737 Flight Crew Operations Manual
Engines, APU -
APU System Description
Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.
7.30.2 D6-27370-866-EGP
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. 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.
APU Start
The automatic start sequence begins by moving the APU switch momentarily to
START. This initiates opening of the air inlet door. When the APU inlet door
reaches the full open position the start sequence begins. After the APU reaches the
proper speed, ignition and fuel are provided. When the APU is ready to accept a
bleed air or electrical load the APU GEN OFF BUS light illuminates.
Note: When the APU is started using battery power only, there is no indication
on the electrical metering panel that the APU generator has come on line
and is ready to be selected. Both the frequency and voltage readings are
zero until the APU generator is placed on line.
Note: During the APU start cycle, the APU EGT indication may fluctuate from
0º to 1100º C prior to normal EGT rise and the LOW OIL PRESSURE light
may cycle on and off several times. These indications have no adverse
effect on starting the APU. It is not necessary to monitor EGT during start.
If the APU does not reach the proper speed with the proper acceleration rate within
the time limit of the starter, the start cycle automatically terminates. The start cycle
may take as long as 120 seconds. Automatic shutdown occurs in the event of EGT
exceedance.
September 24, 2015
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