Additive Effects of Time Awake and Number of Flights on Fatigue in Short-Haul Pilots: Implications for Duty Time Regulation.

Time awake affects airline pilots' fatigue but it is less clear how workload during that time awake impacts fatigue. Although current flight time limitations take the number of flights within a duty period as a measure of workload into account, quantitative evidence for its fatiguing effect is scarce. Here, we analysed fatigue, subjective workload and flight-associated hassles at the end of 553 short-haul flights from 37 airline pilots. Two linear mixed-models with fatigue or subjective workload as the dependent variable were generated, adjusted for time awake, number of completed flights, number of hassles, prior sleep and pilot's responsibility (flying/monitoring). Results showed that fatigue was influenced by time awake, number of flights and pilot's responsibility, whereas no effect was found for number of hassles. The critical fatigue threshold (flying not recommended) was exceeded 2.4 h earlier when operating four flights compared with one flight. Fatigue was lower but subjective workload was higher when the pilot was flying instead of monitoring. Subjective workload increased with number of hassles, but not with number of flights or time awake. European duty time limitations are justified to include a penalty for additional flights but may underestimate the fatiguing effect.