TM 55-1520-240-23-10
15-1
AUXILIARY POWER UNIT
15-1
DESCRIPTION
The auxiliary power unit (APU) is a gas turbine engine
that provides hydraulic and electrical power for engine
starting and ground maintenance. The complete APU
system includes the APU, an electronic sequence unit
(ESU), a switch, relays, a start module, and a start
accumulator. Fuel supply components include a fuel
valve, boost pump, and manual shutoff valve.
NO BREAK - WORK HARDER
A hydraulic motor-pump is mounted on the front of
the APU. To start the APU, the motor-pump is driven
by pressurized hydraulic fluid from the utility hydraulic
system, supplied by the start accumulator. When the
APU is operating, the motor-pump operates as a pump
to pressurize the flight control and utility hydraulic
systems. An ac generator is driven by the APU to
supply power to the electrical system during ground
operation. Refer to TM 55-2835-205-23 for more
detailed information on the APU.
NO BREAK - WORK HARDER
The ESU is a microprocessor with built-in test equipment
(BITE) facilities. The ESU automatically sequences
the start procedure and continuously monitors APU
operation. If the operating sequence is not normal or
the operation is not within normal limits, the ESU stops
the APU and shows fault displays that give the reason
for shutdown. Information to decode the fault display is
on a placard on the ESU.
SEQUENCE OF OPERATION
The APU is started and stopped through a three-position
switch on the overhead ELECT panel. During normal
starts, operating power is obtained from the 28 vdc
essential bus. Emergency start power is obtained from
the battery bus.
NO BREAK - WORK HARDER
Placing the APU switch to RUN provides power to
operate the fuel pump and open the fuel solenoid valve.
It also applies power to the electronic sequencing unit
(ESU) and resets the built-in test equipment (BITE)
indicators. Moving the switch to START energizes the
solenoid valve on the APU start module. The valve
opens to allow pressurized fluid to flow from the start
accumulator to operate the motor-pump on the front of
the APU.
NO BREAK - WORK HARDER
As the motor-pump increases APU rpm, a magnetic
pickup on the APU generates a signal proportional to
APU speed.
NOTE
The starting sequence can be
interrupted at any point and the APU
shut down by closing the manual
shutoff fuel valve.
15-2
