AA.I.E.K5FAA-S-ACS-11A · Nov 2023Knowledge
Altitudes and conditions that require the use of oxygen masks.
Altitudes and conditions that require the use of oxygen masks.
Bottom line
Under 14 CFR 121.333(c)(2), when operating at flight altitudes above flight level 250, one pilot at the controls must at all times wear and use an oxygen mask secured, sealed, and supplying oxygen, in accordance with the following. Under 14 CFR 121.329, at cabin pressure altitudes above 10,000 feet up to and including 12,000 feet, oxygen must be used by each flight crewmember on flight deck duty for that part of the flight at those altitudes of more than 30 minutes duration.
Key numbers
| Number | What it means |
|---|---|
| Above FL250, one pilot wears mask | Mask rule |
| One hand, within five seconds | Quick-donning |
| Exception at or below FL410 | Large airplane |
| Exception at or below FL350 | Small airplane |
| Above 10,000 ft up to 12,000 ft, over 30 minutes | Cabin altitude |
FAA references
- 14 CFR 121.333 (Supplemental oxygen for emergency descent and for first aid; turbine engine powered airplanes with pressurized cabins.) — eCFR current as of 2026-10-01
- 14 CFR 121.329 (Supplemental oxygen for sustenance: Turbine engine powered airplanes.) — eCFR current as of 2026-10-01
- AC 61-138, para. 14.3.1.2 — AC No: 61-138A
- 14 CFR 25.1447 (Equipment standards for oxygen dispensing units.) — eCFR current as of 2026-10-08
- 14 CFR 121.327 (Supplemental oxygen: Reciprocating engine powered airplanes.) — eCFR current as of 2026-10-01
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