General
The APU system contains these subsystems:
* APU power plant (49-10)
* APU engine (49-20)
* APU fuel system (49-30)
* APU ignition and start system (49-40)
* APU bleed air system (49-50)
* APU controls (49-60)
* APU indicating system (49-70)
* APU exhaust system (49-80)
* APU lubrication system (49-90).
APU Power Plant
The APU is a gas turbine engine. It has a single shaft and
operates at a constant speed. It drives an electric generator,
pneumatic load compressor, and a reduction gearbox.
APU Engine
These are the primary APU engine components:
* The power section
* The load compressor
* The accessory gearbox.
The power section has a single stage centrifugal compressor. It
has a reverse flow annular combustor with a two-stage axial
turbine.
The load compressor turns on the same shaft with the power
section compressor. The load compressor also uses the same
air inlet as the power section compressor.
The gearbox provides gear reduction for the high speed torque
of the power section to the accessories on the gearbox.
APU Fuel System
The APU fuel system supplies pressurized and metered fuel to
the combustion chamber. It also supplies fuel to operate the
inlet guide vane actuator and the surge control valve.
APU Ignition and Start System
The APU ignition and start system begins the rotation and
acceleration of the APU. The starter-generator is used as a
starter during APU starting. The start power unit (SPU) and start
converter unit (SCU) change ac or dc power so it can be used
for starting.
APU Bleed Air System
The APU bleed air system supplies pressurized air to the
airplane pneumatic system. Inlet guide vanes control the
quantity of air that goes to the load compressor. A surge
control valve vents unnecessary bleed air overboard through
the exhaust. A bleed air valve (BAV) isolates the APU air
system from the airplane ducts.
AIRBORNE AUXILIARY POWER - GENERAL DESCRIPTION
EFFECTIVITY
GUN ALL 49-00-00
Page 5
D633A101-GUN Oct 10/2002
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
49-00-00-002
APU Controls
An electronic control unit (ECU) controls the APU functions. The
ECU interfaces with other airplane systems.
The ECU monitors APU functions using built in test equipment
(BITE). THE ECU shows unsatisfactory conditions found by BITE
and the cause for protective shutdowns on the control display
unit (CDU). The unsatisfactory conditions are called
maintenance messages. Some of the maintenance messages
and all the protective shutdowns cause one of these lights in the
flight compartment to come on:
* Maintenance light (blue)
* Low oil pressure light (amber)
* Fault light (amber)
* Overspeed light (amber).
APU Indicating System
The APU indicating system consist of these components:
* EGT gauge
* Data Memory Module
* Control display unit (CDU).
The APU indications are on the P5-4 panel and the P9 panel.
APU Exhaust System
The APU exhaust system sends the APU exhaust gases through
the APU muffler. The muffler sends the APU exhaust out the
tailcone.
APU Lubrication system
The APU lubrication system lubricates and cools the APU
bearings, the gearbox, and the starter generator. An eductor
moves air through the oil cooler to control oil temperature. It
also takes in the ambient air to cool the APU compartment.
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