FIRE DETECTION AND EXTINGUISHING SYSTEMS (Continued)

TM 55-1520-240-23-10 12-7 FIRE DETECTION AND EXTINGUISHING SYSTEMS (Continued) 12-7 THEORY OF OPERATION Under normal conditions, the circuit within the magnetic amplifier of a control unit is in balance and no current flows to the warning lights. When the engine compartment temperature around the sensing element reaches about 575 F (300 C), indicating a fire, the insulating material between the sensing element tubing and the internal wire becomes conductive, allowing current to pass through the control unit to the appropriate warning light in the cockpit. When the test switch on the center instrument panel is operated, 28 vdc is delivered to the control units. Within each control unit, a relay operates to ground the system, lighting the warning lamps in the fire pull handles. FIRE EXTINGUISHING SYSTEM DESCRIPTION Two containers filled with fire extinguishing agent are mounted in the base of the pylon, between the engines. In case of an engine fire, extinguishing agent within the containers can be discharged independently to the affected engine from the cockpit. The system includes the two fire extinguishing agent containers, four discharge nozzles (one to each engine from each bottle), controls and switches in the cockpit, and associated tubing, wiring, and circuit breakers. Each container is a metal sphere that holds a three-pound charge of non-toxic extinguishing agent. The agent is pre-charged with nitrogen at 600 psi monitored by a gauge on the bottle. Separate lengths of tubing lead from the bottle to each engine, ending in a pair of discharge nozzles in each engine compartment. The two containers share the nozzles at each engine. The discharge paths are kept separate by a double check valve tee where the tubing from each container joins. Discharge of extinguishing agent to the engines is controlled by two pull handles, one for each engine, and a toggle switch on the center instrument panel. Pulling a handle shuts off fuel to that engine and arms the circuit to the containers. Selecting one of the containers with the switch discharges extinguishing agent from that container to the affected engine. The remaining container may be discharged into the same or the opposite engine as required. Electrical power for the system is supplied from the 28 vdc No. 1 or No. 2 flight essential bus through circuit breakers in the No. 1 and No. 2 power distribution panels. THEORY OF OPERATION When the fire detection system lights the warning lamps in either fire pull handle, the extinguishing system for the affected engine can be armed for operation by pulling out the lighted handle. When this is done, the following sequence of events occurs: a. Current from the 28 vdc flight essential buses is directed by a switch through the engine fuel shutoff circuit breaker to close the engine fuel valve. This shuts off fuel to the affected engine. b. At the same time, the selector switch between the fire pull handles is armed. When the switch is operated to select a container, an explosive cartridge at that bottle is fired. This breaks a brittle retaining disc, releasing extinguishing agent from the bottle through the tubing to the two discharge nozzles at the affected engine. 12-21