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You can start the APU up to an altitude of 41,000 feet (12,500
meters).
The APU electronic control unit (ECU) controls these
components during start:
* APU inlet door
* APU fuel shutoff valve
* APU fuel
* Ignition system
* APU start system.
Prestart
The battery switch must be ON before you can start and operate
the APU.
If AC power is available, turn the aft number 1 fuel boost pump
on. This gives pressurized fuel to the APU. Pressurized fuel
makes the APU start better.
Starting the APU
When you move the APU switch to the START position and
release it, the switch moves back to the ON position. This sends
a signal to the electronic control unit (ECU). The ECU then
opens the APU fuel shut-off valve and the APU air inlet door.
The ECU also causes the low oil pressure light to come on.
When the air inlet door is fully open, the door switch closes. The
door switch sends a door fully open signal to the ECU.
APU Start Sequence
The ECU controls the APU start sequence. This is the start
sequence:
* At 0 percent speed and before the start system is energized,
the ECU energizes the ignition unit
* At 0 percent speed for start or 7 percent speed for restart, the
ECU energizes the starter-generator
* At 7 percent speed, the fuel solenoid valve opens
* At approximately 30 percent speed, the Low oil pressure
light (P5) goes out
* At 60 percent speed the ignition unit deenergizes
* At 70 percent speed, the starter-generator deenergizes
* At 95 percent speed, the APU can supply electrical power up
to 41,000 feet (12,500 meters). The APU can also supply air
up to 17,000 feet (5183 meters)
* The APU accelerates to 100 percent speed.
NOTE: The inlet guide vanes (IGV) close to 15 degrees when
the APU bleed air valve is closed. This will keep the
load compressor cool when it does not have a load.
AIRBORNE AUXILIARY POWER - OPERATION - START
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
49-00-00-006
Training Information Point
The BAT DISCHARGE light on the electrical meters, battery,
and galley power module comes on when the APU start uses
DC power. The BAT DISCHARGE light does not come on when
the APU uses AC power to start.
AIRBORNE AUXILIARY POWER - OPERATION - START
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
49-00-00-006
AIRBORNE AUXILIARY POWER - OPERATION - START
AIRBORNE AUXILIARY POWER - OPERATION - START
EFFECTIVITY
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
49-00-00-006
General
The ECU controls the APU shutdown. The APU has two types of
shutdowns, normal shutdown and protective shutdown.
APU Switch to OFF
When you put the APU switch to OFF, it removes the 28v dc ON
signal to the ECU and gives a 28v dc OFF signal to the ECU.
APU Cool Down
An APU shutdown causes a cool down cycle. The cool down
cycle time is 60 seconds. The time starts when the APU switch
is put to the OFF position.
The ECU does these steps for a cool down:
* Removes the ready-to-load signal
* Closes the bleed air valve
* Closes the inlet guide vanes (15 degrees)
* Opens the surge control valve
* Deenergizes the starter-generator
* Starts the 60 second timer.
Completion of the Shutdown
During shutdown of the APU, these steps occur:
* At 30 percent speed, the APU air inlet door starts to close
(closes immediately for APU fire).
* At less than 7 percent speed, an APU restart can be initiated.
Training Information Point
The APU fuel shutoff valve and air inlet door will close for a
normal or protective shutdown. Wait 40 seconds after the EGT
goes below 300 C for the inlet door and fuel shutoff valve to
close before you move the battery switch to the OFF position.
Do not use the battery switch or fire switches to begin a normal
APU shutdown. The 60 second cool down is required to prevent
coke in the turbine bearing and fuel nozzles.
If the fuel shutoff valve does not close in the required time, the
APU FAULT light will come on and stay on until the APU is
started again or the battery switch is put in the off position.
See the APU engine controls section for more information on
　
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