At high cabin altitudes, the clock that matters most after a pressurisation failure is the time of useful consciousness — how long you have to recognise the problem and act before hypoxia takes the decision away from you. This reference looks up published TUC values by altitude and tells you which oxygen rule applies.
How it works
TUC is read from published FAA and aeromedical training tables, not calculated. The values shorten steeply with altitude: about 20 to 30 minutes at 18,000 ft, roughly 3 to 5 minutes at 25,000 ft, and only 15 to 20 seconds at 40,000 ft. Each altitude lists two figures — seated at rest and during moderate activity — because exertion roughly halves the time available.
The tool also evaluates the cabin altitude against FAR 91.211 and flags the oxygen requirement: crew oxygen after 30 minutes above 12,500 ft, crew oxygen at all times above 14,000 ft, and oxygen for all occupants above 15,000 ft.
Notes
These are gradual-onset values. A rapid decompression can cut the time roughly in half because gas leaving the lungs reverses the normal oxygen gradient. The correct response is immediate: oxygen mask on, then begin an emergency descent to a safe altitude. Treat every figure here as a planning average, never a personal guarantee.