Supplemental Oxygen Requirements

Supplemental oxygen is breathing oxygen carried on board for use when the cabin pressure altitude is too high for crew and passengers to function normally. The operating rules decide when it must be carried, who must use it and how much there must be. They differ between unpressurized airplanes, where the cabin is as high as the airplane, and pressurized ones, which need oxygen only if pressurization fails.

The figures are an exam favorite because EASA and the FAA chose different ones. This article covers the rules. The equipment is described in crew oxygen systems and passenger and portable oxygen, the physiology in hypoxia and hyperventilation and the emergency in decompression.

Why oxygen rules exist

The air contains about 21 percent oxygen at every altitude; what falls with height is its pressure, and with it the oxygen the lungs can pass to the blood. The FAA's AIM describes the progression in a healthy pilot. Night vision deteriorates at a cabin pressure altitude as low as 5,000 ft, but other significant effects usually do not appear below 12,000 ft. Between 12,000 and 15,000 ft judgment, memory, alertness, coordination and the ability to calculate are impaired, and performance can deteriorate seriously within 15 minutes at 15,000 ft. Above 15,000 ft vision narrows and the lips and nails turn blue; the ability to take corrective action is lost within 20 to 30 minutes at 18,000 ft and 5 to 12 minutes at 20,000 ft.

Hypoxia degrades judgment before it causes any discomfort, and a sense of well-being is a recognized early sign, so a pilot cannot be relied on to notice it. The rules are therefore written as altitudes and times, not symptoms. For the same reason the AIM encourages oxygen above 10,000 ft by day and above 5,000 ft at night, well below the legal thresholds.

Every threshold is a cabin pressure altitude: the altitude in the standard atmosphere that corresponds to the pressure in the cabin, as a cabin altimeter shows it. In an unpressurized airplane that is essentially the airplane's own pressure altitude. A pressurized airplane at FL280 with its cabin at 8,000 ft needs no supplemental oxygen in normal flight; it carries a supply for a loss of pressurization.

Unpressurized airplanes

EASA commercial air transport

For airplanes in commercial air transport operated above 10,000 ft, CAT.IDE.A.240 sets the supply to be carried, in bands by who is on board:

Use is a separate rule. CAT.OP.MPA.285 requires the commander to ensure that flight crew members engaged in duties essential to safe operation use supplemental oxygen continuously whenever the cabin altitude exceeds 10,000 ft for a period of more than 30 minutes, and whenever it exceeds 13,000 ft. A passenger flight with no cabin crew, in an unpressurized twin cruising at 12,000 ft for 50 minutes, therefore carries oxygen for the flight crew for the whole 50 minutes and for 10 percent of the passengers for the last 20.

Part-NCO

For non-commercial flights in other-than-complex aircraft, NCO.OP.190 starts from the pilot-in-command's judgment. The flight crew must use oxygen whenever the pilot-in-command determines that a lack of it might impair their faculties, and oxygen must be available to passengers whenever a lack of it might harm them. Where the effect on the occupants cannot be determined, two firm cases apply: crew members on duties essential to safe operation use oxygen for any period over 30 minutes between 10,000 ft and 13,000 ft, and everyone on board uses it above 13,000 ft. The matching equipment rule, NCO.IDE.A.155, adds no figures: it requires equipment, installed or portable, able to store and dispense whatever oxygen NCO.OP.190 calls for. Below 13,000 ft Part-NCO sets no passenger percentage for an unpressurized airplane; the 10 percent band above is a commercial air transport rule.

The rule binds the pilot even against a clearance. A pilot in an unpressurized twin with no oxygen fitted, cleared to climb to FL140 and hold it for 40 minutes, tells ATC at once that the clearance cannot be accepted and stays at the present level until it is amended.

FAA: 91.211 and 135.89

14 CFR 91.211 applies to every US-registered civil aircraft and uses three thresholds of cabin pressure altitude:

• above 12,500 ft MSL up to and including 14,000 ft, the required minimum flight crew use oxygen for any part of the flight at those altitudes beyond 30 minutes;
• above 14,000 ft, the required minimum flight crew use oxygen the whole time;
• above 15,000 ft, every occupant must be provided with oxygen, although passengers need not use it.

A 45-minute cruise at 13,500 ft therefore needs crew oxygen for the last 15 minutes. The trap is a time limit that runs out unnoticed, for example on a mountain IFR route whose minimum en route altitude is 13,000 ft. Commercial operators fly to stricter figures. Under 14 CFR 135.89 each pilot of an unpressurized aircraft uses oxygen above 10,000 ft through 12,000 ft for any part of the flight beyond 30 minutes, and continuously above 12,000 ft.

Pressurized airplanes

In a pressurized airplane the supply is sized for a loss of pressurization and the descent that follows, and the passenger bands step at 14,000 and 15,000 ft instead of 13,000 ft. For EASA commercial air transport the rule is CAT.IDE.A.235; for US airlines it is 14 CFR 121.329 and 121.333.

For an airplane operated at pressure altitudes above 25,000 ft, CAT.IDE.A.235 adds equipment: quick-donning masks for the flight crew; spare outlets and masks or portable units, so that each required cabin crew member has oxygen immediately to hand; a dispensing unit within reach of every other occupant, with at least 10 percent more units than seats; and a warning to the flight crew of any loss of pressurization. In airplanes whose individual certificate of airworthiness was first issued after 8 November 1998, the passenger units must deploy automatically. Transport airplane certification rules require a warning to the crew when the cabin altitude exceeds 10,000 ft and, for high-flying types, automatic presentation of the passenger masks before it exceeds 15,000 ft; many types drop them at about 14,000 ft.

The general operating rule, 91.211(b), requires a pressurized aircraft flown above FL250 to carry a 10-minute supply for each occupant for the descent, in addition to the crew oxygen of 91.211(a). Above FL350 one pilot at the controls must wear a mask, unless two pilots are at the controls, each with a quick-donning mask; that relief ends at FL410. A US pilot acting as pilot in command of a pressurized airplane whose service ceiling or maximum operating altitude, whichever is lower, is above 25,000 ft MSL also needs high-altitude ground and flight training and a logbook endorsement (14 CFR 61.31(g)).

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Quick-donning masks

A quick-donning mask is one that can be put on with one hand within five seconds, sealed and supplying oxygen. The time of useful consciousness is about 3 to 5 minutes at 25,000 ft and 30 to 60 seconds at 35,000 ft, and FAA AC 61-107B advises assuming it is roughly halved after a rapid decompression. The mask is therefore the first action of every depressurization drill, before the descent and before helping anyone else.

EASA requires quick-donning masks for the flight crew of pressurized airplanes operated at pressure altitudes above 25,000 ft (CAT.IDE.A.235). The test is the altitude at which the airplane is operated; certification above 25,000 ft is what sets the two-hour crew supply. The FAA also uses quick-donning masks to decide when a mask must actually be worn. Under 14 CFR 121.333, above FL250 one pilot at the controls must wear and use a mask unless each pilot on flight deck duty has a quick-donning mask. With them, neither pilot need wear one at or below FL410 in airplanes with more than 30 passenger seats or a payload over 7,500 lb, or at or below FL350 in smaller ones. Since a 2020 amendment, a pilot left alone at the controls while the other leaves the flight deck must put a mask on only above FL410; the threshold used to be FL250.

First-aid oxygen

First-aid oxygen is not supplemental oxygen. It is a separate supply of undiluted oxygen for individual passengers who, for physiological reasons, still need oxygen once a depressurization is over and the airplane has descended.

Under CAT.IDE.A.230 it is required in pressurized airplanes operated at pressure altitudes above 25,000 ft when a cabin crew member is required. After a depressurization, it must cover at least 2 percent of the passengers, and never fewer than one person, for the rest of the flight in which the cabin altitude is above 8,000 ft but not above 15,000 ft. There must be at least two dispensing units, with a means for the cabin crew to use the supply. The flow to each person follows the CS-25 certification standard, and the quantity may be calculated on an average of at least 3 liters per minute per person, measured STPD (AMC1 CAT.IDE.A.230(d)). The FAA's 121.333 is similar: enough for 2 percent of the occupants for the entire flight after a depressurization, for those who need undiluted oxygen after a descent from cabin altitudes above FL250.

Cabin crew usually give it from portable oxygen cylinders. A typical cylinder holds 120 liters at about 1,800 psi, with flow settings of 2 and 4 liters per minute, which last about 60 and 30 minutes (see in-flight medical emergencies).

Planning oxygen quantity

In a transport airplane the quantities are built into certification and dispatch. Under 121.333 the two-hour flight crew supply is defined as enough for a constant-rate descent from the maximum certificated altitude to 10,000 ft in 10 minutes, followed by 110 minutes at 10,000 ft. The A320's dispatch minimum covers the greater of an emergency descent (13 minutes for everyone on the flight deck, then 107 minutes at FL100 for the two pilots) and 15 minutes of smoke protection. Because gas pressure varies with temperature, it is tabulated against temperature: 731 psi at 20 °C for two crew. Passenger chemical generators run for a fixed time, commonly about 12, 15 or 22 minutes, the longer ones for routes where high terrain delays the descent; there the minimum safe altitude may be well above 10,000 ft, and operators plan escape routes.

In an unpressurized airplane the pilot does the planning:

1. Profile. Work out the time above each threshold, including the climb, any hold and the highest minimum altitudes on the route.
2. People. Apply the governing rule to each occupant, crew first, then passengers.
3. Consumption. Multiply each user's flow by the number of users and the time; a 120-liter cylinder at 4 liters per minute lasts about 30 minutes.
4. Contents. Read the cylinder pressure, remembering that it also varies with temperature.
5. Margin. Decide in advance where to descend if the supply runs low.

Equipment sets limits too. FAA guidance restricts a nasal cannula to 18,000 ft, above which a mask is needed, and breathing through the mouth or talking reduces what a cannula delivers. Before flight and at intervals at altitude the FAA's PRICE check covers pressure, regulator, indicator, connections and emergency procedures. Only aviator's breathing oxygen, at least 99.5 percent pure and very dry, goes into an aircraft system, and oil and grease are kept away from it because they can ignite in oxygen without a spark. In flight a pulse oximeter gives an objective check before symptoms appear.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.