MD-11 Flight Crew Operations Manual
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APU Chapter APU
Description and Operation Section 10
APU.10.1
APU.10 APU-Description and Operation
General
The Auxiliary Power Unit (APU) provides electrical and pneumatic power for
engine start and air conditioning, and supplies ground and in-flight electrical
power. Pneumatic and electrical power may be used independently or
simultaneously.
The APU is installed in an unpressurized area of the rear pressure bulkhead,
beneath engine 2 and the horizontal stabilizer.
APU RPM, EGT, air output and electrical output are controlled by an Electronic
Control Unit (ECU).
APU controls are on the overhead panel. The external APU control panel is in the
fairing immediately behind the left main gear well. APU parameters are displayed
on the Systems Display (SD).
APU Control System
The APU control system consists of the ECU and the Electronic Fuel Control Unit
(EFCU). The ECU controls starter operation, fuel scheduling, ignition, and surge
bleed airflow and provides a stable, automatic start cycle. The EFCU receives
control input from the ECU and controls fuel supply.
The APU requires battery bus power for start and uses fuel from tank 2. The ECU
receives primary power from the battery bus.
The APU supplies air to operate the three air conditioning packs, before or after
engine start, as required.
APU Door System
The APU installation has an inward-opening door for the intake duct and an
outward-opening half door for the exhaust duct. An electric actuator operates the
doors.
The inlet and exhaust door actuation system receives electrical power from the
battery bus on command from the Miscellaneous Systems Controller (MSC).
If a door starts to close while the APU is operating, the APU shuts down.
October 02, 2006
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