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.
