MD-11 Flight Crew Operations Manual
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Fire Protection -
Description and Operation
Fire.10.2
If one of the loops is inoperative, a fire signal from the remaining operative loop
can provide a valid signal. The outputs are also integrated through a logic circuit
to differentiate between a fault signal and a fire signal.
Engine and APU fire detection and extinguishing is available during battery starts.
Main Engine Sensing Elements
The sensing elements consist of a stainless steel tube filled with pressurized
helium gas. The tube also contains a hydrogen impregnated gas core.
One end of the tube is sealed and the other end has an assembly containing the
pressure switches (alarm and integrity).
During overheat conditions, an increase in temperature from normal to a specified
average will cause thermal expansion of the helium gas with an increase in
pressure until an electrical contact is made to trigger the alarm system.
During a fire condition, hydrogen gas is released from the core material when a
section of the sensing element is heated to a preset temperature. This release of
hydrogen will activate the alarm switch.
The expansion of gases in the sensing element is reversible. The sensing element
will detect the extinguishing of a fire as well as the presence of a fire.
A single break in the element casing or wiring will cause a loop FAULT alert and
will not cause false fire alarms.
The main engine fire detector sensing elements are located in the engine core
compartment.
The sensing elements detect the following types of fire or overheat conditions:
• Fires caused by flammable fluids leaking on hot surfaces or ignited by
electrical sparks.
• Fire/overheat caused by burn through of the combustor case.
• Ruptured bleed ducts (during certain operating conditions only).
APU Sensing Elements
The APU is installed in the aft fuselage. The dual fire detector sensing elements
for the APU are routed along the sides of the compartment wall. There are two
loops, each loop is divided into three segments.
The system is designed to detect flammable fluid fires and hazardous overheat
conditions from ruptured pneumatic ducts.
Besides the APU duct, this compartment also contains the pneumatic duct from
engine 2 (aft). Thus, an APU FIRE warning could also mean that pneumatic duct
2 has ruptured. The APU sensing elements are activated by a uniform temperature
of about 218 ºC.
November 10, 2008
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