737 Flight Crew Operations Manual
Engines, APU -
APU System Description
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
D6-27370-866-EGP 7.30.3
If the start fails or the APU GEN OFF BUS light fails to illuminate by the end of
the start cycle, a system failure has occurred and the FAULT light illuminates.
Operate the APU for one full minute before using it as a bleed air source. This one
minute stabilization is recommended to extend the service life of the APU.
APU Shutdown
Operate the APU for one full minute with no bleed air load prior to shutdown. This
cooling period is recommended to extend the service life of the APU. When the
APU switch is moved to OFF, this time delay is met automatically.
Moving the APU switch to OFF trips the APU generator, closes the APU bleed air
valve and extinguishes the APU GEN OFF BUS light. Shutdown occurs
automatically after 60 seconds. When the APU speed decreases sufficiently
during shutdown, the fuel valve and inlet door close. If the fuel valve does not
close, the FAULT light will illuminate after approximately 30 seconds. An
immediate shutdown can be accomplished by pulling the APU fire switch.
Electronic Control Unit (ECU)
An electronic control unit (ECU) monitors and controls the APU. Automatic
shutdown protection is provided for overspeed conditions, low oil pressure, high
oil temperature, APU fire, fuel control unit failure, EGT exceedance, and other
system faults monitored by the ECU.
The ECU automatically controls APU speed through the electronic fuel control. If
speed or EGT exceed acceptable levels with the APU providing electrical load
only, some electrical load is shed. When electrical load and air extraction raise the
EGT above acceptable levels during engine starting, electrical load shedding
occurs prior to reducing bleed air. When electrical load and air extraction raise the
EGT above acceptable levels other than during engine starting, the inlet guide
vanes move toward a closed position, reducing bleed air extraction while
maintaining electrical load.
APU Automatic Load Shedding
In flight, if the APU is the only source of electrical power, all galley busses and
main buses are automatically shed. If electrical load still exceeds design limits,
both IFE busses are also automatically shed. On the ground, the APU attempts to
carry a full electrical load. If an overload condition is sensed, the APU sheds
galley busses and main busses until the load is within limits. Manual restoration
of galley and main bus power can be attempted by moving the CAB/UTIL Power
Switch to OFF, then back ON.
September 24, 2015
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