Learjet 20 Series Pilot Training Manual page 285

Directional Control Valves (SNs
23-003 through 23-079)
A rotary three-position, four-way directional
control valve is installed in the hydraulic pres-
sure line to each main landing gear. The purpose
of the valves is to close the main gear inboard
doors after the landing gear has extended or
retracted. The valves are repositioned by
a mechanical linkage connected to the main
gear struts.
Operation (SNs 23-003 through
23-079)
Main Gear-Up Cycle
Main hydraulic system pressure enters the landing
gear control valve where it is directed into the up
lines. A 1,100- to 1,150-psi priority valve,
installed in the landing gear actuator retract line,
diverts the initial hydraulic fluid through the door
directional control valves to the main gear inboard
door actuators and to the door uplock actuators.
Pressure unlatches the door uplocks and opens the
inboard gear doors. When the lock actuators and
door operating cylinders "bottom out," the pres-
sure rises above 1,100 psi, the priority valves open,
allowing fluid to enter the retract side of the main
gear actuators, and the main gear retracts. During
the last few degrees of main gear retraction,
mechanical linkage reverses the door directional
control valve and applies pressure to the close side
of the inboard gear door actuators which closes the
doors. The spring-loaded uplatch engages, com-
pleting the main landing gear-up cycle.
Main Gear-Down Cycle
Main hydraulic system pressure enters the landing
gear control valve and is directed into the landing
gear-down lines. Pressure is applied simultaneous-
ly to the uplock actuator, the door actuator, and the
14-18
LEARJET 20 SERIES PILOT TRAINING MANUAL
FOR TRAINING PURPOSES ONLY
gear actuator; however, the return fluid from the
gear actuator is restricted by an orifice. The uplock
opens, the inboard doors open, and the gear slowly
extends. During the last few degrees of main gear
travel, mechanical linkage reverses the position of
the directional control valves. Pressure is applied
to the up side of the inboard door actuators and the
doors close. The downlock in the actuator engages
by spring tension, completing the main landing
gear-down cycle.
Operation (SNs 23-003 through
23-025)
Nose Gear-Up Cycle
Main hydraulic system pressure enters the land-
ing gear control valve and is directed through a
priority valve to the nose gear boost valve. The
boost valve increases the pressure from 1,500 psi
to approximately 2,340 psi to the up side of the
nose gear actuator. The nose gear retracts and
closes the nose gear doors. The spring-loaded
uplock engages to complete the nose gear-up
cycle. Subsequent airplanes do not incorporate a
priority valve and boost valve in the nose gear
retract lines.
Emergency Extension
On SNs 23-003 through 23-024, emergency
extension is initiated with the emergency gear
and brake control handle on the right side of
the pedestal. Pushing the handle down applies
air to the brakes and also opens a valve that
directs air pressure to reposition shuttle valves
and extend the landing gear. On SNs 23-003
through 23-024 with SK 23/24-43, 23-025 and
subsequent, emergency extension is initiated
with an emergency extension lever on the left
side of the pedestal.
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