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wheels chocked prior to releasing brakes.
PARKING BRAKE
The parking brake handle is labeled PARKING BRAKE and is located
on the pedestal below the thrust levers. The handle is mechanically
connected to the parking brake valve through which all pressure from
the primary brake system must pass. The parking brake system is actuated
by pressing and holding the toe brakes (hydraulic system pressurized)
then pulling the parking brake handle which closes the parking
brake valve, thereby locking pressure against the wheel brakes. Returning
the parking brake handle to the off (in) position releases the brakes.
An amberPARK BRAKE annunciator is installed on the pilot's instrument
panel which illuminates (BATTERY Switches On) whenever the
parking brake is applied.
3-12
PM-121
Change 3
Learjet 31A
Pilot's Manual
EMERGENCY BRAKING
In the event of a main hydraulic system failure, the wheel brakes can be
applied pneumatically. Emergency (pneumatic) braking is initiated and
controlled through the red EMERG BRAKE handle located on the pedestal
to the left of the thrust levers . Emergency braking is initiated by
pulling the handle out of the recess and pushing down on the handle .
As the EMERG BRAKE handle is pushed down, air pressure from the
emergency air bottle is directed to the wheel brake shuttle valves
through the lever actuated emergency brake valve. If the emergency air
pressure is greater than the hydraulic system pressure, the wheel brake
shuttle valves will reposition to admit air pressure to apply the brakes.
As the brake handle is released, excess air will be vented overboard and
the brakes will release. Because the emergency air lines are plumbed
into the hydraulic brake system between the anti-skid control valves
and the wheel brakes, anti-skid protection is not available when using
emergency brakes . Also, the parking brake will be inoperative when
using emergency air pressure.
PM-121
3-13
Change 3
Pilot's Manual
Learjet 31A
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3-14
PM-121
Change 3
Learjet 31A
Pilot's Manual
ANTI-SKID SYSTEM
An anti-skid system is integrated into the hydraulic brake system to
provide even braking under all runway surface conditions without
skidding the tires. The system consists of the ANTI-SKID control
switch, anti-skid control box, two anti-skid control valves, monitoring
lights, four wheel-speed transducers (one in each main wheel axle), and
associated aircraft wiring. Each anti-skid control valve is a dual unit capable
of individually modulating brake pressure for both associated
brakes . As the transducers are driven by the main wheels, a frequency
proportional to the wheel speed is induced and forwarded to the control
box. The control box converts the wheel-speed frequency to an analog
signal and compares the analog to a reference representing a near
optimum slip ratio between aircraft velocity and wheel velocity to produce
the maximumbraking coefficient. Should the wheel speed deviate
from the optimum slip rate, the control box will signal the affected
wheel's control valve to reduce braking pressure on the affected wheel.
Braking pressure is reduced bybypassing some of the hydraulic system
pressure into a return line by means of a servo controlled valve in the
control valve. As the wheel speed increases, normal braking pressure is
restored . To ensure full manual control of the hydraulic braking system
and to prevent pressure loss when the parking brake is set, a solenoidoperated
shutoff valve at each control valve return port is deenergized
closed when the ANTI-SKID switch is OFF or the parking brake is set.
Electrical power for the anti-skid system control circuits is 28 VDC supplied
through the 7.5-amp ANTI-SKID circuit breaker on the copilot's
circuit breaker panel.
ANTI-SKID LIGHTS
Four red ANTI-SKID lights on the pilot's instrument panel provide a
continuous cockpit indication of anti-skid system control circuit condition.
The two lights labeled L represent control circuits for the left main
gear brakes and the two lights labeled R represent control circuits for
the right main gear brakes . The anti-skid control box continuously
monitors the system circuits and will illuminate the applicable light(s)
should any of the following conditions arise: loss of input power, open
or shorted transducer circuits, open or shorted control valve circuits,
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