Aircraft oxygen systems exist to prevent hypoxia, a shortage of oxygen that impairs the brain long before a pilot notices. The FAA describes four ways of delivering the oxygen: continuous-flow, diluter-demand, pressure-demand and electronic pulse-demand. The oxygen itself can come from a high-pressure cylinder, a chemical generator or, rarely, liquid oxygen, and which system an aircraft uses depends on how high it flies and who the oxygen is for.
This guide covers why oxygen matters, the four kinds of hypoxia, what 14 CFR 91.211 requires, how each system and each oxygen source works, and the safety rules around handling oxygen. It's built from the FAA's Pilot's Handbook of Aeronautical Knowledge and Aviation Maintenance Technician Handbook, with the regulations quoted from the eCFR.
Key Takeaways
Under 14 CFR 91.211, every occupant must be provided with supplemental oxygen above 15,000 feet cabin pressure altitude, and the required crew must use it above 14,000 feet.
Continuous-flow systems are usually for passengers. Diluter-demand and pressure-demand systems are for crew.
Airliners may use chemical oxygen generators for emergency passenger oxygen, which the FAA's maintenance handbook says supply typically 10 to 20 minutes of oxygen.
Cannulas are comfortable, but FAA guidance says they may not supply oxygen as reliably as masks at higher altitudes.
Oxygen and oil or grease don't mix: the FAA warns that materials that are nearly fireproof in air can burn in oxygen.
Why Aircraft Carry Oxygen
Hypoxia means reduced or insufficient oxygen. The FAA's pilot handbook explains that the percentage of oxygen in the air stays the same as an aircraft climbs, but its partial pressure falls with atmospheric pressure. Fewer oxygen molecules are available at the pressure needed to pass into the blood. The maintenance handbook puts numbers on it: oxygen pressure in the lungs at sea level is approximately three psi, enough to saturate the blood, and the chart below shows how the pressure of oxygen in the atmosphere falls with altitude.
Pressure of oxygen in the atmosphere, from the FAA Aviation Maintenance Technician Handbook (Figure 16-2). Original chart for PilotPassion.
The handbook says that below 7,000 feet the oxygen quantity and pressure stay sufficient to saturate the blood. Above that they become increasingly insufficient. At 10,000 feet saturation is only about 90 percent of normal, and long durations there can bring headache and fatigue. At 15,000 feet it drops to 81 percent, which typically brings sleepiness, headache, blue lips and fingernails, and impaired vision and judgment. It's 68 percent at 22,000 feet, and the handbook says remaining at 25,000 feet for 5 minutes, where saturation is about 50 percent, causes unconsciousness.
The pilot handbook gives approximate times of useful consciousness, the maximum time a pilot has to make rational, life-saving decisions and carry them out at a given altitude without supplemental oxygen:
Time of useful consciousness by altitude (FAA Pilot's Handbook, Figure 17-1)
Altitude (MSL)
Time of useful consciousness
20,000 feet
30 minutes or more
22,000 feet
5 to 10 minutes
25,000 feet
3 to 5 minutes
28,000 feet
2.5 to 3 minutes
30,000 feet
1 to 2 minutes
35,000 feet
30 to 60 seconds
40,000 feet
15 to 20 seconds
45,000 feet
9 to 15 seconds
The Four Kinds of Hypoxia
The FAA sorts hypoxia by its cause. Supplemental oxygen counters the altitude kind directly, and the FAA also recommends it when carbon monoxide poisoning is suspected, but a pilot needs to know all four, because carbon monoxide and alcohol can produce the same symptoms at low altitude.
Forms of hypoxia (FAA Pilot's Handbook, Chapter 17)
Form
What goes wrong
Example
Hypoxic
Insufficient oxygen is available to the body as a whole.
Climbing, where the partial pressure of oxygen falls. Also a blocked airway.
Hypemic
The blood can't take up and carry enough oxygen, although plenty is breathed in.
Carbon monoxide poisoning, the most common form. Also blood loss, anemia, and blood donation, after which blood volume can take several weeks to recover.
Stagnant
Oxygen-rich blood isn't moving to the tissues that need it.
Excessive G-forces, shock, a failing heart, or cold temperatures that reduce circulation.
Histotoxic
The cells can't use the oxygen that reaches them.
Alcohol and other drugs. The handbook says one ounce of alcohol can equate to an additional 2,000 feet of physiological altitude.
The first symptoms can include euphoria and a carefree feeling, which is part of the danger. The handbook lists headache, decreased response to stimuli and increased reaction time, impaired judgment, visual impairment, drowsiness, lightheadedness, tingling in the fingers and toes, numbness and cyanosis, a blue color in the fingernails and lips. As hypoxia worsens the field of vision narrows and instrument interpretation gets difficult, and the pilot can be deceived into believing everything is normal.
Hyperventilation is the look-alike. The FAA says its symptoms are similar to those of hypoxia, so if you're on supplemental oxygen, check the equipment and flow rate to make sure the symptoms aren't from hypoxia. Treatment for hypoxia is flying at lower altitudes, using supplemental oxygen, or both.
When Oxygen Is Required
14 CFR 91.211 sets the minimums for civil aircraft of U.S. registry, and it uses two bases: paragraph (a) is written in cabin pressure altitude, while the pressurized-aircraft requirements in paragraph (b) use flight altitude. The FAA handbook adds that some people feel the effects of oxygen deprivation at much lower altitudes: some flying above 10,000 feet during the day may be disoriented, and at night, especially when fatigued, the effects may occur as low as 5,000 feet. It encourages pilots to use supplemental oxygen above 10,000 feet cabin altitude during the day and above 5,000 feet at night.
Supplemental oxygen requirements, 14 CFR 91.211
Altitude basis and threshold
What the rule requires
Cabin pressure altitude above 12,500 up to and including 14,000 feet
The required minimum flight crew is provided with and uses oxygen for the part of the flight at those altitudes that lasts more than 30 minutes
Cabin pressure altitude above 14,000 feet
The required minimum flight crew is provided with and uses oxygen for the entire time at those altitudes
Cabin pressure altitude above 15,000 feet
Each occupant of the aircraft is provided with supplemental oxygen
Pressurized aircraft, flight altitude above FL250
At least a 10-minute supply for each occupant, in addition to other required oxygen, available for use if a descent is needed after loss of pressurization
Pressurized aircraft, flight altitude above FL350
One pilot at the controls wears and uses a secured, sealed oxygen mask, with an exception listed in the rule for two pilots with quick-donning masks at or below FL410
Airline and charter operations have stricter rules. Under 14 CFR 135.89, each pilot of an unpressurized aircraft must use oxygen continuously above 12,000 feet MSL, and above 10,000 through 12,000 feet for the part of the flight over 30 minutes. For turbine-powered airline airplanes, 121.329 starts the crew requirement at 10,000 feet and requires oxygen for a share of passengers, 10 percent of them above 10,000 feet through 14,000 feet after 30 minutes and 30 percent between 14,000 and 15,000 feet, with every passenger above 15,000 feet.
Where the Oxygen Comes From
The maintenance handbook describes four sources. Gaseous oxygen in cylinders is common throughout civilian aviation, and chemical generators are used as the backup on airliners.
Nearly pure oxygen stored as a gas in high-pressure cylinders, typically painted green, at atmospheric temperature.
Portable and built-in systems on light aircraft up to transport category aircraft. Common throughout civilian aviation.
Chemical (solid) oxygen
A sodium chlorate candle that makes oxygen as it burns, started by a firing pin or electric current. Once lit it can't be put out and burns for typically 10 to 20 minutes.
Backup on pressurized aircraft. The handbook says this use is most common on airliners, where the generators sit in the overhead passenger service unit.
Liquid oxygen (LOX)
Oxygen kept as a liquid in a Dewar bottle and vaporized into a gaseous system. A small quantity makes an enormous amount of gas.
The handbook says it's rarely used in civilian aviation, but it's used in military aviation and some medical helicopters.
Onboard oxygen generating system
A molecular sieve filters nitrogen out of engine bleed air, leaving nearly pure oxygen.
Some military aircraft. The handbook says civil use is anticipated.
Emergency oxygen cylinders and valves in a jet aircraft.
A few details about the gaseous kind. Traditional cylinders are heavy steel tanks rated at 1,800 to 1,850 psi, and the pilot handbook says aircraft oxygen is usually stored at 1,800 to 2,200 psi. Lighter cylinders have an aluminum shell wrapped in Kevlar. Cylinders must be hydrostatically tested periodically, which the maintenance handbook describes as filling them with water and pressurizing them to five-thirds of their rating. The handbook's table lists test intervals of 3 or 5 years depending on the certification, and says a cylinder is considered empty below 50 psi, because that keeps moist air from getting in and causing corrosion or ice in the valves.
Not all oxygen in a container is the same. Aviator's breathing oxygen is tested for water, with a maximum of 0.02 milliliter per liter, so that it can't freeze in the small passages of valves and regulators at subzero temperatures, and the words "Aviator's Breathing Oxygen" should be marked clearly on the cylinder. The pilot handbook says the oxygen should meet or exceed SAE AS8010, the aviator's breathing oxygen purity standard.
Because pressure varies directly with temperature, a cylinder in an unheated area can show a drop in pressure without having lost oxygen. The maintenance handbook adds that fill charts compensate for temperature: cylinders are filled to a higher pressure than the standard rating when it's hot, and to less than the maximum on a cold day.
The Four Oxygen Delivery Systems
All four are described in the FAA handbooks. The regulator is the heart of each one: it decides how much oxygen reaches the mask at a given cabin altitude.
Aircraft oxygen delivery systems (FAA Pilot's Handbook, Chapter 7, and FAA Aviation Maintenance Technician Handbook, Chapter 16)
System
How it works
Typical use
Continuous-flow
Oxygen flows constantly once the valve is open, even when the user exhales. The passenger mask has a reservoir bag that collects oxygen while the user exhales, so more is available on the next breath. Ambient air is added once the bag runs out. The flow to each mask is set by a calibrated orifice.
Usually provided for passengers.
Diluter-demand
Supplies oxygen only when the user inhales. An automix lever mixes cabin air and oxygen, or supplies 100 percent oxygen, depending on altitude. The tight-fitting mask can be used safely up to 40,000 feet.
Crew.
Pressure-demand
Like diluter-demand, but oxygen is supplied to the mask under pressure at cabin altitudes above 34,000 feet. The positive pressure helps the lungs, which makes it safe above 40,000 feet.
Crew at high altitude. The maintenance handbook says these regulators are used on aircraft that regularly fly at 40,000 feet and above, and are also found on many airliners.
Electronic pulse-demand
Senses the user inhaling and delivers oxygen during the first part of the breath. The FAA says this can cut the oxygen needed by 50 to 85 percent compared with continuous flow. Most also have an internal barometer that adjusts the dose with altitude.
Portable systems, and a recent development in general aviation.
Two more regulator details from the handbooks. Most regulators approved for use up to 40,000 feet give no cylinder oxygen and 100 percent cabin air at cabin altitudes of 8,000 feet or less, shifting to 100 percent oxygen at about 34,000 feet. Most regulators approved up to 45,000 feet start at 40 percent cylinder oxygen and 60 percent cabin air at lower altitudes. In a demand system the oxygen is first reduced to roughly 60 to 85 psi, then fed to an individual regulator for each user, and each user can also select 100 percent oxygen at any time with the regulator's selector.
Why Airline Masks Drop
On an airliner the system is a backup for pressurization. The maintenance handbook says that when depressurization occurs or the crew flips a switch, a compartment door opens and the masks fall out in front of the passengers. Pulling a mask down to a usable position triggers a current or an ignition hammer that lights the chemical oxygen candle, and oxygen flows. Typically 10 to 20 minutes is available for each user, calculated as enough time for the aircraft to descend to a safe altitude for unassisted breathing. A chemical generator doesn't make oxygen until it's needed, which allows safer transport with less maintenance, and the handbook says it weighs less than a gaseous system for the same number of people.
The rules behind this are in 14 CFR Part 25. For pressurized cabins, 25.841 generally requires equipment to provide a cabin pressure altitude of not more than 8,000 feet under normal operating conditions, and designs for operation above 25,000 feet must keep occupants from cabin pressure altitudes over 15,000 feet after any probable failure of the pressurization system, with exceptions in paragraph (c) for airports at or above 8,000 feet. After a decompression from a failure not shown to be extremely improbable, occupants may not be exposed to cabin altitudes above 25,000 feet for more than 2 minutes or above 40,000 feet for any duration. If certification above 30,000 feet is requested, 25.1447 generally requires the dispensing units to be automatically presented to occupants before the cabin pressure altitude exceeds 15,000 feet, with a manual means for the crew, and a different threshold applies for operations at airports above 13,000 feet. Section 25.841(b)(6) also requires a warning to the flightcrew when cabin pressure altitude limits are exceeded, and says an alert at 10,000 feet meets that requirement, subject to the high-elevation airport provisions of paragraph (d).
Masks and Cannulas
Crew masks fit the face with minimal leakage and usually include a microphone, and most are oronasal, covering only the nose and mouth. Passenger masks may be a simple, flexible, cup-shaped molding that needs no individual fitting. For transport category airplanes, 25.1447 requires an individual dispensing unit for each occupant to be supplied with oxygen, designed to cover the nose and mouth, and flight crew masks must allow communication equipment. In planes certified above 25,000 feet, each flight crewmember on duty needs a quick-donning mask that can be placed on the face with one hand, properly secured and sealed and supplying oxygen, within five seconds.
Cannulas are plastic tubes that run under the nose. They're usually more comfortable but may not deliver an adequate flow as reliably as masks at higher altitudes. Current regulations require aircraft with oxygen systems installed and certified for operations above 18,000 feet to have masks instead of cannulas. Many cannulas include a green flow detector that belongs in the pilot's scan. The FAA says to check that a mask is compatible with the oxygen system, and to trim any beard or mustache so it doesn't break the mask's seal.
Pulse Oximeters
A pulse oximeter clips onto a fingertip and measures blood oxygen saturation and heart rate by shining a special light through the fingertip. The FAA says it can calculate oxygen saturation to within one percent of directly measured blood oxygen, and calls it very useful for pilots in nonpressurized aircraft above 12,500 feet, because it lets crew and passengers check their actual need for supplemental oxygen instead of guessing. The maintenance handbook notes that pilots can use the reading to adjust the oxygen flow and avoid hypoxia.
Fire Safety and Servicing
The FAA lists the fire risk first. Materials that are nearly fireproof in ordinary air can burn in oxygen, oils and greases may ignite on contact, and smoking is prohibited around any oxygen equipment. The maintenance handbook explains the physics behind slow-opening valves: when high-pressure oxygen rushes into a low-pressure area and meets an obstruction, it compresses so quickly that no heat escapes, and the temperature can exceed the ignition point of the material it hits. Even stainless steel can ignite in 100 percent oxygen under high pressure. That's why oxygen shutoff valves are designed to open slowly, and technicians open all of them slowly.
Before each flight, inspect and test the equipment with clean hands: the mask and tubing for tears, cracks or deterioration, the regulator, the oxygen quantity, and the gauges, flow indicators and connections. Don the mask and test the system, and afterward check that all valves are shut off.
The green blowout disc is the relief valve's indicator. It's on the fuselage where a walk-around can see it, and the handbook says its absence means the relief valve has opened and the cause should be investigated before flight.
Servicing should be done only outside hangars, using the aircraft service manual. The FAA says to avoid it during fueling, near other work that could ignite, or with passengers on board. Hands, clothing and tools must be free of oil, grease and dirt, including lip salve and hair oil.
For the pressurized structures the cabin air sits inside, see our guide to fuselage types, and for the electrical side of crew and passenger systems, our electrical systems guide.
Frequently Asked Questions
What are the main types of aircraft oxygen systems?
Continuous-flow, diluter-demand, pressure-demand and electronic pulse-demand. Continuous-flow is usually for passengers, and the demand systems are for crew.
At what altitude do pilots need supplemental oxygen?
Under 14 CFR 91.211, the required crew must use oxygen after 30 minutes above 12,500 feet cabin pressure altitude, and for the whole time above 14,000 feet. Every occupant must be provided with oxygen above 15,000 feet. The FAA encourages using it above 10,000 feet by day and 5,000 feet at night.
What are the four types of hypoxia?
Hypoxic (not enough oxygen available), hypemic (the blood can't carry enough, as in carbon monoxide poisoning), stagnant (the blood isn't moving to the tissues) and histotoxic (the cells can't use oxygen, as with alcohol).
How long does airliner passenger oxygen last?
The FAA's maintenance handbook says a chemical oxygen generator typically supplies 10 to 20 minutes of oxygen for each user, calculated as enough time to descend to a safe altitude.
What's the time of useful consciousness at 35,000 feet?
The FAA gives 30 to 60 seconds. At 25,000 feet it's 3 to 5 minutes, and at 40,000 feet it's 15 to 20 seconds.
What's the difference between diluter-demand and pressure-demand oxygen?
Both supply oxygen when the user inhales. A pressure-demand system also supplies oxygen under pressure at cabin altitudes above 34,000 feet, which makes it safe above 40,000 feet.
Is a nasal cannula as good as an oxygen mask?
The FAA says cannulas are more comfortable but may not provide an adequate flow as reliably as masks at higher altitudes. Aircraft with oxygen systems certified above 18,000 feet are required to have masks.
Why does the oxygen cylinder pressure drop in cold weather?
Pressure varies directly with temperature when volume is constant. A cylinder in an unheated area can show lower pressure without having lost oxygen.
Can any oxygen be used in an aircraft system?
No. Aviator's breathing oxygen is tested for water so it can't freeze in valves and regulators, and the FAA's pilot handbook says it should meet SAE AS8010. The cylinder should be marked Aviator's Breathing Oxygen.
Why must oxygen valves be opened slowly?
Oxygen rushing into a low-pressure area can compress against an obstruction so quickly that the heat ignites materials, even stainless steel. The FAA's maintenance handbook says oxygen shutoff valves are designed to open slowly for that reason.
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