Night Freight at 11,000 ft after Four Hours' Sleep
It is 22:40 and you are cruising at 11,000 ft in an unpressurised Cessna 208 on a night freight sector, two hours into a duty that began at 08:00. You slept about four and a half hours in the last 24 because of a family problem, and you flew four sectors yesterday. There is no oxygen on board and no autopilot. Over the last ten minutes you have noticed tingling in your fingertips, a feeling of warmth and mild euphoria, a slight headache, and the instrument panel seems to be losing contrast. You feel pleasantly relaxed and unusually confident that everything is fine; the destination is 35 minutes away and the weather there is fine. Your colleague in the right seat has said nothing. The aircraft is running smoothly, the fuel state is as planned, and there is no abnormal indication on the engine instruments.
1. What is the most likely explanation, and why is the pilot the last person to notice it?
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This is hypoxia, not fatigue alone, and the relaxation and euphoria are the giveaway. At 11,000 ft the alveolar oxygen partial pressure falls to roughly 55–60 % of sea-level values; the result is impaired judgement, reduced visual acuity and colour vision, tingling, headache and a false sense of well-being - but not unconsciousness. That is precisely what makes it dangerous: you are impaired without being incapacitated, so you believe you are handling it well.
The pilot is the last to notice because:
- The organ that would detect the problem - the brain - is the one being starved.
- Euphoria is a symptom that suppresses self-criticism; 'everything is fine' is the symptom speaking.
- The panel contrast loss is your own visual acuity degrading, not an electrical fault.
- Fatigue and the circadian low (22:40, after a 14-hour duty on 4.5 h sleep) lower the threshold at which hypoxia bites and slow every corrective decision.
Regulatory point to note: in an unpressurised aircraft, flight crew must use supplemental oxygen for any period above 10,000 ft cabin altitude exceeding 30 minutes, and continuously above 13,000 ft. At 11,000 ft with no oxygen on board, this flight should not have been planned.
2. What are your immediate actions?
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Do not troubleshoot, descend. In order:
- Call ATC and state the problem - suspected hypoxia at 11,000 ft, request an immediate descent to 10,000 ft or below. Use the words; do not merely ask for 'lower'.
- Start the descent now at a comfortable rate with power reduced, keeping a positive attitude and trimmed aircraft; do not chase the airspeed or the panel you cannot see clearly.
- Involve the other pilot - tell your colleague to monitor the instruments, the heading and the altitude, and to take control if you become unreliable. A second person is your only cross-check when your own judgement is compromised.
- Do not go around, hold or continue to destination to save the sector. If symptoms do not resolve quickly on descent, land at the nearest suitable aerodrome.
- Confirm the diagnosis afterwards - the same picture can come from hyperventilation, but hypoxia is the one that kills if you treat it as anxiety.
After landing: do not simply go home. This is a reportable occurrence and a duty-planning failure as much as a physiological one.
3. How did this error chain form, and what should happen after the flight?
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The chain is visible in hindsight and each link was reasonable at the time:
- Insufficient sleep (4.5 h in 24) taken voluntarily, so no one flagged it.
- A duty rostered to 14 hours with four sectors the previous day - cumulative fatigue, not a single bad night.
- A night sector into the circadian low, when alertness and self-monitoring are naturally weakest.
- A cruise level of 11,000 ft chosen for terrain and winds, without asking whether the aircraft had oxygen or whether the crew should be that high for that long.
- Symptoms misread as comfort - the euphoria shut down the option to question the plan.
After flight: write an occurrence report covering the oxygen equipment, the flight level and the duty history, and be honest about the sleep. Then address the system: an unpressurised freighter cannot routinely cruise above 10,000 ft for more than 30 minutes without oxygen. The practical fixes are cruise at or below 10,000 ft, portable oxygen on board, and a fatigue-reporting process that a pilot will actually use.