737 MAX Flight Crew Operations Manual
Engines, APU -
APU System Description
7.30.2
MN-FLT-OH-201
When the APU is operating:
• The APU door will automatically move from the ground position to the
flight position at liftoff.
• The APU door will automatically move from the flight position to the
ground position at touchdown.
APU Fuel Supply
Fuel to start and operate the APU comes from the left side of the fuel manifold
when the AC fuel pumps are operating. If the AC fuel pumps are not operating,
fuel is suction fed from the No. 1 tank. During APU operation, fuel is
automatically heated to prevent icing.
APU Engine and Cooling Air
APU engine and cooling air routes to the APU through the air inlet door. The APU
air inlet door has 3 positions, closed, flight open (approximately 17 degrees) and
ground open (approximately 45 degrees). APU exhaust gases discharge overboard
through an exhaust muffler.
The cooling 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
main battery. 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.
March 1, 2021
