Chemical Oxygen Generators
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A chemical oxygen generator is a device which produces oxygen for emergency use by means of a chemical reaction.
Aviation regulations require that an emergency oxygen supply is available to passengers in the event that there is a loss of cabin pressurisation at an altitude where the partial pressure of oxygen would be insufficient to sustain consciousness. The weight, complexity and maintenance issues associated with an oxygen tank system mean that the majority of transport aircraft use chemical oxygen generators to provide emergency oxygen for occupants of the passenger cabin.
Oxygen generators are usually installed above each seat row. If the cabin altitude reaches a predetermined height (14,000' is standard), or if the system is activated by the flight crew, overhead panels open and oxygen masks drop out. To put the mask on, it is necessary for the intending user to pull it down and this action releases the firing pin and activates the generator.
The oxidizer core of an oxygen generator usually consists mainly of Sodium Chlorate (NaClO3) mixed with <5% Barium Peroxide (BaO2) and <1% Potassium Perchlorate (KClO4). This core is activated by the application of heat, which is normally generated by a mixture of lead styphnate and tetracene which itself is activated by a small explosive charge in a percussion cap. This explosive charge is set off by the release of a spring-loaded initiation mechanism which is restrained until released by a pull on the lanyard which is visible when oxygen masks are released from overhead panels. Once activated, the chemical reaction and production of oxygen will continue until the generator has been exhausted, typically in the range 12 to 20 minutes depending upon the type and size of generator installed. The reaction of the chemicals produces a significant amount of heat and the generator canister in the overhead compartment can reach temperatures above 250°C. The effect of this is that an often-unanticipated burning smell may become apparent in the passenger cabin and cause alarm.
The amount of oxygen which must be able to be produced by an installed cabin oxygen generator, as a function of time, is not constant. The requirement depends on the maximum operating altitude of the aircraft and the emergency descent profile which it is expected to follow after a loss of cabin pressure before it reaches an altitude, generally 10,000 feet amsl, where supplementary oxygen is no longer necessary. Also, since the amount of oxygen required at higher altitudes will be greater than that needed at lower altitudes, the chemical core of the generator has a larger diameter at the initiation end than at the outlet end so that relatively more oxygen is produced at the beginning of the reaction.
Each oxygen generator is attached to a number of emergency oxygen masks by plastic tubes. There is a regulatory requirement to provide at least 10% more masks than there are seats so some seat rows will have an extra mask available. This allows an additional mask in the event that someone has an infant in their lap or that someone in the aisle requires one. If the event of an onboard fire, passenger oxygen masks should not be deployed as the production of oxygen may worsen the situation.
Further information on the wider aspects of aircraft oxygen supply systems and their use is available.