Commercial pilot · ACS CA.VIII.A · lesson pack
Supplemental Oxygen
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.
1 · Know these cold
- •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)
2 · The lesson
The rules
- 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
- 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
- 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))
Want it all on one page, with scenarios? Read the full lesson.
3 · Say it out loud
Answer each one out loud, then check yourself and mark how it went.
Remember Recall it
“When must required crew use supplemental oxygen?”
Understand Explain why
“Why is hypoxia so dangerous to a pilot?”
Apply Use it on a flight
“You plan to cruise at 16,500 feet for 2 hours with three passengers and a portable system. What do you plan?”
4 · 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.
5 · Quick check
Answer, then reveal. Flag anything you want to see again.
1. Above what altitude must passengers be provided oxygen?
2. Above what altitude are cannulas not permitted?
3. Name the four types of hypoxia.
4. What is the approximate time of useful consciousness at 25,000 feet?
Scenario5. At 14,500 feet you feel tingling and tunnel vision. What do you do?
Trained it? Now prove it: practice supplemental oxygen out loud with our examiner.
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Connect it
One situation that runs across this lesson and its neighbors in the same ACS area. Decide first, then check.
Connect it · High Altitude Operations
A pressurized single at FL 230 with a cabin at 10,000 feet. A loud bang, fog in the cabin and the cabin altitude climbing fast.
What happens to you, and in what order do you act? Decide, then see the worked answer
Rapid decompression: the cabin altitude climbs to the airplane's altitude, about 23,000 feet. The FAA's figure for time of useful consciousness there is roughly 5 to 10 minutes at rest, a rapid decompression can cut it by as much as half, and impairment can start at once. Don't hold your breath. Your own mask first: on, 100% oxygen, and flow verified. Then make sure the passengers' masks are on and flowing. Start the emergency descent immediately under your 91.3(b) emergency authority, without waiting for ATC, and squawk 7700 and declare as soon as you can. Fly the POH checklist down to 10,000 feet MSL or the lowest safe altitude, whichever is higher. 12,500 feet is only the 91.211(a)(1) threshold for crew oxygen, not a descent target. Then land as soon as practicable. Hypoxia can cloud your judgment without warning, so act on the checklist rather than on how you feel, and brief the passengers on their masks before every high-altitude flight.
PHAK (FAA-H-8083-25C), Aeromedical Factors; AFH (FAA-H-8083-3C), High Altitude Operations; 14 CFR 91.3, 91.211
Brings together: Supplemental Oxygen · Pressurization
Teaching this? The instructor lesson plan →
Study aid, not FAA guidance. Sources: Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C); Aeronautical Information Manual (AIM); 14 CFR 91.211; AC 61-107; Commercial Pilot ACS (FAA-S-ACS-7B); Your airplane's POH/AFM. Checked against current sources 2026-11-13.