14-17
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
Engage steer lock mode by momentary actuation
of the STEER LOCK switch. Depressing either
wheel master switch disengages steer lock mode.
With steering engaged in either mode, the green
STEER ON light illuminates. Rudder pedal
movement provides the displacement and direc-
tional signals to the computer-amplifier. Signals
from the wheel speed transducers provide
groundspeed information. The computer-ampli-
fier applies a signal to the steering actuator in
the appropriate direction until it is stopped by a
signal from a followup located in the drive gear-
box. In the event the steering system is inopera-
tive, differential power and braking can be used
to taxi the airplane.
MODEL 23
DIFFERENCES
LANDING GEAR
Indication
On models 23-003 through 23-009, the landing
gear control panel located on the pilot’s instru-
ment panel has one red UNSAFE light instead of
three (Figure 14-15).
If the UNSAFE light remains on at the comple-
tion of the retract cycle, one or both main gear
inboard doors are not latched or the nose gear is
not locked.
With the nose gear up and locked and the main
gear inboard door latched, all lights on the panel
are out. With all three gear down and locked, the
green LOCKED DOWN lights will be on and
the UNSAFE light out.
A warning horn sounds if the thrust levers are
retarded below 70% rpm with the gear up. The
TEST–MUTE switch positioned to MUTE will
silence the horn, but the UNSAFE light will
remain on.
After an emergency extension, the three green
LOCKED DOWN lights will illuminate, and the
red UNSAFE light will be off.
Priority and Boost Valves
On SNs 23-003 through 23-025, a priority valve
and a pressure boost valve are installed in the
nose gear retract line. The priority valve pro-
vides instantaneous pressure to the pressure
boost valve, which, in turn, increases pressure to
accelerate nose gear retraction.
Gear Control Valve
The gear control valve is a four-way, two-
position, solenoid controlled valve. The valve,
electrically controlled by the gear selector
switch on SNs 23-003 through 23-079 and the
door-down microswitches on all other Learjets,
directs hydraulic fluid to retract or extend the
landing gear. A pneumatic cylinder is provided
on the valve for positioning to “gear down”
when using the pneumatic extension system.
Door Control Valve (SNs 23-080
and Subsequent)
The gear door control valve is a solenoid-operat-
ed valve controlled by the gear selector switch
and the gear down-and-locked or gear-up
switches. It directs pressure to close the main
gear inboard doors after extension or retraction.
Figure 14-15. Landing Gear Control Panel
(Model 23 Airplanes)
