Hypoxia and Decision Quality in a Pressurised Cabin Failure
You are flying a light twin at FL180 with three passengers. The cabin altitude warning illuminates and the cabin climbs to 14,000 ft. You feel mildly euphoric and unusually confident, and you notice your handwriting on the flight log is becoming untidy. Your passenger in the right seat is laughing at nothing and has taken off his jacket. The aircraft has a 45-minute oxygen supply for four people if the masks are used continuously. The nearest suitable airport is 90 NM away, requiring a descent to 8,000 ft. Terrain along the direct track rises to 11,500 ft. You have a current instrument rating.
1. Identify the condition and explain why the pilot's own judgement is unreliable in this situation.
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The picture is hypoxia from a cabin altitude of 14,000 ft, with the pilot showing early symptoms: euphoria, inappropriate confidence, and deteriorating fine motor control evidenced by the handwriting. The passenger's laughter and removal of clothing (a sensation of warmth from peripheral vasodilation) are consistent with the same cause.
The critical point is that hypoxia impairs the very faculty needed to recognise it. The brain's oxygen supply falls, and the first functions lost are those involving judgement, self-criticism and complex reasoning, while a superficial sense of wellbeing remains. The pilot therefore feels 'fine' and may rationalise the symptoms as fatigue or stress. Because the pilot's own assessment is the impaired instrument, the correct response is to act on the objective indications - the warning, the cabin altitude, the symptoms in others - rather than on how the pilot feels. Waiting to feel worse is a trap, because the deterioration is insidious and can progress to incapacitation without the pilot noticing.
2. Set out the immediate actions in priority order, including oxygen management.
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- Don oxygen masks immediately, all occupants, 100% if available, and confirm flow. Do not delay to diagnose.
- Initiate an emergency descent to a cabin-safe altitude, typically 10,000 ft or lower, while remaining within the aircraft's limits. If structural or terrain considerations prevent an immediate descent, level off at the highest altitude that restores useful consciousness and continue descending as soon as possible.
- Squawk 7700 and declare an emergency with ATC, stating the cabin altitude problem and the intended descent and routing. ATC can clear traffic and provide terrain-safe headings.
- Manage terrain: the direct track has high ground to 11,500 ft. A descent to 8,000 ft must follow a route clear of terrain, so request a heading or airway that provides terrain clearance, and do not simply descend on track.
- Consider the passengers: brief them calmly, ensure their masks are on, and check for anyone who is unresponsive.
- Plan the diversion to the airport 90 NM away only after the descent and oxygen are under control, and account for the 45-minute oxygen supply - it is generous for a descent but not for a long level cruise.
3. Explain how the oxygen supply duration and the diversion distance interact, and what decision that drives.
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45 minutes of oxygen for four people at continuous use is the limiting resource once the cabin altitude is high. The descent to 8,000 ft should take only a few minutes, after which the masks may not be needed continuously if the cabin altitude is low, but you cannot assume that until the descent is complete.
If the aircraft cannot maintain a safe cabin altitude, the diversion must be flown at an altitude where oxygen is not required for the whole flight. 90 NM at, say, 180 KT groundspeed is 30 minutes; at a lower altitude with a reduced true airspeed it may be 35–40 minutes. That is within the 45-minute supply, but only if the descent is prompt and the masks are not wasted during a long period at high cabin altitude while you troubleshoot.
The decision is therefore to descend first and troubleshoot second. If the problem cannot be fixed and the diversion is longer than the oxygen allows, the correct choice is to land at the nearest airfield at all, even if less suitable, rather than stretch the supply. Fuel, terrain and weather all matter, but the oxygen clock is the one that runs out first.