FIRE DETECTION AND EXTINGUISHING SYSTEMS (Continued)
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
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
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:
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.
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.