Night Hypoxia and Fatigue on an Unpressurised Cross-Country Flight
You are pilot-in-command of an unpressurised light aircraft on a night VFR flight. You have been on duty for 14 hours, including a 3-hour delay, and you are 2 hours into the flight at 10,500 ft. You have no supplemental oxygen on board. You begin to feel euphoric, with a mild headache, tingling in your fingers and difficulty focusing on the instrument panel. Your passenger is asleep. The nearest suitable aerodrome is 25 nm ahead, with a 1,200 m lit runway; the destination is 70 nm ahead. Weather is clear, wind light, and fuel is sufficient for 2 h 30 min. You must decide whether to continue, descend, or divert, and how to manage the passenger and the aircraft.
1. What is the most likely explanation for your symptoms, and what immediate action is required?
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The symptoms - euphoria, headache, tingling fingers and difficulty focusing - are classic for mild hypoxia, worsened by fatigue. At 10,500 ft in an unpressurised aircraft without supplemental oxygen, the partial pressure of oxygen is low enough that after 30 minutes the crew's judgement and vision deteriorate. The delay and 14-hour duty increase oxygen demand and reduce tolerance. The immediate action is to descend below 10,000 ft, ideally to 8,000 ft or lower, and to use oxygen if any is available. Do not wait to see whether the symptoms pass; hypoxia impairs the very judgement needed to recognise it. Reduce power, descend at a safe rate, and maintain a visual scan. If symptoms do not improve below 10,000 ft, treat it as an emergency and land at the nearest aerodrome. Inform ATC if able, and do not continue climbing.
2. What are the regulatory and operational considerations for oxygen use at this altitude?
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In most ICAO-aligned regulations, crew members must use supplemental oxygen when above 10,000 ft cabin pressure altitude for more than 30 minutes, and continuously above 12,000 ft. At 10,500 ft, you are in the band where oxygen is required after 30 minutes; you have none, so the flight is already outside the intended oxygen envelope. Operationally, the lack of oxygen means the only compliant options are to descend below 10,000 ft or land. The passenger is asleep and may also be hypoxic, so wake the passenger, check orientation, and provide oxygen if available. Do not rely on 'feeling fine' after a short descent; fatigue and mild hypoxia can persist. The correct decision is to land at the 25 nm aerodrome, not to press on to the destination 70 nm ahead, because a second episode or worsening fatigue could lead to incapacitation.
3. How should fatigue and night conditions affect your decision-making and approach planning?
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Fatigue reduces attention, slows reaction time and increases the tendency to accept risk, which is especially dangerous at night when visual cues are poor and spatial disorientation is more likely. The 14-hour duty period and 3-hour delay mean you are beyond a normal efficient state. You should therefore choose the nearest suitable aerodrome, 25 nm ahead, rather than the destination. Before descent, brief the approach: set the altimeter, check runway lighting, confirm the 1,200 m runway is adequate, and plan a stabilised approach with a go-around if not. Use autopilot if available to reduce workload, but monitor it. Wake the passenger to help with radio calls or lookout. After landing, do not continue the flight the same night; rest before the next sector. The key judgement is that continuing is not worth the risk, even though the weather is clear.