Commercial pilot · ACS CA.VIII.A
Supplemental Oxygen
Supplemental oxygen keeps the crew and passengers alert at altitude. Knowing the rules, the systems and your own symptoms turns a silent hazard into a checklist item.
Why this one matters
Hypoxia has killed pilots who never knew they were impaired; the airplane flew on until the fuel ran out. A commercial pilot flying passengers high to top weather or for performance must plan the oxygen, use it early, and brief the people in the back, because they can't judge their own condition either.
Can’t miss
- •At cabin pressure altitudes (the airplane's altitude when unpressurized): required crew must use oxygen above 12,500 feet up to and including 14,000 for any time beyond 30 minutes, and at all times above 14,000; every occupant must be provided oxygen above 15,000.(14 CFR 91.211)
- •Hypoxia is insidious: the first thing it impairs is the judgment you'd use to notice it.(AIM, Chapter 8, Medical Facts for Pilots)
- •Time of useful consciousness: roughly 3-5 minutes at 25,000 feet and 1-2 minutes at 30,000; less after a rapid decompression.(AIM, Chapter 8, Medical Facts for Pilots)
- •Continuous-flow systems suit most piston airplanes: cannulas are approved only to 18,000 feet, and continuous-flow masks are effective to about 25,000; above that, diluter-demand or pressure-demand systems.(PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization))
- •Use aviator's breathing oxygen (99.5% pure, very dry). Medical oxygen can hold enough moisture to freeze in the lines and regulator; welding oxygen isn't a breathing grade and may contain impurities.(PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization))
- •Oxygen feeds fire: no smoking, no oil or grease on fittings, secure the bottles, and keep it away from heat and sparks.(PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization))
- •Brief passengers before you need it: how to don the mask or cannula, when, and the signs of hypoxia.(Commercial Pilot ACS (FAA-S-ACS-7B), CA.VIII.A.S3)
What you should know
The rules K1 · R1
- Cabin pressure altitude above 12,500 feet up to and including 14,000: required flight crew use oxygen for any portion beyond 30 minutes. Above 14,000: crew uses it at all times. Above 15,000: every occupant must be provided oxygen.(14 CFR 91.211)
- Pressurized airplanes, 91.211(b): above FL250, at least a 10-minute supply for each occupant, in addition to the oxygen above, for a descent after losing pressurization; above FL350, one pilot at the controls must wear and use a sealed mask, except at or below FL410 if two pilots are at the controls with quick-donning masks (or if one pilot leaves the controls, the other wears the mask).(14 CFR 91.211)
- The rules are minimums. Many pilots use oxygen from 10,000 feet by day and lower at night, when tired, or after a cold.(AIM, Chapter 8, Medical Facts for Pilots)
Physiology K2
- Hypoxia is too little oxygen reaching the body. Hypoxic hypoxia comes from altitude; hypemic from carbon monoxide or blood loss; stagnant from poor circulation (including G-loads); histotoxic from alcohol or drugs.(PHAK (FAA-H-8083-25C), Aeromedical Factors)
- Symptoms vary by person and come on insidiously: euphoria, poor judgment, headache, tunnel vision, tingling, drowsiness, slowed reactions, cyanosis. Learn your own (an altitude chamber course helps).(AIM, Chapter 8, Medical Facts for Pilots)
- Time of useful consciousness shrinks fast with altitude: about 20-30 minutes at 18,000 feet, 3-5 minutes at 25,000, 1-2 minutes at 30,000, 30-60 seconds at 35,000. A rapid decompression can cut it roughly in half.(AIM, Chapter 8, Medical Facts for Pilots; AC 61-107, Aircraft Operations at Altitudes Above 25,000 Feet MSL and/or Mach Numbers Greater Than .75)
- Night vision degrades from as low as 5,000 feet; smoking, alcohol, fatigue and some medications lower your tolerance. A pulse oximeter shows trends but doesn't replace the rules.(AIM, Chapter 8, Medical Facts for Pilots)
Systems and equipment K3 · R2 · R3 · R4
- Continuous flow: a steady flow to a cannula or mask, simple and common; cannulas are limited to 18,000 feet. Diluter-demand delivers oxygen as you inhale, mixed with cabin air. Pressure-demand pushes oxygen under pressure, for very high altitudes.(PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization))
- Calculate the quantity: the bottle size, the flow rate for the altitude and the number of users give the endurance; plan a reserve.(PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization))
- Check before flight: pressure, mask or cannula condition, flow indicators working, the bottle secured, hydrostatic test date current.(PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization))
- Oxygen-rich environments burn fiercely: keep oil, grease, lip balm and cigarettes away; open valves slowly.(PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization))
Scenarios
Decide what you would do first, then see the worked answer.
At 15,500 feet, your passenger becomes giggly and stops making sense.
What's likely and what do you do? See the worked answer
Hypoxia: check their oxygen flow and mask, check your own, and descend if there's any doubt.
AIM, Chapter 8, Medical Facts for Pilots
A friend offers you a welding oxygen bottle for your portable system.
Do you use it? See the worked answer
No: use aviator's breathing oxygen. Welding oxygen isn't a breathing grade and may contain impurities; medical oxygen can hold moisture that freezes in the regulator.
PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization)
A passenger wants to apply lip balm while on oxygen.
What do you tell them? See the worked answer
Not a petroleum-based one: oil and grease near oxygen are a fire hazard. Use an approved water-based product or none.
PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization)
Common mistakes
Waiting for symptoms before using oxygen.
Judgment goes first; you may not notice.
Instead: Use it by altitude and plan, not by feel.
Not calculating the supply.
You may run out at altitude with passengers aboard.
Instead: Compute endurance for everyone with a reserve.
No passenger briefing.
They won't know when or how to use it.
Instead: Brief before departure.
Now train it
Say it out loud, hear it asked, and take the quick check in this lesson’s practice pack. It tracks your shaky spots.
Got it?
When it feels solid, practice supplemental oxygen out loud with our examiner.
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Sources
- Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C): Aeromedical Factors; Aircraft Systems
- Aeronautical Information Manual (AIM): Chapter 8, Medical Facts for Pilots
- 14 CFR 91.211: Supplemental oxygen
- AC 61-107: High-altitude operations
- Commercial Pilot ACS (FAA-S-ACS-7B): Area VIII
- Your airplane's POH/AFM: oxygen and pressurization systems
Study aid, not FAA guidance. Regulations change; check the current text at eCFR and the ACS. Checked against current sources 2026-11-13.