Refer to figure.
In modern transport aeroplanes, there are two independent oxygen systems. The flight crew emergency oxygen system is gaseous, and the passenger system is supplied by chemical oxygen generators – continuous flow system.
Chemical Oxygen generators are relatively light self-contained devices and are located in each passenger, cabin attendants and lavatory service units. When a depressurization occurs, or the flight crew activates a switch, a compartment door opens and the masks fall out in front of the passengers. To activate the system, the passenger pulls on the mask, which pulls a lanyard cord that triggers the mechanism to ignite the chemical. Once started, the chemical process cannot be stopped, and continues until the chemical is fully consumed, typically in the range 12 to 20 minutes depending upon the type and size of generator installed. The chemical reaction used for oxygen generation creates heat (the generator itself incorporates a heat shield). Therefore, the smell of burning or smoke, and cabin temperature increase, may be associated with the normal operation of the oxygen generators.
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