TM 1-1520-238-T-5
Change 4
7–23
7–7.
SYSTEM DESCRIPTION (cont)
7–7
(c) During normal operation, pressurized air (30 5 psi) from the PAS manifold enters through the air
inlet check valve of the reservoir. The pressurized air acts upon the reservoir piston creating low pressure
hydraulic fluid and helps prevent pump cavitation. The low pressure relief valves open at 215 psi, relieving excess
trapped fluid and air. When the reservoir reaches the lowest permissible fluid level volume, the low level switch
closes and lights the pilot’s caution/warning panel OIL LOW PRI HYD indicator. The low pressure fluid is drawn by
pump suction to the primary hydraulic pump, pressurized to 3000 psi, and returned to the primary hydraulic
manifold. High pressure entering the manifold is sensed by the pressure switch. When pressure falls below 1250
psi, the pressure switch closes lighting the pilot’s PRI HYD PSI indicator and the CPG PRI HYD indicator on the
respective caution/warning panels. The pressurized fluid is filtered by a five micron filter. If the filter is partially
clogged, the differential pressure of 70 10 psi is sensed by an electrical switch in the dirty filter indicator. The
switch closes, causing the OIL BYP PRI HYD indicator on the pilot’s caution/warning panel to light and a pop–up
visual indicator on the primary hydraulic manifold to extend (the pressure filter has no bypass capabilities). Fluid is
isolated from GSE ports during normal operation by the GSE check valve. A high pressure relief valve returns
excessive pressure to the return side of the primary hydraulic manifold. Prior to output to the flight control
servocylinders, the fluid pressure is monitored by the pressure transducer. The pressure transducer provides
signals to the PRI side of the pilot dual hydraulic pressure indicator. Fluid returning from the flight control
servocylinders is filtered by a five micron filter. If the filter is partially clogged, differential pressure of 70 10 psi is
sensed by an electrical switch in the dirty filter indicator. The switch closes, causing the OIL BYP PRI HYD
indicator on the pilot’s caution/warning panel to light, and extends a pop–up visual indicator in the primary
hydraulic manifold. At a differential pressure of 100 15 psi, a bypass valve opens and allows fluid to be routed
around the return filter to the primary hydraulic manifold reservoir and the primary hydraulic pump.
(d) During GSE operation, an AGPU sends high pressure fluid to the GSE pressure port. A flow limiting
orifice restricts the flow to a maximum flow of eight gpm at 3000 psi. The back pressure from the GSE return
isolation valve piston forces the GSE check valve open. The GSE return isolation valve opens the reserve return
system to the GSE return port so that hydraulic fluid flows evenly out. A pressure bleed orifice between the GSE
pressure and return ports prevents hydraulic lock of the GSE return isolation valve. Hydraulic lock prevents the
GSE check valve from reseating. GSE pressure flows through the GSE check valve and through the normal and
return primary hydraulic manifold system. Normal operating leakage across the GSE check valve leaks back into
the return system through the GSE return isolation valve which prevents pressure build up and actuation of the
isolation piston.
