APU Engine and Cooling Air
APU engine air routes to the APU through an automatically operated air inlet door located
on the right side of the fuselage. APU exhaust gases discharge overboard through an
exhaust muffler. Air for APU cooling enters through a cooling air inlet above the APU
exhaust outlet. This air circulates through the APU compartment, passes through the oil
cooler and vents through the exhaust outlet.
Electrical Requirements for APU Operation
APU operation requires the following:
- APU fire switch on the Overheat / Fire panel must be IN
- APU fire control handle on the APU ground control panel must be IN
- Battery switch must be ON.
Electrical power to start the APU comes from No. 1 transfer bus or the airplane battery(ies).
With AC power available, the starter generator uses AC power to start the APU. With no AC
power, the starter generator uses battery power to start the APU.
Moving the battery switch to OFF on the ground or in the air automatically shuts
down the APU because of power loss to the electronic control unit.
