Aircraft emergency oxygen systems or air masks are emergency equipment fitted to pressurized commercial aircraft, intended for use when the cabin pressurisation system has failed and the cabin altitude has climbed above a safe level. It consists of a number of individual yellow oxygen masks stored in compartments near passenger seats and near areas like lavatories and galleys, and an oxygen source, like a centralized gaseous cylinder or decentralized chemical oxygen generator.
Most commercial aircraft that operate at high flight altitudes are pressurized at a maximum cabin altitude of approximately 8,000 feet. On most pressurized aircraft, if cabin pressurization is lost when the aircraft is flying at an altitude above 4.267 m (14,000 feet), compartments containing the oxygen masks will open automatically, either above or in front of the passenger and crew seats, and the oxygen masks will drop down in front of the passenger. Oxygen masks may also drop on extremely rough landings or during severe turbulence if the oxygen mask panel becomes loose. Rows of seats typically have an extra mask (e.g. 3 seats, 4 masks), in case someone has an infant in their lap, or someone in the aisle needs one.
An oxygen mask consists of a yellow, soft, silicone facial cup with white elastic bands for securing the mask to the passenger's face. This band is adjustable by pulling two ends looped through the facial cup. The mask may also have a concentrator or re-breather bag that may or may not inflate depending on the cabin altitude, which has (in some instances) made passengers nervous the mask was not providing adequate oxygen, causing some to remove them, who thereby suffered hypoxia. All airlines now make a point in the safety video or demonstration to point out that the bag may not inflate. The bag is attached to a tube, connected to the oxygen source in the compartment, allowing for it to drop down and hang in front of the passengers. To operate on all aircraft, they must be pulled sharply toward the user to un-clip the flow pin and start the process of transporting the oxygen to the passenger. Passenger oxygen masks cannot deliver enough oxygen for sustained periods at high altitudes. This is why the flight crew needs to place the aircraft in a controlled emergency descent to a lower altitude where it is possible to breathe without emergency oxygen. While the masks are being used, passengers are not allowed to leave their seat for any reason until it is safe to breathe without the emergency oxygen. If there is a fire on board the aircraft, masks are not deployed, as the production of oxygen may further fuel the fire.
Aircraft safety cards and in-flight safety demonstrations shown at the beginning of each flight explain the location and use of oxygen masks.
Some aircraft, such as the Saab 340/2000 and the Beechcraft 1900D, have a mask system where either a mask is stored under the seat or is distributed by the cabin attendant. These masks are removed from packaging and plugged into the socket for oxygen supply.
On the Boeing 787, oxygen masks only consist of the mask and tube. Passengers breathe into the mask to start oxygen flow, and there are no side straps, as the mask adjusts automatically.
Portable oxygen supplies for cabin crew are not a suitable substitute for medically-necessary therapeutic oxygen, even in an emergency, because it does not provide enough oxygen to overcome Dalton's Law.
There are currently three systems that are typically found on aircraft:
See also: Fire triangle
Combustion is the exothermic chemical reaction between oxygen and a fuel, producing a flame and smoke. Because oxygen is necessary to fuel a fire, oxygen-generating equipment on aircraft pose a significant fire hazard and have contributed to several aircraft fires, both on the ground and while in flight.
Aircraft accidents and incidents related to oxygen supply systems include: