§ 91.211 Supplemental oxygen.

Section 91.211 — Supplemental oxygen — sets the altitudes at which oxygen is required for crew and passengers.

Paragraph (a) — General aviation altitudes (the 12-14-15 rule):

• (a)(1): Above 12,500 MSL up to and including 14,000 MSL — required for required minimum flight crew when flight at those altitudes exceeds 30 MINUTES.
• (a)(2): Above 14,000 MSL — required for required minimum flight crew at all times.
• (a)(3): Above 15,000 MSL — required for ALL OCCUPANTS (crew AND passengers).

What 'required minimum flight crew' means: In a Cessna 172, the required minimum is one pilot. In a King Air, it's typically two. The oxygen requirement applies to the pilots, not passengers, until 15,000 MSL where everyone must have oxygen.

Paragraph (b) — Pressurized aircraft: For pressurized aircraft, the rule is based on CABIN pressure altitude, not the aircraft's actual flight altitude. The 12,500/14,000/15,000 thresholds apply to the cabin altitude.

Paragraph (b) — Mask quick-don for pressurized aircraft above FL250: If operating above FL250 with passengers, the crew member at controls must have a quick-donning oxygen mask within reach. This is the famous '30-second mask' rule for cabin-altitude emergencies.

Hypoxia awareness: Hypoxia at 12,500 MSL after 30 minutes is real and measurable. Studies show measurable cognitive degradation at altitudes as low as 10,000 MSL after several hours. Many GA pilots use pulse oximeters and supplemental oxygen at altitudes below the legal minimum to maintain alertness.

Reference: FAA-H-8083-25 (PHAK) Chapter 17 on aeromedical factors; AC 61-107B on operations at high altitudes.

The 12,500 / 14,000 / 15,000 Rule — § 91.211 Decoded

Section 91.211 is one of the most-tested regs on the oral exam — and one of the most misremembered. It sets three altitude bands in MSL (not AGL — the whole reg is referenced to mean sea level), and at each band the rule changes both who needs oxygen and when. Get the bands straight and you've got the reg.

Three distinctions a lot of pilots fumble:

• "Required minimum flight crew" vs "each occupant." Below 15,000 the reg only talks to the crew. At and above 15,000 it pulls every soul on board into the picture.
• "Provided" is not "used." Above 15,000 you have to offer O₂ to every passenger. If they refuse the mask, that's their call — you've met the reg by having it available.
• Cabin pressure altitude, not aircraft altitude. Per (b), if you're flying pressurized, the trigger is what the cabin is reading — not what's on the altimeter. A jet at FL350 with a cabin holding 8,000 is nowhere near the § 91.211 thresholds.

One more thing worth filing: the reg lags physiology. Hypoxia onset for an unacclimatized pilot typically starts somewhere between 8,000 and 12,500 MSL — well below the legal trigger. The FAR sets a floor for compliance. Restorative airmanship sets a higher floor for performance. The competent pilot uses O₂ before the reg says they have to.

What an Examiner Asks About § 91.211

This is one of those regs your DPE will lean on because the answers separate pilots who memorized a number from pilots who actually understand the structure. Be ready for these:

• "When does crew oxygen become required?" Above 12,500 MSL, after 30 minutes at that altitude. Above 14,000, immediately and continuously.
• "What's the difference between 14,000 and 15,000?" 14,000 is a crew threshold — required to use. 15,000 pulls in every occupant — required to be provided. Crew vs everybody. Use vs provide.
• "What's the 30-minute rule about?" It only applies in the 12,500-to-14,000 band, and it's cumulative time at altitude — not a once-per-flight clock you can reset by descending and re-climbing.
• "At 13,000 MSL for 20 minutes — do you need O₂?" Legally, no — you're inside the 30-minute window in that band. Operationally, probably yes if you're feeling anything. Don't argue the reg when your body's telling you something.
• The trick question: "Same scenario but pressurized — does it change?" Yes. Per (b), the rule keys to cabin pressure altitude. If the cabin is at 8,000, you're not in § 91.211(a) territory at all.

Flying High in a Normally-Aspirated 172, Under § 91.211

Picture a real scenario, because the reg comes alive when you put it in a cockpit. High-DA mountain day in the West. Density altitude is brutal, terrain is unforgiving, and the only safe cruise altitude that clears the ridges is 11,500 MSL — with a stretch where you need to step up to 12,500 for about 25 minutes to keep margin over a saddle.

Walk it through. At 11,500 you're under the threshold — § 91.211 doesn't say a word. At 12,500 for 25 minutes you're at the floor of the 30-minute band, not above it — and you're under the cumulative 30 minutes. Legally, no O₂ required.

Operationally? Different question. Up there in a normally-aspirated 172 you're already burning oxygen the engine can't deliver — and neither can your bloodstream as well as you'd like. Reaction time stretches. Color perception flattens. Decision-making gets slow in a way you don't notice until you debrief on the ground. And here's a number most checkride study guides skip: night vision starts degrading above about 5,000 MSL — well below any legal O₂ requirement. If that mountain leg is anywhere near sunset, the calculus shifts hard.

The move a competent pilot makes on a leg like that: carry a pulse oximeter. Cheap, pocket-sized, runs on a couple of AAAs. Spot-check yourself every 10–15 minutes above 8,000. If your SpO₂ drops below 90%, you're done debating the reg — you're on O₂ or you're coming down. The FAR is the legal floor. Airmanship is the actual ceiling.

Throttle On!