Typical fixed systems in the types of aircraft for which fixed fire extinguisher systems are specified, it is usual for the extinguishant to be stored in the containers under pressure and to be discharged by electrically firing cartridge units within the extinguisher discharge heads. The firing circuits are controlled by switches or fire control handles in the flight crew compartment; some types may also be automatic as in the case of an APU. The layout of a system and the number of components required, depend largely on the type of aircraft and number of power plants and also on whether fire protection is required for auxiliary power units, landing gear wheel bays and baggage compartments.
A secondary function of the Engine fire handles is to isolate the engine from other aircraft systems to prevent them from making the fire worse, and also to stop the fire spreading. The systems usually affected are fuel, hydraulics, and air systems. See figure. There are two types of fixed systems used for power plant fire protection.