SYSTEM OPERATION
APU Fire Detection System
The APU fire detection system is monitored
and controlled by a fire detector and loop. The
APU fire/overheat detector monitors the tem-
perature inside the APU firebox, and is con-
nected to a sensor tube attached to the APU
turbine housing. The detector is an electrical
device that provides a 28 VDC signal to the
APU maintenance panel in the event of an
APU fire or overheat condition. The APU
maintenance panel provides the fire and over-
heat warnings to the operator and initiates the
fire extinguishing sequence.
The fire detector/sensor unit is a sealed tube,
containing a material that produces gas when
it is heated. The detector/sensor unit has a
built in pressure switch, that is normally in the
open position. As the detector/sensor unit is
heated, a gas is produced and closes the con-
tacts of the pressure switch. The closed con-
tacts of the pressure switch emits a signal to
the APU maintenance panel that initiates an
automatic shutdown of the APU, and dis-
charges the Halon extinguishing agent into
the fire box.
APU Fire Extinguishing System
In the event that an overheat or fire is detected
in the APU firebox the APU maintenance panel
initiates an automatic shutdown and discharge
of the extinguishing agent from the Halon
container through the injection tube/nozzle
into the fire box. The APU maintenance panel
also sends a signal to the Electronic Control
Unit (ECU) which relays the signal to the
cockpit for an APU fire warning. The APU
shutdown and extinguisher discharge sequence
can also be manually initiated by depressing
the red “FIRE PUSH” button located on the
APU control panel.
FIRE DETECTION
SYSTEM TEST
Rotating the airplane system test knob on the
center pedestal to the FIRE DET position and
depressing the center knob activates the fire
detection system test. The FIRE DET test
checks continuity in the engine fire detection
circuits, the extinguisher circuits, the bleed air
leak detection circuits and the anti-ice duct
leak detection circuit. Also, if the optional
APU is installed, this test checks the APU fire
detection and warning system, and the extin-
guishing system. Holding the FIRE DET test
on for more than 30 seconds will result in an
APU FAIL indication. If the APU is running
while this test is activated, the APU will shut-
down and illuminate the APU FAIL annunci-
ator.
The FIRE DET test activates the following
indications:
• Master WARN chimes sound followed
by aural voice message “Left Engine
fire followed by “Right Engine Fire.”
• L e f t a n d r i g h t e n g i n e F I R E P U S H
switches illuminate steady red.
• All four extinguisher switches illuminate
“ARMED.”
• Red CAS messages appear:
“L R BLEED AIR LEAK”
“WING/STAB LEAK”
“APU FIRE”
• CWP annunciators illuminate:
“L BLEED AIR LEAK”
“R BLEED AIR LEAK”
“WING/STAB LEAK”
• Red FIRE EI’s flash in the left and right
ITT displays on EICAS.
L E A R J E T 4 5
P I L O T T R A I N I N G M A N U A L
8-10
FOR TRAINING PURPOSES ONLY
FlightSafety
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