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.
Why TUC shortens so dramatically with altitude
At sea level the atmosphere is about 21% oxygen and pressure is high enough to drive oxygen into the bloodstream efficiently. Above 18,000 ft the partial pressure of oxygen drops to the point where the blood cannot absorb enough even with full lung ventilation. The brain, which consumes roughly 20% of the body’s oxygen despite being only about 2% of its weight, is the first organ affected. Cognitive impairment — poor decisions, slow reactions, false confidence — begins before the pilot is aware of any problem, which is why TUC is so operationally critical.
The two-column table (rest vs. activity) matters because the difference is large. A pilot seated calmly at 25,000 ft may have 3 to 5 minutes; the same pilot moving through the cabin has roughly half that. Cabin altitude is not the same as aircraft altitude when the pressurisation system is working normally, but after a failure they quickly converge.
Rapid decompression and the reduction factor
A slow or gradual pressurisation failure produces the TUC values in the table. A rapid or explosive decompression — where the cabin altitude rises in seconds — can cut those values to roughly one-third to one-half, because gas rushing out of the lungs temporarily reverses the oxygen gradient and accelerates the onset of hypoxia.
The correct response in either case is immediate: mask on first, then initiate an emergency descent. Do not wait for symptoms to confirm hypoxia; by the time you feel obviously impaired, useful time may already have run out.
Regulatory context
FAR 91.211 sets the legal minimums for supplemental oxygen in unpressurised aircraft. The tool maps your entered altitude to the appropriate rule. Note that these are legal minimums, not physiological optima — many pilots choose to use oxygen below the legal threshold, particularly on long cross-country flights above 10,000 ft, to maintain alertness.
Treat every figure in this reference as a population average for planning and training. Individual TUC varies substantially based on fitness, anaemia, smoking history, recent alcohol use, fatigue, and adaptation to altitude. Hypoxia is insidious partly because it degrades judgment without the sufferer noticing; training to recognise personal symptoms is a valuable complement to knowing the published numbers.