definition by contract negotiation. Second, the practice of requiring early report times makes it more difficult for pilots to obtain adequate sleep, even during relatively long layovers. This is because circadian rhythms impede falling sleep earlier than usual, except after major sleep loss. Third, in the trips studied, duty began progressively earlier across the days of the trip. Because of the difficulty of falling asleep earlier, this has the effect of progressively shortening the time available for sleep across the days of the trip. In addition, because the innate "physiological day" determined by the circadian system is longer than 24 hr, it adapts more readily to schedule delays than to advances. Thus, where possible, successive duty days should begin progressively later. Fourth, the widespread use of alcohol as a means of relaxing before going to sleep has deleterious effects on subsequent sleep. It thus seems likely that the quality of sleep on trips could be improved in many cases by providing pilots with information on alternative relaxation techniques which have been well-tested in the treatment of sleep disorders.

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