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pass through a portion of the valve and on to the utility
side of the tandem servo actuators.
(4) Override Solenoid Valve. Normally de-energized
closed, permits crew management of the accumulator
reserve pressure. When the pilot or CPG
places his EMER HYD switch ON, the override solenoid
valve will energize open and accumulator fluid
will pass to the accumulator isolation valve via emergency
routing. In this case, another portion of the accumulator
isolation valve will permit accumulator fluid to
flow to the utility side of the servo actuators. The
EMER HYD switch is powered through the EMERG
HYD circuit breaker on the pilot overhead circuit
breaker panel.
(5) Accumulator Hydraulic Pressure Transducer.
Located in the manifold will provide the pilot with
a continual indication of accumulator pressure on the
accumulator hydraulic pressure indicator. During normal
operation, the indicated pressure will be the same
as that of the utility hydraulic system.
TM 1-1520-238-10
M01-089A
Figure 2-30. Utility Hydraulic System
Change 4 2-53
TM 1-1520-238-10
(6) Rotor Brake Solenoid Valves. Valves are
controlled by a three-position switch OFF, BRAKE,
and LOCK on the pilot PWR lever quadrant adjacent
to the PWR levers (fig 2-19). When the switch is positioned
to BRAKE, utility system pressure regulated to
337 psi is applied to stop the rotor brake disc on the
main transmission. When positioned to LOCK, the
brake off solenoid valve traps 3000 psi pressure between
the manifold and the rotor brake actuator.
b. Utility Accumulator. The accumulator is a multi-
purpose installation unique to the utility hydraulic
system. It is located on the right side of the helicopter
directly beneath the APU. It stores hydraulic fluid at
3000 psi. The accumulator is charged by nitrogen gas.
A gas storage tank supplies the charge and is serviced
through a charging port (fig 2-31). The utility hydraulic
manifold pressure is used for rotor brake application,
APU starting, and emergency use of flight control operation.
2.38.3 Hand Pump. A hand pump is installed next to
the primary system GSE panel on the right side of the
helicopter. The pump provides one method of charging
the fluid pressure in the utility accumulator as well as
a method for the ground crew to fill both the primary
and utility reservoirs. Low pressure fluid entering the
fill port is filtered by a 45 micron screen filter (before
MWO 1-1520-238-50-52) or a 5 micron cartridge filter
(after MWO 1-1520-238-50-52). A lever may be moved
to any of three positions. This, in turn, will open one of
three mechanically operated check valves to the accumulator
or to either reservoir. Section XV contains
servicing instructions.
2.39 PRESSURIZED AIR SYSTEM (PAS).
The PAS cleans, pressurizes, regulates, and distributes
air to the air turbine starters, fuel boost pump, fuel
transfer pump, external fuel tanks, hydraulic reservoirs,
heat exchangers, defog nozzles, engine cooling
louver actuators, utility receptacle, ice detector sensor,
and environmental control unit (ENCU). The pressurized
air system has three sources of air: The primary
source is the shaft driven compressor; the other sources
are bleed air from the No. 1 engine and from an external
air source.
Figure 2-31. Utility Manifold
2-54 Change 4
TM 1-1520-238-10
2.39.1 Shaft Driven Compressor (SDC). The SDC is
mounted on the main transmission accessory case and
uses outside air. The air passes through a particle separator
and then through a butterfly-type throttle valve.
This valve is closed by a time-delay solenoid when the
APU START switch is engaged. The valve remains
closed for 60 seconds after the APU START switch is
released. This reduces APU loads during starting. The
shaft driven compressor will compress air at 3:1 ratio
and route it through a surge valve. This valve, mounted
on top of the SDC, will open or close as necessary to
keep SDC air pressure constant with variable system
demand. Pressurized air then passes through a springoperated
check valve and into the PAS manifold. This
check valve prevents pressurized air, from sources other
than the SDC, from reverse routing back to the SDC.
The SDC is lubricated by the main transmission accessory
oil pump and has its own scavenge. If the SDC oil
temperature exceeds 182 °C (360 °F), or if the SDC
fails, the pilot SHAFT DRIVEN COMP caution light
illuminates.
2.39.2 No. 1 Engine Bleed Air. The No. 1 engine provides
an alternative source of pressurized air in case of
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