Hypoxia and Decision-Making on a Pressurized Sector
You are the pilot flying a pressurized light turboprop at FL250 on a 3 h sector. Cabin altitude is indicating 8 000 ft. About 1 h into cruise you notice a mild headache, slight euphoria, and that your colleague's speech seems slow. The pressurization controller shows normal differential, but the cabin altitude gauge has been reading steady at 8 000 ft since top of climb. You have a portable oxygen bottle with 30 min of supply at your current flow rate, and the aircraft has a fixed oxygen system with masks at both seats. The nearest suitable diversion airfield is 20 min away at FL250 descent.
1. Identify the most likely physiological problem and explain why the symptoms are easy to misattribute.
Read the worked answer
The symptoms - headache, euphoria, slowed speech - at a cabin altitude of 8 000 ft are consistent with hypoxia, likely aggravated by fatigue and the sustained cabin altitude. At 8 000 ft cabin altitude, oxygen saturation is reduced but usually tolerable for fit crew; the fact that symptoms are appearing suggests either the cabin altitude is higher than indicated (a gauge/controller fault) or the crew's tolerance is reduced.
The insidious part is that hypoxia impairs the very judgement needed to recognize it: euphoria and slowed cognition make the crew less likely to suspect the problem. The colleague's slow speech is a classic early sign. Do not dismiss it as fatigue.
2. State your immediate actions, including oxygen use and descent, and the reasoning behind the sequence.
Read the worked answer
- Don oxygen masks immediately (100% if available) - this is the fastest way to reverse hypoxia and confirm the diagnosis; if symptoms clear on oxygen, hypoxia is confirmed.
- Initiate descent toward a lower level, targeting a cabin altitude that restores comfortable saturation (e.g., 10 000 ft cabin or below). Descending also reduces the consequences if the pressurization fault is real.
- Cross-check the cabin altitude against the differential pressure and outside altitude; a steady 8 000 ft reading with symptoms suggests an instrument or controller fault. Treat the symptoms as real until proven otherwise.
- Divert to the 20-min airfield - the safest option is to get on the ground and have the aircraft checked, rather than continue a 3 h sector with a possible pressurization fault.
- Notify ATC and request priority as appropriate.
3. Explain the time-of-useful-consciousness concept and how it should shape your planning at FL250.
Read the worked answer
Time of useful consciousness (TUC) at FL250 is on the order of 3–5 minutes for a sudden decompression - the time before you become too impaired to act effectively. That is why the oxygen mask must go on first, before any radio call or checklist. At FL250 a rapid decompression leaves very little margin; the correct drill is mask on, establish communication, then descend. Planning-wise, this means briefings must emphasize mask-first, and the crew should know the nearest diversion before top of climb.