TM 55-1520-240-23-6
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UTILITY HYDRAULIC SYSTEM
(Continued)
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THEORY OF OPERATION
Pressure to operate all utility subsystems comes through
the pressure control module. Input to this module is from
the utility pump, the APU motorpump, or an external
ground power unit.
If the APU is operating, hydraulic power comes from
the motorpump. Output from the pump is 3,350
psi, to ensure enough pressure for engine starting.
If the rotors are turning, 3,000 psi hydraulic pressure
becomes available from the utility pump on the aft
transmission.
If engines and APU are shut down, 3,000 psi
hydraulic power can be obtained from an external
ground power unit through ground service panel
connections at the aft right side of the helicopter.
In the control module, pressure is routed through
separate ports to the individual subsystems. Solenoid
valves on the module allow pressure to individual
subsystems to be cut off and the subsystems isolated.
NO BREAK - WORK HARDER
Return fluid from each of the subsystems is routed to
a common return control module and through a return
filter. It then returns at a pressure of 60 to 90 psi to
the source (utility pump or APU motorpump), by way of
the reservoir cooler module. When a standard ground
power unit is connected to the utility system, the utility
reservoir dumps into the power units reservoir. System
operation is then conducted using the power units fluid
reservoir. The aircraft’s reservoir will then need to be
re-serviced prior to disconnecting the ground power unit.
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