737 MAX Flight Crew Operations Manual
Engines, APU -
APU System Description
MN-FLT-OH-201
7.30.3
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 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:
While the APU is starting using battery power only, the CPS FREQ
indication will be blank and the AC VOLTS indication will be 0 when the
AC Meters Selector is set to APU GEN. After the APU completes the start
sequence and the blue APU GEN OFF BUS light illuminates, the CPS
FREQ will show actual APU frequency and AC VOLTS will show actual
APU voltage.
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.
If the start fails or the APU GEN OFF BUS light fails to illuminate by the end of
the start cycle, a system failure has occurred and the FAULT light illuminates.
Operate the APU for two full minutes before using it as a bleed air source. This
two minutes stabilization is recommended to extend the service life of the APU.
APU Shutdown
Operate the APU for one full minute with no bleed air load prior to shutdown. This
cooling period is recommended to extend the service life of the APU. When the
APU switch is moved to OFF, this time delay is met automatically.
Moving the APU switch to OFF trips the APU generator, closes the APU bleed air
valve and extinguishes the APU GEN OFF BUS light. Shutdown occurs
automatically after 60 seconds. When the APU speed decreases sufficiently
during shutdown, the fuel valve and inlet door close. If the fuel valve does not
close, the FAULT light will illuminate after approximately 30 seconds. An
immediate shutdown can be accomplished by pulling the APU fire switch.
Electronic Control Unit (ECU)
An electronic control unit (ECU) monitors and controls the APU. Automatic
shutdown protection is provided for overspeed conditions, low oil pressure, high
oil temperature, APU fire, fuel control unit failure, EGT exceedance, and other
system faults monitored by the ECU.
March 1, 2021