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hydraulic fluid contamination and incorporate bypass valves
which will open in the event a filter becomes clogged. An auxiliary hydraulic
pump is installed to provide system pressure in the event of a
malfunction or during engine-off ground operations .
Two motor-driven firewall shutoff valves can stop hydraulic fluid flow
to the engine-driven hydraulic pumps in the event of an emergency or
engine fire. Each shutoff valve is operated by the corresponding ENG
FIRE PULL T-handle on the glareshield. (Refer to ENGINE FIRE EXTINGUISHING.)
The valves operate on 28 VDC supplied through the
7.5 amp Land R FW SOV circuit breakers on the pilot's and copilot's
circuit breaker panels respectively. Loss of power to the shutoff valves
causes the valves to remain in their last position . The firewall shutoff
valves are operative during EMER BUSmode.
The system is serviced through ground service quick-disconnects located
in the tailcone. The service access includes quick-disconnect ports
for pressure and return lines, an air valve for accumulator charging,
and a direct-reading accumulator pressure gage.
PM-121
3-1
Change 3
Pilot's Manual
Learjet 31A
HYD PUMP SWITCH
The auxiliary hydraulic pump is controlled by the HYD PUMP switch
located on the center switch panel. When the switch is placed in theOn
(HYD PUMP) position, the auxiliary hydraulic pump is cycled by a
pressure sensing switch plumbed into the high-pressure side of the system.
The pressure switch will energize the auxiliary hydraulic pump if
system pressure drops below approximately 1000 psi (6895 kPa) and
then deenergizes the pump when system pressure rises above approximately
1125 psi (7757 kPa). The auxiliary hydraulic pump operates on
28 VDC supplied through a 30-amp current limiter. The auxiliary hydraulic
pump is operative during EMER BUS mode.
LO HYD PRESS LIGHT
Illumination of the amber LO HYD PRESS light on the glareshield annunciator
panel indicates low hydraulic system pressure. The light is
operated by the same pressure switch that controls the auxiliary hydraulic
pump operation. The pressure switch will illuminate the light
when hydraulic system pressure drops below approximately 1000 psi
(6895 kPa) and extinguish the light when hydraulic system pressure rises
above approximately 1125 psi (7757 kPa).
HYDRAULIC PRESSURE INDICATOR
The HYD PRESS indicator is a vertical scale instrument incorporated
into a combination HYD PRESS/EMERG AIRunit. The unit is located
on the center switch panel adjacent to theHYD PUMP switch. The indicator
face consists of a vertical scale marked from 0 to 2500 psi in 500
psi increments and a pointer at the left margin of the instrument. The
instrument is operated by a pressure transducer plumbed to the highpressure
side of the hydraulic system. The indicator operates on 28
VDC supplied through the 1-amp HYD PRESS IIvD circuit breaker on
the copilot's circuit breaker panel. Refer to Airplane Flight Manual for
instrument limit markings.
3-2
PM-121
Change 3
Learjet 31A
Pilot's Manual
HYDRAULIC SYSTEM SCHEMATIC
Figure 3-1
PM-121
3-3
Change l
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Pilot's Manual
Learjet 31A
EMERGENCY AIR SYSTEM
An emergency air bottle (3000 psi [20,685 kPa]) is installed in the nose
compartment to provide alternate gear extension and emergency braking
in the event of an electrical or hydraulic system failure. The bottle
provides air pressure to operate the alternate gear extension blow
down system and the emergency brakes. Refer to LANDING GEAR
ALTERNATE EXTENSION and EMERGENCY BRAKES for system operation
.
EMERGENCY AIR PRESSURE INDICATOR
The EMERG AIR pressure indicator is a vertical scale instrument incorporated
into a combination HYD PRESS/EMERG AIR unit. The unit is
located on the center switch panel adjacent to the HYD PiJMP switch.
The indicator face consists of a vertical scale marked from 0 to 4000 psi
in 1000 psi increments and a pointer at the right margin of the instrument.
The instrument is operated by a pressure transducer plumbed to
the emergency air bottle . The indicator operates on 28 VDC supplied
through the 1-amp AIR PRESS 11VD circuit breaker on the copilot's circuit
breaker panel. If air bottle is serviced near the high end of yellow
segment, pressure may drop during flight as system cools; satisfactory
gear extension and braking can still be expected. Refer to Airplane
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