A device that provides oxygen to passengers or crew when necessary, usually due to a loss of cabin pressure.
An oxygen mask, as indicated by the name, is a masklike device used to provide oxygen to aircraft occupants. They can be divided into three basic categories, depending on their intended users:
- Passenger oxygen masks are carried on pressurized aircraft, to provide the passengers with an emergency source of oxygen if the aircraft loses cabin-pressure; they are usually stored in compartments above the passengers' heads (a few large airliners have ceilings that are too high for this to be practicable for passengers near the centerline of the aircraft, and instead oxygenate those passengers with masks stored in the backs of the passenger seats), and automatically drop down from their compartments if the aircraft depressurizes in flight. They consist of a soft plastic cup, one or more elastic straps to hold the mask on the passenger's face, and some tubing that connects the mask to its oxygen source.
- All smaller aircraft with passenger oxygen masks, as well as some large pressurized aircraft, use a large cylinder of compressed oxygen located somewhere in the aircraft; if the aircraft depressurizes, oxygen flows from the cylinder through a pressure-reducing valve (to keep from exploding the passengers' respiratory tracts) and thence to the masks via a series of tubes.
- Many large aircraft instead use individual oxygen generators located in the mask compartments, with each generator supplying from one to four masks (depending on what generator is installed and where said generator is installed). Pulling a mask from its compartment releases a firing pin inside the generator, which detonates a small explosive charge, which initiates a chemical reaction in the main body of the generator, which generates oxygen along with a great deal of heat. Chemical oxygen generators are cheap and long-lasting and eliminate the need to install long lengths of oxygen tubing and periodically recharge an oxygen bottle (to compensate for leakage), but pose an extreme fire hazard if improperly installed, stored, or transported.
- Cabincrew oxygen masks, carried on the same aircraft as passenger masks, are intended for use by flight-attendants and other cabin crew; they use masks similar or identical to those used by passengers, but are fed from portable oxygen bottles in order to allow flight attendants to move around the cabin while on oxygen.
- Flightcrew oxygen masks are used on all pressurized aircraft, plus all unpressurized aircraft flying higher than approximately 10,000 feet above mean sea level (AMSL); they provide oxygen to the pilots, as well as to the flight-engineer and/or navigator on aircraft so equipped. (Some very early high-altitude bomber aircraft were unpressurized, with the entire crew - not just the flightcrew - wearing flightcrew-style oxygen masks, but these have all long since been replaced in service by pressurized aircraft.) Unlike passenger or cabincrew oxygen masks, flightcrew masks are large, cover the entire face of the wearer (the front of the mask is transparent, to let its user see their flight-instruments), and seal tightly against the user's face, allowing them to be used not only in the event of a depressurization (or a flight in an unpressurized aircraft), but also if the ambient air in the cockpit becomes unbreathable for some other reason, such as smoke or hot gasses from a fire, or toxic fumes from improperly-stored hazardous cargo. The tight seal allows the mask to exclude smoke and toxic gas, while still minimizing escape of oxygen from the mask (which could help feed a fire); on aircraft flying at very high altitudes (above approximately 40,000 feet AMSL), it also allows the mask to deliver oxygen at higher-than-ambient pressure, and, thus (as the air at these altitudes is so thin that even 100% oxygen at ambient pressure would be insufficient to keep the user functional), continue to serve its purpose.
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