TM 55-1520-240-23-6
7-135
UTILITY HYDRAULIC SYSTEM
(Continued)
7-135
Engine Start System Description
A hydraulic starter is mounted on the front of each
engine at the 12 o’clock position. The starter uses
hydraulic pressure from the APU motorpump, the utility
pump, or a ground power unit to turn the engine.
NO BREAK - WORK HARDER
The pressure source is coupled to each starter through
the utility system pressure control module by a hydraulic
pressure line through a coupling at the fuselage
quick-disconnect shelf. Return lines run from each
engine to the utility hydraulic system return module. An
overrunning clutch and an overspeed protector prevent
the starter motor from becoming a pump when engine
speed exceeds motor speed. The starter shuts off at
50 percent N1.
NO BREAK - WORK HARDER
Engine starting is controlled by a cockpit switch
operating through solenoid (pilot) and pressure-operated
(start) valves for each engine. The valves are located in
the pressure control module.
Engine Start System Theory of Operation
When the cockpit ENGINE 1 or ENGINE 2 START
switch (without 74 ) is set to MOTOR or the ENG START
switch on the FADEC panel is set to 1 or 2 (with 74 , the
associated solenoid pilot valve is energized, allowing
hydraulic pressure to flow through the engine start
control valve. Both valves are in the pressure control
module. The hydraulic pressure may come from one
of three sources:
APU Motorpump (3,350 psi)
Utility Pump (3,000 psi) (Emergency Use)
External Ground Power Unit (3,350 psi )
NO BREAK - WORK HARDER
Hydraulic pressure from the control valve turns the
hydraulic starter, motoring the engine compressor (N1)
section to 18 to 20 percent to start the engine. As the
engine accelerates, an overspeed clutch disengages
the starter from the engine and an overspeed protector
shuts off the flow of pressurized fluid to the starter.
Return fluid from the starter motor is discharged through
the return control module to the reservoir cooler.
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