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FADEC, is a function of weight-on-wheels, airplane MACH number and APU
speed
• Compressor - Drives a single-stage centrifugal impeller. Delivers approximately
45 psi bleed pressure at normal operating speed
• Combustor - Contains 10 fuel nozzles and 2 igniters
• Turbine - Two-stage axial flow turbine which drives the gearbox
• Gearbox - Provides the sump for the lubrication system and mounting of
component and accessories
• Exhaust - The flow across the APU and oil cooler is created by the eductor using
the exhaust discharge velocity
• Monitor zones for fire/overheat/smoke events, FIDEEX system health and status
4-4 For Training Purposes Only
Sept 04
P I LOT T R A I N I N G GU I D E
AUXILIARY POWER UNIT
SYSTEM SCHEMATIC
LUBRICATION SYSTEM
The lubrication system incorporates an integral oil reservoir. The two main areas that
are lubricated are the gearbox and turbine bearings.
The gearbox sump serves as a reservoir and can be filled through a fill cap. The APU
can also be serviced using the airplane oil replenishment system, simplifying post
flight maintenance actions.
The lubrication system consists of the following components:
• Oil filters - Lube and generator scavenge
• Oil temperature sensor - Sends an input to the FADEC for low and high oil
temperature protection. When oil temperature increases to a high predetermined
value for 10 seconds, the APU (on ground only) will shut down
• Low Oil Pressure (LOP) switch - When pressure drops below a predetermined
PSIG after the APU has reached on-speed (not less than 99%) for 15 seconds, the
APU (on ground only) will shut down automatically
• Oil cooler - Will bypass oil at a predetermined lower than normal operating
temperature, and flow through the cooler when temperatures are greater than the
normal operating range
• Magnetic Chip Collector (visual inspection) - Provides a visual indication of metal
particles within the system
Turbine
EXHAUST
EDUCTOR
Surge Duct
Note: The air system is shown with an APUbleed load.
Aircraft
Skin
FROM/TO
AIRCRAFT
SYSTEMS
Inlet
Combustion
Bleed
Diffuser
Bleed Air
Duct
INLET AIR
COMPRESSED AIR
ELECTRICAL
CIRCUITS
APU COMPARTMENT
AIRFLOW
FADEC
FROM/
TO APU
SYSTEMS
Load Control
Valve
APU
Compressor
AIR/OIL COOLER
Gearbox
TO APU
COMPARTMENT
Bleed Air
Check Valve
OIL
IN
OIL
OUT
Surge Valve
GX_04_002
TO AIRCRAFT
BLEED AIR
SYSTEM
For Training Purposes Only
Sept 04
4-5
P I LOT T R A I N I N G GU I D E
AUXILIARY POWER UNIT
OIL PRESSURE
On the prestart BIT check, the LOP switch is monitored by the FADEC. During
operation, if oil pressure decreases below a predetermined PSIG for approximately 15
seconds (on ground), automatic shut down will occur. An “APU OIL LO PRESS”
message will be on for 15 seconds and will be replaced by a cyan “APU
SHUTDOWN” advisory.
A low oil pressure condition in flight will not cause an APU protective shut down.
OIL REPLENISHMENT SYSTEM
The oil replenishment tank volume contains 5.7 liters (6 US quarts). APU oil level is
measured using a sensor which is located in the APU oil tank and provides quantity
information on the EICAS STAT display. Electrical failure of this sensor will be
detected during the BIT check and the indicated quantity will change from numerical
value to amber dashes on EICAS. An “APU OIL LO QTY” advisory message will
occur on EICAS (on ground) when the APU oil level reaches 3.5 U.S. quarts and the
APU has not been operating for 15 minutes.
4-6 For Training Purposes Only
Sept 04
P I LOT T R A I N I N G GU I D E
AUXILIARY POWER UNIT
An oil replenishment tank is located in the aft equipment bay and contains an electrical
pump and sensor probe for quantity level. The oil replenishment (pressure filling)
system is designed for ground use only and serves both main engines and APU. A
three way electrical valve is incorporated in the replenishment system to allow transfer
of oil to the system being topped up (either engine or APU).
Honeywell
APU Oil Tank
APU/Engine Oil
Replenishment Tank
APU OIL QUANTITY
Indicates APU oil tank
quantity in quarts.
GX_04_003
APU
RPM EGT
BRAKE TEMP
90%
100 650
03 03 03 03
OIL QTY (QTS)
ENG
APU
RES
12 . 3 12.3
5.0
3.2
　
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