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BOEING 737 SYSTEMS REVIEW
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AUXILIARY POWER UNIT (APU)
1. GENERAL
¡ The auxiliary power unit (APU) is a self-contained gas turbine engine. It is installed within a fireproof,
sound-reducing shroud located in the tail of the aircraft.
¡ The APU supplies bleed air that may be used for engine starting and air conditioning.
¡ An AC electrical generator is provided which may be used as an auxiliary AC power source.
¡ Electrical power from the aircraft battery and fuel from the No.1 tank are used to start and operate
the APU.
¡ The APU is monitored and controlled by an Electronic Controller.
¡ Automatic shutdown protection is provided in case of APU fire or when certain limits are exceeded.
737 NG
The APU is equipped with an automatic fire extinguishing system.
2. COMPONENTS
¡ The APU consists of a load compressor, engine compressor, turbine and a gearbox which drives the
generator and other accessories. The APU is monitored and controlled by an Electronic Controller.
Automatic shutdown protection is provided in the event of low oil pressure, system fault, overspeed,
overtemperature or fire.
¡ For the 737NG, automatic shutdown protection is also provided for a high oil temperature or a Fuel
Control Unit failure.
¡ The APU oil cooler and electrical generator are provided with positive cooling airflow by a geardriven fan. Inlet air is ducted to the fan through a shutoff valve which opens when pressure is
sensed in the bleed feeder duct.
¡ Fuel for the Fuel Control Unit (FCU) is available from the no.1 fuel tank whenever the APU is
operating. The fuel is automatically heated, if required, to prevent icing. The fuel solenoid valve
opens when APU speed reaches 7%.
¡ APU speed is automatically controlled by signals from the Electronic Controller to the FCU, which
modulates the fuel flow to control speed.
¡ When electrical load and air extraction combine to raise the EGT above acceptable levels, the Inlet
Guide Vanes (IGV) to the load compressor trim to a more closed position, reducing air extraction but
still maintaining the electrical load. In the event of overtemperature, the APU will shut down and the
Fault Light will illuminate.
¡ The Surge Control Valve (SCV) prevents the load compressor from stall, by dumping bleed air, if flow
demands do not match with IGV position.
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¡ APU fire control/discs :
- External fire warning panel in right wheel well
- B737 Classics only : 2 discs below leading edge of right stab (for single APU fire bottle)
red disc = thermal discharge
yellow disc = pilot discharge
¡ battery provides fire detection and protection :
- Detection = Battery Bus
- Protection = Hot Battery Bus
¡ Air inlet aft of aft cargo door for cooling :
- B737 NG only : cooling air is routed through a cooling air inlet above the APU exhaust outlet
3. OPERATION
¡ APU Altitude Limits :
- up to 10 000 ft for APU Bleed + Electrical load
- up to 17,000 ft for APU Bleed only
- up to 35,000 ft for APU Electrical load (B737 Classics only)
- up to 41000 ft for APU Electrical load (B737 NG only)
¡ APU bleed valve must be closed when :
- Ground air is connected & isolation valve is open
- Engine No 1 Bleed valve open
- Isolation & Engine No 2 Bleed valves open
APU Bleed valve may be open during engine start, but avoid engine power above idle (to avoid APU
back-pressure)
¡ Do not start or shut down APU during refueling operations.
¡ APU Fuel Consumption :
- 115 kg / hr (B737 Classics / normal OPS) & 90 kg/hr (B737 Classics / in flight)
- 105 kg / hr (B737 NG / normal OPS) & 80 kg/hr (B737 NG in flight / 1 ENG)
¡ In flight start may be attempted at any altitude & may take up to 135 sec.
¡ The Battery Switch must be ON during normal APU operation. Positioning the Battery Switch to
OFF, while the aircraft is on ground, will automatically shut down the APU.
¡ APU SWITCH
- Normal position when the APU is not running.
- Positioning the switch to OFF with the APU running trips the APU generator off the bus(es) (if
connected) and closes the APU bleed air valve. The APU continues to run for a 90 seconds
cooling period before the APU automatically shuts down. The APU shuts down immediately if the
APU bleed air valve is never opened after APU start. An immediate shutdown can also be
accomplished by pulling the APU Fire Switch. The APU air inlet door automatically closes after
shutdown.
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ON : Normal position with the APU running.
START (Momentary) : Positioning the APU Switch from OFF to START and releasing it to ON initiates
an automatic start sequence.
¡ APU EGT INDICATOR
- shows APU exhaust gas temperature. Remains powered 5 minutes after shutdown (B737 NG only)
- B737 Classics = Max EGT 760°C (red line), Max Cont 710°C (top of green band)
- B737 NG = No limitations on EGT gauge. System self-monitors exceedance
¡ APU GENERATOR AC AMMETER
- shows APU generator current load
- B737 NG = AC ammeter removed. APU generator amperage can be obtained by placing the AC
meter selector on the electrical panel to APU GEN and reading AG amperage on the digital AC
Ammeter
¡ APU Start Sequence :
1) Automatic start sequence initiated when the APU switch is momentarily positioned to the START
position & then release to ON. (APU fire handle must be pushed down and APU fire handle in
wheel well in UP position)
Note : the auxiliary 28v DC power receptacle can be used to start without battery (with battery
switch still ON)
2) 90 sec. internal timer (The starter duty cycle is time limited to prevent possible cutout failure &
excessive battery drain)
Note : battery charger is isolated from battery during start (reason why 23 Volts is needed)
3) LOW OIL PRESSURE light comes on
4) APU door opens in 20 seconds
5) Tank No 1 APU Fuel shutoff valve opens
6) When door is full open, then starter engages & ammeter indicates full discharge
7) 4 psi oil pressure switch opens fuel solenoid valve, starts ignition
8) LOW OIL PRESSURE light out at 55 psi
9) 50% RPM speed switch cuts out starter and ignition (90 second timer de-energized)
10) Indicated by drop in DC load, EGT rising
11) If in 90 seconds the APU RPM is not high enough to disengage the starter, the start will be
aborted to prevent damage to the starter & the battery (in this case OVERSPEED light will be on
until start switch is turned off)
12) At operational speed (95% speed switch) APU GEN OFF BUS light comes on indicating the APU
can take a load (electric or pneumatic)
13) It is recommended the APU be operated one full minute before using as a pneumatic source.
¡ Whenever the APU is operating & AC power is on the aircraft busses, extended service life of the
APU FCU can be achieved by operating at least 1 fuel boost pump to supply fuel under pressure to
the APU
Note : Whenever the APU is operating to provide electrical power only, switching the APU Bleed
switch to OFF further off-loads the APU resulting in lower fuel flow, EGT &… noise !
4. AUTOMATIC GALLEY LOAD SHEDDING
¡ Regarding of aircraft options (see AFM), Galley electrical loads will automatically be shed should the
total aircraft electrical power requirements exceed design limits with the APU generator providing
electrical power to all busses.
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BOEING 737 SYSTEMS REVIEW
5. FAULTS & INDICATIONS
VALID FOR
LIGHT
FAULT
LOW OIL
PRESSURE
MAINT
OVER
SPEED
INDICATION
APU FAULT LIGHT (amber)
- APU malfunction exists, causing the APU to initiate
an automatic shutdown.
- Additional restarts may be attempted.
- When the APU switch is in the OFF position, 737
Classics light is disarmed, 737 NG light
extinguishes after 5 minutes.
- If fuel valve has failed, light remains illuminated
- Also illuminated if OVERSPEED protection is lost
APU LOW OIL PRESSURE LIGHT (amber)
- APU oil pressure is low, causing the APU to initiate
an automatic shutdown (after the start cycle is
complete).
- Light is illuminated during start until the APU oil
pressure is normal.
- When the APU switch is in the OFF position, 737
Classics light is disarmed, 737 NG light
extinguishes after 5 minutes.
- On 737 NG, if light is ON when APU is placed to
OFF, light extinguished after 5 min.
APU MAINTENANCE LIGHT (blue)
- APU maintenance problem exists.
- APU maybe operated.
- Light is disarmed when the APU Switch is in the
OFF position.
(low oil qty, INOP speed sensor or thermocouple
starter brush wear)
APU OVERSPEED LIGHT (amber)
- APU speed is excessive (110%), causing the APU
to initiate an automatic shutdown.
- During APU shutdown indicates overspeed
shutdown protection is lost or an APU start is rejected
prior to obtaining normal operating speed.
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BOEING 737 SYSTEMS REVIEW
APU LOCATION
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