Climb high enough and the air no longer holds enough oxygen to keep a brain working properly. The supplemental-oxygen rules draw a line, in terms of altitude and time, at which carrying and using oxygen stops being optional. They are short rules with sharp numbers, and the numbers are not the same on both sides of the Atlantic.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
Why oxygen, and why a threshold at all
As you climb, atmospheric pressure falls, and with it the partial pressure of oxygen your lungs can take up. The result is hypoxia, a shortage of oxygen at the tissues that creeps up without obvious warning: judgement dulls, vision and colour perception fade, reactions slow, and a cheerful sense that everything is fine often masks the lot. The insidious part is that it does not announce itself, which is exactly why the rules use a fixed altitude rather than leaving it to how you feel.
Because the effect builds gradually with height, any threshold is a chosen point on a sliding scale, and that is why authorities pick slightly different numbers. The crucial detail in every rule is that the altitude that matters is the cabin pressure altitude, the pressure altitude inside the passenger compartment, not necessarily the height of the aircraft. In an unpressurised light aircraft the two are the same. In a pressurised aircraft the cabin is held far lower than the flight level, so a jet at FL370 with a cabin at 7,000 ft is nowhere near the oxygen thresholds, until something depressurises the cabin.
The FAA rule: 14 CFR 91.211
The United States sets out three steps in 14 CFR 91.211(a), all in terms of cabin pressure altitude MSL:
- Above 12,500 ft up to and including 14,000 ft: the required minimum flight crew must use oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration. Below 30 minutes in that band, no oxygen is required by the rule.
- Above 14,000 ft: the required minimum flight crew must use oxygen during the entire flight time at those altitudes.
- Above 15,000 ft: each occupant of the aircraft must be provided with supplemental oxygen.
Note the careful wording at the top end: above 15,000 ft every occupant must be provided with oxygen, while the crew must use it. The rule guarantees the supply to passengers and the use by crew.
The EASA rule: NCO.OP.190
For general-aviation flying under EASA, the equivalent is NCO.OP.190, the use of supplemental oxygen rule for non-commercial operations with other-than-complex aircraft. Its baseline duty is judgement-based: the pilot-in-command ensures crew use oxygen whenever the lack of it might impair them, and that oxygen is available to passengers when a lack of it might harm them. Where the commander cannot make that judgement, NCO.OP.190 falls back to fixed cabin-altitude thresholds:
- crew members essential to safe operation use oxygen for any period in excess of 30 minutes when the cabin pressure altitude is between 10,000 ft and 13,000 ft;
- all occupants use oxygen for any period the cabin pressure altitude is above 13,000 ft.
So EASA starts the crew clock 2,500 ft lower than the FAA, at 10,000 ft rather than 12,500 ft, and brings everyone onto oxygen at 13,000 ft rather than 15,000 ft.
The two systems side by side
The shapes match: a crew threshold with a 30-minute grace, then a higher level at which everyone is covered. The numbers differ, and EASA is the more cautious throughout.
- Crew, with a 30-minute allowance: FAA above 12,500 ft; EASA above 10,000 ft.
- Crew, continuously: FAA above 14,000 ft; EASA effectively from the top of its band, with everyone covered above 13,000 ft.
- Everyone on board: FAA above 15,000 ft; EASA above 13,000 ft.
The practical effect of the gap shows up most in the band between 10,000 ft and 12,500 ft, where a long cruise needs crew oxygen under EASA but none under the FAA. Fly a European aircraft to American numbers in that band and you would be short of oxygen the rule expects you to be using.
Pressurised aircraft and decompression
For a pressurised aircraft the whole calculation hangs on the cabin, which is exactly why the rules are written that way. An airliner cruising in the thirties holds its cabin at a few thousand feet, comfortably below every oxygen threshold, so no routine requirement bites. The danger is a decompression: lose pressurisation and the cabin altitude climbs rapidly towards the flight level, and now the thresholds are very much in play. The standard response is immediate, crew oxygen on and an emergency descent to a safe level, because at high cabin altitudes the time a person can act usefully without oxygen, the time of useful consciousness, shrinks from minutes to seconds.
That risk is why the oxygen rules for commercial and higher-performance operations go further than the general-aviation thresholds above. The ICAO framework in Annex 6, and the EASA commercial air transport rules, require enough oxygen to be provisioned for a proportion of the occupants for the time the cabin altitude would exceed the thresholds after a failure, along with quick-donning masks for the flight crew on aircraft operating above defined levels. The detail differs by operation and is beyond the scope of a private-flying summary, but the principle to take away is that pressurisation does not remove the oxygen requirement; it moves it to the failure case.
Hypoxia and the individual
The thresholds are deliberately conservative because hypoxia does not affect everyone equally, and it does not wait politely for the legal trigger. Its early effects, a mild euphoria, slower thinking, fading colour vision and a creeping loss of judgement, are easy to miss precisely because impaired judgement is one of the symptoms. The EASA guidance to NCO.OP.190 lists the factors a pilot should weigh alongside the raw altitude: the duration of the flight, and individual conditions such as the altitude of your normal residence, smoking, fitness and experience at altitude, medical conditions and medication, and age. A smoker, or someone tired or unwell, can feel the effects well below the numbers, which is the case for treating the legal threshold as a ceiling on complacency rather than a safe floor.
A worked example
You are flying an unpressurised single, so the cabin altitude is the aircraft altitude.
You cruise at 11,000 ft for 45 minutes in smooth air. Under EASA NCO.OP.190, the cabin is in the 10,000-to-13,000 ft band and you are there for more than 30 minutes, so the crew must use supplemental oxygen, because the time in that band exceeds 30 minutes. Under the FAA's 91.211, 11,000 ft is below the 12,500 ft crew threshold, so no oxygen is required at all. Same flight, same height, opposite answers, purely because of where each authority drew its lower line.
Now climb to 16,000 ft. Here the systems agree on the outcome if not the wording: under the FAA you are above 15,000 ft, so every occupant must be provided with oxygen and the crew must use it continuously; under EASA you are above 13,000 ft, so all occupants must use it. Either way, at 16,000 ft cabin altitude everyone on board is on oxygen.
Finally, imagine the same 16,000 ft in a pressurised aircraft holding a cabin altitude of 6,000 ft. Neither rule is triggered, because both are read against the cabin, and a 6,000 ft cabin is well below every threshold, unless the pressurisation fails and the cabin climbs to meet the aircraft.
Common pitfalls
- Reading the thresholds as aircraft altitude. They are cabin pressure altitudes; in a pressurised aircraft the cabin, not the flight level, decides.
- Carrying the FAA numbers into EASA airspace, or the reverse. The crew threshold differs by 2,500 ft and the all-occupants threshold by 2,000 ft.
- Forgetting the 30-minute window is cumulative time in the band. It is the time spent at those altitudes, not a fresh clock each climb.
- Treating the threshold as a safe floor. Hypoxia begins below the legal triggers, and individual factors such as fitness, fatigue, smoking and age can bring its effects on sooner.
- Confusing provided with used. The top FAA step requires oxygen to be provided to every occupant and used by the crew; do not assume one means the other.
In Pilot EFB
Pilot EFB is a study and planning companion for high-altitude flying and the standard atmosphere the thresholds are built on, alongside your notes on pressure altitude and the rest of your offline-first briefing. It does not measure your cabin altitude, track your time in a band, or tell you to put oxygen on, and the binding requirements are those in the current rule for your operation. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and fly the oxygen rules from your official source of record.