Emergency oxygen system

Aircraft emergency oxygen systems are emergency equipment fitted to commercial aircraft, intended for use when the cabin pressurisation system has failed and the level of oxygen in the cabin atmosphere drops below a safe level. It consists of a number of personal oxygen masks by passenger seats, and some form of central oxygen generator.

Use

Most commercial aircraft are pressurized at a maximum cabin altitude of 8,000 feet, where it is possible to breathe normally without an oxygen mask. If the cabin altitude reaches 14,000 feet or higher, or a decompression occurs and hypoxia is possible, compartments containing the oxygen masks will open automatically, either above or in front of the passenger and crew seats, and in the lavatories. Oxygen masks may also drop on extremely rough landings or during severe turbulence if the oxygen mask panel becomes loose.

Most oxygen masks have a yellow facial cup with elastic bands for securing the mask to the passenger's face. The mask may also have a concentrator bag that may or may not inflate depending on the cabin altitude. 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. 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.

Mechanism

There are two systems that are typically found on aircraft:
* A "gaseous manifold system", which connects all oxygen masks to a central oxygen supply, usually in the cargo hold area. Pulling down on one oxygen mask starts the oxygen supply for that mask only. The entire system can usually be reset in the cockpit or in some other location in the aircraft.
* A "chemical oxygen generator system" connected to all masks in the compartment. Pulling down on one oxygen mask removes the firing pin of the generator igniniting a mixture of sodium chlorate and iron powder, opening the oxygen supply for all the masks in the compartment. Oxygen production cannot be shut off once a mask is pulled, and oxygen production typically lasts for 12-15 minutes. During the production of oxygen, the generator becomes extremely hot and should not be touched. A burning smell may be noted and cause alarm among passengers, but this smell is a normal part of the chemical reaction.

Usage history

Remarkably, for a widely-deployed piece of safety equipment, some research has suggested that no lives are known to have been saved by use of an emergency oxygen mask - nor any lives lost through the absence of one - whilst carrying oxygen generating apparatus, albeit as cargo, has caused at least one fatal accident. [Original article by William Langewiesche in "The Atlantic", 1988; discussed in [http://yarchive.net/air/airliners/emergency_oxygen.html "Re: Cabin Depressurization" on sci.aeronautics.airliners] . Mary Shafer, Dryden Flight Research Center, "et al." 2000.] The fatal accident was ValuJet Flight 592, in 1996, where expired chemical oxygen generators were loaded as cargo on board the aircraft without being safely deactivated; in transit, it is believed these generators activated; the heat generated from the activated generators caused the boxes they were improperly stored in to catch fire.

In the three cases of in-flight explosive decompression studied, one took place at a sufficiently low altitude for atmospheric oxygen to be sufficient, whilst in the other two cases the systems failed in the accident and did not provide oxygen to the passengers. However, in several other cases, oxygen masks have kept passengers cautious and alert during a decompression and have protected passengers from injury.

The cockpits of aircraft generally contain a separate oxygen system for the flight crew, and effective use of these has no doubt saved many aircraft. Hypoxia, which can cause severe disorientation and unconsciousness, sets in quickly; if a flight crew does not realise the cabin has decompressed, or is too slow to respond, they can quickly lose control of the aircraft. For example, on Helios Airways Flight 522 in 2005, the cabin depressurized slowly during the ascent to cruising altitude, and whilst the passenger oxygen masks were released at 14,000 feet, the crew were disoriented and failed to realise the significance of this; they lost control within a few minutes, having not put on their own oxygen masks. In the 1999 South Dakota Learjet crash, the NTSB report concluded that only a few seconds delay in using their masks following decompression would be enough to incapacitate a flight crew.

In one case, in 2000, a Boeing 737-800 suffered a slow depressurization, coupled with the failure of the cabin altitude warning system. The depressurization was only discovered by the crew due to the automatic deployment of the passenger oxygen masks; this gave them time to respond appropriately. [ [http://www.aaiu.ie/upload/general/3496-0.pdf PDF report] ]

References