FIRE PROTECTION
Volume 2
Flight Crew Operating Manual
Sep 13/2004
09-01-08
CSP 100-6
REV 1
AUXILIARY POWER UNIT FIRE PROTECTION
DESCRIPTION
The auxiliary power unit (APU) fire detection system functions by detecting an overheat condition within the APU enclo-
sure. An APU fire can only be extinguished from bottle No. 2.
The APU fire protection has a ground mode for unattended operation and a flight mode.
On the ground, when a condition is detected, the APU shuts down automatically. Five seconds later, the APU fire bottle
automatically discharges Halon into the APU enclosure.
In flight, when a fire condition is detected and the APU FIRE switch is pushed, the APU shuts down and arms the discharge
cartridge on the number 2 container. The flight crew must discharge the bottle by pushing the FIRE EXT 2 switch.
COMPONENTS AND OPERATION
DETECTION LOOPS
The APU enclosure is fitted with dual fire detection loops. The loops are connected to the fire detection and extinguishing
control unit. The loops are constructed of an outer metal sheathing and two internal wires that are separated by an insulating
material (thermistor). One of the wires carries electrical current, the other is grounded to the outer sheathing at the control
unit. The resistance of the thermistor decreases with an increase in temperature.
When the electrical resistance decreases to a trip point, electrical current from the live internal wire conducts to the second
internal wire and a fire indication is generated.
This dual loop system minimizes the possibility of false fire warnings. Both loops must sense the fire or overheat condition
under exacting parameters before flight deck fire indications are generated.
ENGINE
L ENG FIRE
FIRE
FIRE
2
ARMED
1
ARMED
APU FIRE
OFF
ON
FIRE EXT
IGNITION
MACH HOLD
SYNC
N1
N2
OFF
EVENT
ON
AUTO APR
OFF
R STARTER
CRANK
START
OFF
CRANK
START
L STARTER
R ENG FIRE
FIRE
FIREX
CONTROL
UNIT
FLIGHT COMPARTMENT
APU
ECU
FIRE
APU FIRE
APU SHUT DOWN
FIRE ELEMENT
APU COMPARTMENT
APU
C
F
O
09
010
02
_00
3
APU
AUTO
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