Fuel System
The APU fuel system is a fully automatic elec-
tronically controlled system. During start the
fuel system provides the correct amount of
fuel to support combustion to governed speed.
Once governed speed is reached, fuel flow is
controlled as necessary to meet the demands
of varying pneumatic and electrical loads
while maintaining a constant speed.
Fuel for the APU is supplied from the right
wing tank. A solenoid actuated fuel shutoff
valve, located on the forward baggage com-
partment bulkhead, is powered open at 5%
rpm during start and commanded closed for
shutdown (either normal or protective). The
fuel solenoid valve requires 28 VDC to open.
The right wing standby fuel pump is com-
manded ON during APU START, and the cor-
responding CAS message and S/I on the fuel
control panel illuminate. An integral fuel fil-
ter with bypass capability is also provided.
Following an unsuccessful start attempt, an
EPA drain tube collects the unused fuel and
vents it out the exhaust on the next success-
ful start. Unused fuel is not discharged on the
ground.
APU fuel consumption (approximately 100
pounds per hour) is dependent upon pressure
altitude, outside air temperature, and load im-
posed. Fuel is used from the right wing tank
and is not reflected on the EICAS fuel system
schematic fuel “used” display, nor is it ac-
counted for in the FMS fuel monitoring.
APU Protection System
The ECU provides all electrical commands
for both normal and protective operation of the
APU. Once the ECU is powered it completes
a pre-start BITE test to ensure no faults exist,
which would inhibit start. Upon start activa-
tion, the ECU provides signals for automatic
fuel and ignition initiation, starter relay
dropout, and ignition termination. The ECU
controls APU speed at approximately 70,000
rpm.
When system faults are detected, the ECU au-
tomatically shuts down the APU by closing the
fuel solenoid valve. Shutdowns due to a mon-
itored fault illuminate an amber “APU FAIL”
light on the APU control panel and display an
amber “APU FAIL” caution CAS message on
EICAS. The shutdown is recorded in the ECU
memory for later maintenance download.
APU Fire Detection &
Extinguishing System
The APU fire protection system is designed to
terminate APU operation and automatically
dispense a self-contained Halon extinguishing
agent whenever the detection loop inside the
APU enclosure has detected a fire. Manually
depressing the “FIRE PUSH” switch/indica-
tor on the APU control panel also activates the
system.
When an APU fire is detected, a FIRE indi-
cation illuminates on the control panel, ac-
companied by a MASTER WARNING
indication and a red flashing “APU FIRE”
warning CAS message on EICAS. An APU fire
warning horn located in the nose wheelwell
also sounds to alert ground personnel. When
an APU fire is detected, the APU is com-
manded to shutdown, the APU fuel valve is de-
energized closed, the generator is disabled to
prevent re-ignition, and the fire extinguishing
agent is discharged.
The APU fire detection system resets itself
when a fire is no longer detected. Reset does
not include re-use of the fire bottle after dis-
charge. The APU fire protection system is in-
tegrated into the fire detection test selection
on the System Test Knob. The APU fire de-
tector test occurs simultaneously with the air-
craft fire warning tests.
Refer to Chapter 8, Fire Protection for addi-
tional information.
6-6
FOR TRAINING PURPOSES ONLY
LEARJET 45 PILOT TRAINING MANUAL
FlightSafety
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