Diurnal rhythmicity and Air Force flight accidents due to pilot error

In order to evaluate the possible role of an endogenous rhythmic factor in Air Force flight accidents a retrospective study was carried out. The study included all Air Force (aircraft) flying accidents which have been attributed to pilot's error and which occurred, in peace time missions, over a period of 12 years (1968-1980). The frequency of hourly accidents was computed separately, for each year, for each month, for each day of the week, and for each calendar day. Identical computations were carried out for the frequency of hourly flights. When the hourly ratios of these two parameters were computed, by dividing the value of one parameter to the other at each hour, a rhythmic (rather than constant) diurnal pattern was obtained. The pattern was defined as the "Hourly Accident Coefficient (HAC)". The HAC values ranged from 1.58 to 0.68 (pooled data for all surveyed aircrafts) and from 4.12 to 0.74 (data for fighter planes). The pattern, which exhibited a diurnal rhythm, was independent of the frequency of flights and appeared to be related to the sleep-wake cycle of the pilots, especially to the time of waking from the night sleep. The results are used as a directive for a progressive study aimed at evaluating the practical implications of the presented observations.