The door position switch is located on the air inlet duct, above the door actuator, near the lower hinge position of the inlet door. The purpose of the switch is to detect that the inlet door has opened, working in conjunction with, but independently from the internal switches of the actuator. Should the door fail to open within 60 seconds, or open to a position less than the minimum 15-degree position, the electrical circuitry of the APU control unit is such that the APU will not start and the EICAS fault message APU DOOR will be displayed.
The battery switch on the P5 overhead panel is set to the ON position before an APU start. The APU control switch on the P5 overhead panel is set to the START position, hold at the START position for one second and manually set to the ON position. The air inlet door actuator opens the air inlet door to the fully open position (45°). If the air inlet door does not open in 60 seconds, the EICAS display shows "APU DOOR" on the status page. During an in-flight operation, the ground safety relay, R263, is energized and the air inlet door closes to the partly open position (15°). The APU fuel shutoff valve opens on the rear spar No. 2 main tank. If ac power is available, the ac aft fuel boost pump supplies fuel from the No. 2 main tank of the airplane to the fuel pump. If ac power is not available, the dc fuel boost pump operates from the airplane main battery. After the air inlet door opens, the APU controller does the APU start sequence and supplies 28V dc power to the starter motor. The starter motor starts to turn the low pressure (N1) and high pressure (N2) turbine rotor assemblies. The speed sensor (N1) measures the speed of the input shaft of the load gearbox. The speed sensor (N2) measures the speed of the oil pump drive. When the N2 rotor speed is 8%, the APU controller energizes the ignition exciter and sends a signal to the fuel metering unit to supply fuel to the combustion chamber. The ignition exciter causes the two igniter plugs to make sparks. The APU engine has combustion and starts to increase in speed. When the N2 rotor speed is 40%, the starter motor and ignition exciter are de-energized by the APU controller. When the N2 rotor speed is 55%, the APU controller sets the inlet guide vane (IGV) demand to a minimum. Two seconds after the N1 rotor speed is 95%, the APU is prepared for pneumatic and/or electrical loads