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Chemical Oxygen Generators

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Article Information
Category: Flight Technical Flight Technical
Content source: SKYbrary About SKYbrary
Content control: EUROCONTROL EUROCONTROL

Description

A chemical oxygen generator is a device which produces oxygen by means of a chemical reaction.

Aircraft Applications

Aviation regulations require the provision of emergency oxygen to passengers to sustain them during an emergency descent following loss of pressurisation at high altitude. For any aircraft which carries more than a very few passengers, the weight, complexity and maintenance issues associated with a compressed gas system would be prohibitive. As a consequence, the industry relies on chemical oxygen generators to meet this requirement.

Oxygen generators are typically mounted in an overhead compartment above each seat row or in a compartment in the back of a seat in the row ahead. If the cabin altitude reaches a predetermined height (typically 14,000') or if the system is activated manually or electronically by the crew, the compartment opens dropping or exposing the masks. A lanyard connects the mask(s) to the firing pin of the oxygen generator. Pulling a mask fully out of the compartment releases the firing pin and activates the generator.

A chemical oxygen generator produces oxygen as the result of the reaction between two or more chemicals. In most cases, some or all of sodium chlorate, barium peroxide, potassium perchlorate and iron powder are used as the reactants in oxygen generators. When the generator is activated, the chemical reaction is initiated and oxygen is released. Once activated, a generator cannot be turned off and the chemical reaction and production of oxygen will continue until the generator has been exhausted - generally 12 to 20 minutes depending upon the type and size of generator installed. The reaction of the chemicals also produces a significant amount of heat and the canister of the generator can reach temperatures above 250°C. A burning smell may be noted and could cause alarm among passengers. However, this smell is a normal part of the chemical reaction.

The oxygen generator is attached to a number of emergency oxygen masks by plastic tubes. There is a 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.

The oxygen mask consists of a soft, yellow silicone cup fitted with elastic bands for securing the mask to the face. The bands are adjustable to accomodate passengers of different sizes. The mask may also have a clear concentrator or re-breather bag. Depending upon the cabin altitude, the concentrator bag may or may not inflate. Airlines make a point during their safety presentation of pointing out that the bag may not inflate as, in the past, lack of bag inflation has lead some passengers to believe that their mask was not working and to remove it resulting in hypoxia. Due to a potentially limited time of useful consciousness, it is critical that masks be put on immediately and kept on until advised by the crew that it is safe to remove them. Passengers should always don their own mask prior to assisting others (such as children) with their mask.

If the event of an onboard fire, masks will not be deployed as the production of oxygen may worsen the situation.

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