The Silent Descent
A wide-body is on a 9-hour eastbound night flight with a three-pilot crew. The cruise is uneventful and the aircraft is on a datalink route with no frequency changes for 40 minutes. The relief pilot is resting in the crew bunk; the captain and first officer are on the flight deck. It is 04:10 body-clock time. The autopilot is engaged in altitude hold. The first officer is monitoring; the captain is completing paperwork. Over about 6 minutes, the aircraft drifts 900 feet below cleared level before the controller calls. Neither pilot noticed. Both later say they were 'watching the instruments' and felt alert. There were no warnings because the deviation was below the altitude alert threshold until the controller's call.
1. Explain how this deviation occurred without either pilot feeling drowsy.
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This is a classic underload and vigilance failure rather than a dramatic lapse into sleep. A stable, automated cruise phase with no frequency changes provides very low stimulation; attention to a monitoring task decays quickly when nothing changes, and the eyes can be directed at the instruments without the information being processed. Both pilots described feeling alert, which is expected: subjective alertness is a poor indicator of monitoring performance, and boredom masks the drift. The paperwork task and the absence of external prompts removed the natural cues that would have re-engaged attention. The result is a slow, unnoticed deviation that no single event corrected.
2. What defences should the crew have used, and what does this say about 'monitoring' as a task?
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The crew should have applied active monitoring discipline: call out the flight mode annunciator and altitude at defined intervals, alternate who is 'eyes on' while the other works, and avoid both pilots being engaged in secondary tasks at once. The relief pilot's rest period should have been timed so a fully alert pilot was on the flight deck during the circadian low. Monitoring is not a passive state; it needs structure, a schedule, and a shared verbal loop. When the task is 'watch the aircraft', the crew should create artificial change points so that a silent drift becomes visible.
3. What organisational factors contributed, and how would you change the operation?
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The operator created the conditions: a long night sector, a datalink route that removed radio interaction, a low-workload cruise, and a rest schedule that left two potentially fatigued pilots on the flight deck at the circadian low. The altitude alert threshold gave no warning for a slow drift. Changes should include fatigue-informed bunk scheduling, a monitoring policy with explicit callouts and task-sharing, and a review of how quiet datalink segments are managed. The point is that 'underload' is a real hazard with the same consequences as overload, and it must be designed against, not left to individual willpower.