Operation
Pushing the emergency gear extension lever
down opens a valve to release air bottle pressure
to position the gear control and door control
valves to the extend position (Figure 14-10).
This provides a return flow path for fluid in the
retract side of the gear and door actuators.
The air pressure repositions the shuttle
valves, which:
• Release the nose gear uplatch and the
main gear door uplatches
• Open the main gear inboard doors
• Extend all three gear
Since no provision is made to close the main
inboard doors, the two main gear red UNSAFE
lights will remain illuminated. The three green
LOCKED DOWN lights will illuminate.
After the gear is down and locked (provided
there is no hydraulic pressure indicated), air
pressure can be relieved in the gear system by
pulling an adjacent metal tab while lifting the
emergency gear extension lever to the normal
position. This closes the valve on the emergency
air bottle and isolates the remaining air pressure
from the gear system, preventing a possible leak
in the gear system from depleting air pressure
that might be required for emergency braking.
Attempting to retract the landing gear after using
the emergency extension system may cause
excessive air pressure to be introduced into the
hydraulic system return lines, thereby rupturing
the reservoir.
BRAKES
GENERAL
The brake system is powered by hydraulic sys-
tem pressure from the nose gear down (extend)
line. The brakes can be applied by either pilot.
The system has four multidisc, self-adjusting
brake assemblies, one for each main gear wheel,
operated by power brake valves linked to the top
section of the rudder pedals. Braking force is in
direct proportion to pedal application unless the
antiskid system is activated. The antiskid system
permits stopping in the shortest possible distance
for any given runway condition. Parking brakes
can be set by pulling a handle located on the left
side of the throttle quadrant. A pneumatic emer-
gency brake system is used to stop the airplane if
hydraulic pressure is lost. Neither antiskid pro-
tection nor differential braking is available dur-
ing emergency braking.
The brake assemblies are designed to stop for-
ward movement of the airplane. If the brakes are
applied with the airplane rolling backward, the
brakes may be damaged.
NORMAL OPERATION
When either pilot depresses both brake pedals,
the two associated brake valves meter hydraulic
system pressure (from the nose gear down line)
through shuttle valves (one in each main pres-
sure line), parking brake valves, antiskid valves,
brake fuses, and a second set of shuttle valves,
one for each set of four brake assemblies
(Figures 14-11 and 14-12). The pilot applying
the most pressure has control of the brakes.
Pistons in each brake assembly move a pressure
plate, forcing the stationary and rotating discs
together against a backing plate to produce the
braking action. Depressing one pedal applies
both brakes on the corresponding main gear;
therefore, differential braking is available,
if required.
Releasing pedal pressure repositions the brake
valve, and springs in the brake assembly force
fluid back through the brake valves to the reser-
voir, thereby releasing the brakes.
During gear retraction, a restrictor in the return
line creates back pressure on the brakes, which
is sufficient to stop the wheels from rotating
prior to their entering the wheel well.
When taxiing through slush or snow, frequent
brake application creates friction heat, which
may prevent the brakes from freezing.
14-10
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international