MD-11 Flight Crew Operations Manual
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APU -
Description and Operation
APU.10.2
APU Start and Shutdown
The APU START/STOP alternate action (pushbutton) switch on the aft overhead
panel commands APU start through the MSC, after determining APU running
status.
A DC electric starter motor, mounted on the APU gearbox, uses aircraft battery
power for APU start. A battery-powered DC pump provides starting fuel flow to
the APU from main fuel tank 2. This pump is used for APU start when AC power
is not available. When AC power is available, the AC pump in main fuel tank 2
supplies fuel to the APU.
Following APU start, the ECU controls the APU through the EFCU, maintaining
a constant APU N2 rpm speed with all varying loads. The ECU also regulates the
surge control valve and the load bleed valve to meet demand for pneumatic power.
The EGT reaches set values with combined bleed air and power loads. The ECU
commands the low pressure (N1) compressor speed to drop. A variable turbine
nozzle actuator adjusts N1 speed by adjusting the position of the vanes, which are
located upstream of the N1 turbine.
Following APU shutdown, EGT is monitored continuously until the APU is shut
down. The EGT value is displayed on the APU data window of the SD. Should
EGT limits set by the ECU be exceeded, N1 speed is slowed to N1/EGT cutback.
Shut down is automatic if there is still no control.
The following automatic shutdowns, processed through the ECU, protect the APU
against hazardous conditions and potential damage:
• N1 or N2 overspeed.
• Low oil pressure.
• High oil temperature.
• Reverse flow.
• Loss of EGT signal.
• Fire warning.
• Loss of N1 or N2 speed signal.
• High EGT.
• Loss of DC power.
• Slow start.
• No flame during start.
• Inlet door not open.
• Starter motor energized over 1 minute.
• Overcurrent.
October 02, 2006
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