Fatigue and Circadian Low on a Pre-Dawn Duty
You are rostered for a report time of 04:10 local for a three-sector day, finishing at 15:30. You slept from 22:30 to 03:00, about four and a half hours, after a late shift the previous day. Your body clock has been stable on local time for two weeks. The first sector is a 55-minute flight in good weather; the second is a 2-hour sector with a demanding arrival into a high-altitude airport. By the second sector, around 07:00 local, you notice heavy eyelids, a strong urge to close your eyes during the cruise, and you have missed two radio calls until the controller repeats. You have had one coffee at report and no food since the previous evening.
1. Assess the fatigue risk using circadian and sleep-loss concepts, and rank the sectors by risk.
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Two factors are acting together. First, acute sleep restriction: about 4.5 hours against a typical need of 7 to 9 hours leaves a measurable sleep debt, and the previous late shift suggests the debt may be cumulative rather than one night. Second, circadian phase: 04:00 to 07:00 local falls in the early-morning circadian low, when alertness is at its lowest and the drive to sleep is strongest, even in people who have slept well. The combination is multiplicative, not additive.
Risk ranking:
- Highest: the second sector, because it coincides with the circadian low in cruise, has a long low-workload cruise phase, and ends in a demanding high-altitude arrival where workload spikes just as fatigue peaks.
- Moderate: the first sector, short but at the very bottom of the circadian trough during departure and climb.
- Lower: the third sector, later in the day when circadian alertness recovers, though cumulative duty time and the missed food intake still matter.
The missed radio calls are objective evidence of reduced vigilance and a genuine safety signal, not a minor lapse. Hypoglycaemia from no food since the previous evening amplifies the problem.
2. What would you do before and during the second sector, and what would justify declaring yourself unfit?
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Before the sector: eat a proper meal, hydrate, and take a short planned nap of 20 to 30 minutes if the turnaround allows, since a brief nap improves alertness without significant grogginess. Use caffeine strategically, timed 20 to 30 minutes before the high-risk period rather than continuously, and agree with the other pilot a shared plan: who flies, who monitors, and how you will cross-check each other.
During the sector: maximise monitoring discipline. Both pilots make and respond to radio calls aloud, use altitude and heading callouts, and avoid long silent cruise periods. Open the flight deck to light and air, avoid passive monitoring roles, and if the other pilot is also tired, consider splitting the cruise so one pilot is actively engaged at all times. Report the fatigue through the company system after flight; a fatigued report at 04:10 is a rostering issue, not a personal failing.
Declaring unfit is justified if you cannot maintain vigilance despite these measures: repeated missed calls, microsleeps, difficulty holding a heading or altitude, or an inability to recall the last few minutes. The threshold is not feeling tired, which is expected, but losing the ability to monitor reliably. On the ground, that means refusing the sector before departure; in flight, it means handing control, declaring the situation, and landing as soon as practicable.