LANDING GEAR AND
BRAKES
AIRPLANE
OPERATIONS
MANUAL
2-12-10
Page
4
Code
01
REVISION 40
AOM-145/1114
ANTI-SKID PROTECTION
The anti-skid protection controls the amount of hydraulic pressure
applied by the pilots on the brakes. The anti-skid provides the
maximum allowable braking effort for the runaway surface in use. It
minimizes tire wear, optimizes braking distance, and prevents
skidding.
To perform this function, the BCU computes the wheel speed signal
from the four speed transducers.
BCU monitors constantly wheels velocity through wheel speed
transducers. If a skid occurs, then wheel velocity decreases suddenly
compared with a calculated deceleration ratio proportional of the pedal
deflection, then BCU will command brake pressure release until the
wheel velocity returns to the expected one. This anti-skid protection is
monitored and controlled for each wheel, independently.
When one of such signals decreases below the average of the
remaining wheels, a skidding is probably occurring, and braking
pressure has to be relieved. After that wheel speed has returned to the
average speed, normal braking operation is restored.
The anti-skid does not apply pressure on the brakes, but only relieves
it. So, to perform a differential braking technique, the pilot should
reduce pressure on the side opposite to the turn, instead of applying
pressure to the desired side.
The anti-skid system incorporates the locked wheel protection and
touchdown protection features.
LOCKED WHEEL PROTECTION
Locked wheel protection is activated for wheel speeds above 30 kt. It
compares wheel speeds signals. If one wheel speed is 30% lower than
that of another, a full brake pressure relief is commanded to the
associated wheel, allowing wheel speed recovery. The 30% tolerance
between the wheel speeds is provided to permit an amount of
differential braking, for steering purposes.
For wheel speeds below 30 kt, the locked wheel protection is
deactivated and the brake system actuates without the wheel speed
comparator. For wheel speeds below 10 kt, the anti-skid protection is
deactivated, allowing the pilot to lock and pivot on a wheel.
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