These systems are provided for power plants, APUs and baggage compartments. A system generally consists of:
· A number of metal containers or bottles, containing an extinguishant eg. methylbromide, bromotrifluoromethane or bromochlorodifluoromethane (BCF) also known as Halon 1211.
· Tubing to carry the extinguishing agent to areas that require protection.
· Control valves.
· Indicators.
· Control circuitry.
Systems vary considerably on different aircraft but the basic elements are similar. HRD or high rate discharge is the term applied to most systems in common use.
The extinguishant is pressurised with an inert gas and sealed in the container by means of a discharge or operating head. When operated, either by selector switches (fire handles) on the flight deck or crash switches, an electrically fired cartridge or squib ruptures a metal diaphragm within the discharge head and the extinguishant is released. It then flows through spray pipes, spray rings or discharge nozzles into the appropriate firezone. The electrical power is 28 volts dc and is supplied from an essential services busbar.
Most aircraft use a ‘two shot’ extinguishing system for the power plants. This uses connections between the individual power plant systems. In this system the fire extinguishers for each power plant are interconnected. This allows two separate discharges of extinguishant into any one power plant. On many aircraft two fire bottles are installed in each engine nacelle. This allows two separate discharges of extinguishant into each power plant. Indication that a fire extinguishing circuit has been operated is indicated by a warning light.
In some installations special switches are incorporated to automatically operate the extinguishers in the event of a crash. These switches also connect cabin emergency lights to the aircraft battery power supply. Two types of crash switch are in common use:
· The inertia control type
· The frangible type.
An inertia controlled switch generally consists of a heavy piston supported on its own spring and so arranged that at the required degree of deceleration (a typical value is 3g), it compresses the spring and causes a bow spring to snap over thereby bridging contacts connected in the extinguishing system circuit. To allow resetting of the switch after operation or rough handling during transit, a reset plunger is incorporated.
Frangible switches consist of two electrical contacts mounted in a hermetically sealed glass envelope. The contacts are prevented from closing by a spring, but in the event of the glass envelope being shattered, the contacts will close and complete the circuit to the extinguishing system. The switches are located in positions such as the wing tips, the underside of engine nacelles, at various points on the underside of the fuselage, etc. This is so that in the event of a crash, at least one of the switches will be shattered.
