Duty Time, Rest, and a Mechanical Delay That Crosses the Limit
A two-pilot crew reports for duty at 06:00 local for a four-sector day. Planned finish is 15:30, giving 9 hours 30 minutes of duty. The second sector is delayed 2 hours 40 minutes by a hydraulic leak that requires a component change. The crew is re-planned and the company dispatcher proposes completing all four sectors, which would put the finish at 18:10 - 12 hours 10 minutes of duty. The captain has already flown 6 hours 20 minutes of flight time in the preceding 24 hours, and 22 hours 40 minutes in the preceding 7 days. The operator's manual sets a maximum flight time of 8 hours in any 24 consecutive hours and 30 hours in any 7 consecutive days, and a maximum duty period of 13 hours. The captain has a 10-hour rest period scheduled before the next duty.
1. Determine whether the proposed re-plan is within the flight and duty time limits, showing the arithmetic and identifying any margin.
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Work each limit separately; the binding constraint is the one with the least margin.
Flight time, 24 hours. The captain has flown 6 h 20 min in the preceding 24 hours. The four-sector day originally planned would add flight time; assume the planned flight time is roughly 5 h 30 min for the day. Adding that to 6 h 20 min gives about 11 h 50 min, which already exceeds the 8-hour limit if the preceding 24 hours are counted as a rolling window. The limit is the binding one. The arithmetic must be done on the actual rolling 24-hour window at the time of each sector, not on the calendar day.
Flight time, 7 days. 22 h 40 min plus the day's flight time approaches or exceeds 30 hours depending on the exact figure. This is close and must be computed precisely.
Duty period. 06:00 to 18:10 is 12 h 10 min, which is inside the 13-hour maximum, leaving 50 minutes of margin. Duty alone does not block the plan.
Rest. A 10-hour rest before the next duty is adequate for a standard rest requirement, but if the duty period is extended the operator's manual may require compensatory rest, which should be checked.
Conclusion. Duty time is legal with 50 minutes to spare, but the 24-hour flight time limit is likely to be exceeded. The plan cannot be flown as proposed; sectors must be dropped or the flying reassigned.
2. The dispatcher argues that the delay was caused by maintenance and should therefore be treated as an unforeseen circumstance. How should the captain respond?
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The captain should reject the argument, and the reasoning matters more than the conclusion.
- The cause is irrelevant to the limit. Flight and duty time limits exist because of fatigue risk, and fatigue accumulates the same way whether the delay was mechanical, weather, or traffic. A limit that could be waived by attributing the cause to maintenance would not be a limit.
- 'Unforeseen' is narrowly defined. Where an operator's manual permits an extension for unforeseen circumstances, it typically requires that the circumstance could not reasonably have been foreseen, that the extension is limited in duration, that it is used to reach a point where the crew can be replaced, and that it is recorded. A hydraulic leak found on the ground during a turnaround, with a known component change, is a maintenance event the operator plans for, not an unforeseeable one.
- The extension is not a tool for completing the schedule. It exists to avoid stranding a crew or passengers in an unsafe position, not to preserve four sectors of commercial flying.
- Command authority. The captain's refusal is a safety decision and should be recorded as such, with the dispatcher's request noted.
The correct response is to state the limit, state the arithmetic, and propose the operational solution: drop the fourth sector, or arrange a crew change at the next station.
3. What fatigue-risk and human-factors factors should the captain weigh beyond the bare numbers when deciding how to finish the day?
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Compliance with the numbers is the floor, not the standard. Factors to weigh:
- Time of day. A duty that began at 06:00 and extends to 18:10 covers the early-afternoon circadian dip, when alertness is lowest, and the crew has been on duty through a stressful 2 h 40 min maintenance delay.
- Delay stress. Sitting in a hot or noisy environment waiting on a component change is not rest. The crew arrives at the aircraft more tired than the duty clock suggests.
- Sector count and approach exposure. Four sectors means four departures and four arrivals, and the later sectors are the ones flown when fatigue is highest. The final approach is where the residual risk concentrates.
- Cumulative load. 22 h 40 min in 7 days is a heavy week. Ask whether the crew has had a day off in that period and how the previous night's sleep was.
- Self-assessment. Use a structured check rather than a feeling. If either pilot reports significant fatigue, that is decisive regardless of the arithmetic.
- Mitigation. Where the flight proceeds, brief the approach threats explicitly, assign the higher-workload tasks deliberately, and set a low threshold for a go-around.
If the honest assessment is that the crew is significantly fatigued, the correct decision is to stop for the day, and the flight and duty time limits should be treated as confirming that decision rather than as the only reason for it.