The Last 35 Minutes: Fatigue on a Weekend Charter

The cues are cumulative, not isolated:

• Sleep debt and timing: about 4.5 hours of sleep after a late event, with an early wake. The current time is likely mid-morning, but the pilot already shows impairment after a short stop, suggesting the sleep loss is significant.
• Physical signs: heavy eyelids, repeated yawning, and an altitude drift of 150 ft show degraded attention and motor control.
• Cognitive signs: a missed radio call and a 30-second lapse are the most serious cues. That lapse is consistent with a microsleep, not simply boredom.

The 35-minute argument fails because risk is not proportional to time remaining. The last 35 minutes contain the highest-workload phase: descent, checks, radio calls, traffic, configuration and landing. A microsleep during approach or landing is far less recoverable than one at cruise. Fatigue also impairs self-assessment, so the pilot is a poor judge of how impaired they are. The thought 'only 35 minutes' is itself a warning sign of plan-continuation bias: the plan is being defended rather than reassessed. The correct question is not how long the flight is, but whether the pilot can reliably perform the next 35 minutes.

A simple detect-estimate-choose-act sequence works:

1. Detect: recognise the microsleep, missed call and altitude drift as a fitness-to-fly problem, not a minor concentration lapse.
2. Estimate: the probability of another lapse is high, and the consequence during arrival is severe. Continuing has little operational benefit.
3. Choose: divert to a suitable aerodrome short of destination, or land at the nearest suitable one, and stop flying for the day or take a substantial rest before any further flight.
4. Act: reduce workload immediately, use the autopilot if serviceable, and request priority or assistance from ATC if symptoms are severe. Do not press on to the planned destination simply because it is close.

Passenger communication should be brief and honest: 'I am not fit to continue safely, so we are landing at [place] to rest.' Passengers do not need a technical lecture, but they do need to know the decision is deliberate. After landing, record the event and review the duty-planning failure: a late family event before an 0600 departure, insufficient sleep, and no personal fatigue limit. Prevention includes a personal rule not to fly early after less than a defined amount of sleep, planning the previous evening around the flight, using a strategic nap before duty, and treating a microsleep as a stop-flying event rather than a warning to push through.

At 3500 ft in an unpressurised aircraft, significant hypoxia is unlikely in a healthy pilot. Hypoxia would normally worsen with altitude and improve with descent or oxygen, and it is often accompanied by tingling, visual disturbance, euphoria or headache rather than a clear sleep-debt pattern. Carbon monoxide is also less likely here because there is no report of heater use, headache, nausea or a sleepy passenger; the dominant signs are yawning, eyelid heaviness and a lapse after short sleep.

The distinction matters for diagnosis but not for the conservative action. If there is any doubt, treat it as a physiological problem: open fresh-air vents, turn off the heater if it is on, use supplemental oxygen if available, descend and land as soon as practicable. Do not continue while symptomatic. After landing, do not fly again until symptoms have resolved and the cause is understood. If carbon monoxide were suspected, medical assessment and aircraft inspection would be mandatory before further flight.