14 CFR 91.211 (general aviation)
For an unpressurized (or cabin-altitude) flight under Part 91:
- Cabin pressure altitude above 12,500 through 14,000 ft MSL: the required minimum flight crew must use supplemental oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration.
- Above 14,000 ft: the required minimum flight crew must use oxygen for the entire time at those altitudes.
- Above 15,000 ft: each occupant must be provided with supplemental oxygen.
91.211(b) adds pressurized-cabin rules above FL250 (a 10-minute supply, quick-donning masks) and tighter mask discipline at higher flight levels. Read the current text of the rule for the exact conditions, including the FL350 / FL410 provisions.
Parts 135 and 121 are stricter
135.89 requires the pilot at the controls to use oxygen above 12,000 ft cabin altitude, and after 30 minutes above 10,000 through 12,000 ft. 121.329 requires occupant oxygen to be provided for more than 30 minutes between 10,000 and 14,000 ft, provided above 14,000, and provided and used above 15,000. 121.333 adds pressurized-cabin descent and mask-deployment requirements above FL250.
Time of useful consciousness
The FAA / Civil Aerospace Medical Institute table of typical TUC after a sudden exposure is the source for the ranges this page reports. It is a physiology briefing aid, not a countdown clock. Published values include 20–30 minutes at 18,000 ft, 3–5 minutes at 25,000 ft, 1–2 minutes at 30,000 ft, and 15–20 seconds at 40,000 ft. This calculator names the nearest published row and, when you are between rows, the neighboring brackets. It does not interpolate a false-precision curve.
Sources
14 CFR 91.211, 135.89, 121.327–121.333 (public eCFR text); FAA Pilot’s Handbook of Aeronautical Knowledge, Chapter 17 (aeromedical factors, hypoxia); FAA Civil Aerospace Medical Institute hypoxia guidance (TUC table). Always check the current CFR, not a memory item, before you rely on a threshold.
Frequently asked questions
Does Part 91 require oxygen at 12,500 ft?
Not at 12,500 ft exactly. 91.211(a)(1) applies above 12,500 through 14,000 ft, and only after more than 30 minutes. Many instructors still recommend using oxygen earlier, especially at night.
Why is TUC so short at 25,000 ft?
The partial pressure of oxygen is already low, and a sudden exposure (for example after a decompression) leaves little reserve. The published FAA range at 25,000 ft is 3–5 minutes, and it is shorter if you are physically active. That is why pressurized-cabin rules emphasize immediate descent and masks.
Are the Part 135 and 121 numbers the same as Part 91?
No. Crew oxygen under 135.89 starts at a lower cabin altitude (10,000 / 12,000 ft). Part 121 occupant-equipment rules also start at 10,000 ft. This page lists each part separately so you can see the difference.