Cabin altitude warning, an unsealed mask and high ground ahead
Climbing through FL370 in a twin-jet, the cabin altitude warning sounds; the cabin is climbing through 14,000 ft. The commander dons and seals his mask and selects 100%. The first officer's mask is on but hangs loose at one side, and his regulator is at NORMAL. He has stopped answering the checklist, is fumbling with his headset, then says 'I'm fine, I'm fine' twice. The aircraft has drifted into a 15° right bank and is 400 ft high. The commander feels warm and slightly euphoric and has had to re-read the same checklist line twice. The minimum sector altitude on the route ahead is 12,800 ft with high ground 20 NM ahead; behind the aircraft the sector MSA is 6,000 ft. ATC has not been contacted.
1. Interpret the first officer's condition. Why is his own statement unreliable, and what is the significance of the loose mask and the NORMAL regulator position?
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His behaviour is a textbook picture of hypoxia: cessation of checklist response, loss of fine motor control (fumbling with the headset), repetition of a short reassurance phrase, plus the aircraft drifting in bank and altitude - the pilot flying has stopped flying and does not know it.
The critical point is that hypoxia destroys the very faculty needed to recognise it. The brain is the first organ affected, judgement and self-assessment fail early, and euphoria produces a false sense of competence. 'I'm fine' is therefore not evidence of anything; it is a symptom. Never accept a hypoxic crew member's self-assessment, and never argue with him.
The mask being unsealed at one side means he is entraining cabin air with every breath whatever the regulator says, so his oxygen concentration is far below what the gauge suggests. The NORMAL (diluter) position on top of that supplies a reduced-oxygen mixture as cabin altitude rises. Effective protection requires a sealed mask on 100%, held by the harness, with the regulator set deliberately - not left in a default position.
2. Set out the commander's actions in order, taking account of the terrain ahead.
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Order matters because the commander's own time of useful consciousness is limited and degrading.
- Own mask first - already sealed on 100%. Do not remove it at any point for any reason. Confirm it by squeezing the bag/checking the flow indicator.
- Take control. Disconnect the autopilot or take it through the mode controls, level the wings, stop the climb; brief yourself aloud if it helps.
- Get oxygen to the FO - check and reseat his mask, select 100% on his regulator, or fit a portable bottle. This takes seconds, not minutes, and is done with your own mask on.
- Communicate before descending: 'Mayday, Mayday, Mayday, decompression, descending to FL120, 20 NM south of ...'. Squawk 7700. Communication first is deliberate: it protects the aircraft from other traffic and from terrain while you are single-pilot and possibly impaired.
- The terrain problem. A standard 'descend to 10,000 ft' is wrong here: the MSA ahead is 12,800 ft. Either descend to FL130 in the sector ahead while remaining above MSA, or turn to the 6,000 ft sector behind and descend there. Do not simply select a lower level and let the autopilot fly into high ground.
- Then the checklists: outflow valve/manual pressurisation, bleed configuration, and the decision to land at the nearest suitable aerodrome.
3. The commander's own symptoms are the same as the FO's, and the descent will take several minutes. What must he guard against, and what should happen after landing?
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Guard against three specific failure modes:
- Doing too much. With early hypoxia and euphoria, there is a strong pull to troubleshoot the pressurisation system, run checklists, or 'sort out' the FO - all while single-pilot and descending. The priority is to fly a safe path and get the aircraft down, not to fix the cause.
- Going to help. The instinct to lean over and adjust the FO's harness or fetch a bottle is the classic fatal error; it means breaking the seal on the only functioning mask. Use the intercom, use the PA to summon cabin crew, or have the FO's mask adjusted by feel with your own mask in place.
- Mis-reading improvement. Once below about 10,000 ft the symptoms resolve quickly, which can create a false sense of full recovery and a temptation to continue the sector. Cognitive performance lags recovery; the crew is not fit to continue.
After landing: declare an emergency or request priority as appropriate, land at the nearest suitable aerodrome, and have both pilots medically assessed, including the FO, who may have been hypoxic for longer than is apparent. Preserve the pressurisation data, make the mandatory occurrence report, and treat the event as an airworthiness matter (outflow valve / pressurisation controller) rather than a crew-error matter until proven otherwise.