The antiskid system consists of three basic elements:
- Antiskid Detectors
Wheel speed sensors/transducers in each main wheel axle. - Antiskid Control Shield
A transistorized computer located in the left main wheel well. - Four Dual Antiskid Valves
Hydraulic pressure control valves, one for each wheel. Located in the main wheel wells, above the lockout deboost valves.
Antiskid Detector
The wheel speed is transduced by means of a rotor and a magnetic stator. The supplied signal is therefore proportional to the wheel speed. This signal is fed to the first stage of a control circuit, one for each wheel.
Control Shield
With the Antiskid Control Switch in the DN position, electrical power is available to the Antiskid Control System if the green left and right MLG lights are on.
In-flight arming: Prior to touchdown, power is supplied to the in-flight arming circuit through both LH and RH safety switches. This releases all brakes.
If the landing gear is left down after takeoff, release indications normally appear after approximately 90 seconds. An early release indication, for instance after 20 seconds, points to a scraping brake.
The control shield interprets signals from the antiskid detectors and produces signals for the antiskid hydraulic pressure control valves. Brake pressure is corrected in two different ways:
- SKID — Deceleration Rate Control
If a sudden deceleration occurs due to a skidding wheel, a signal is sent to the antiskid valve. The hydraulic brake pressure to that skidding wheel is reduced in proportion to the detected skid.As the wheel speed recovers, partial or full metered pressure is applied to the brake until another skid causes a pressure reduction. In this way, optimum hydraulic brake pressure is attained.There will be no REL indication when a skid occurs. - Locked Wheel Control
All inboard wheel detectors feed the inboard memory circuit, and all outboard wheel detectors feed the outboard memory circuit.The locked wheel control becomes active only if at least one wheel from a memory circuit reaches approximately 17.5 mph.If one or more wheels tend to lock, a full-strength signal is applied to the antiskid valve(s), fully releasing the brake until the wheel spins up again.If all wheels from a memory circuit (inboard or outboard) report a wheel speed below 17.5 mph, there will be no brake release due to locked wheel control. This applies, for instance, when the aircraft is not moving. However, the skid control remains active.
Dual Antiskid Valve
The valve is a pressure-control servo, spring-loaded in the “pressure to brake” position.
In the interest of weight saving, two units are housed in a single body. They therefore share common pressure and return lines but operate independently.
Valves of this type have relatively high internal leakage. For this reason, a return-line shutoff solenoid is included in the same body.
Switching the antiskid system OFF or setting the parking brakes cuts power to the solenoid, which then blocks the return line. This allows the airplane to be parked using accumulator energy.
Antiskid System Test Switch
Placing the test switch toward INBD introduces an AC signal into the antiskid control system, simulating the inboard wheels as rotating.
Simultaneously, the inboard memory circuit is connected to the outboard memory circuit. Comparison in the control shield then results in the outboard wheels being sensed as “locked.”
Consequently, both OUTBD indicators display REL on the ground.
In flight, with the gear down and locked, the same action makes the brakes available on the “rotating” inboard wheels, resulting in the same indication: the INBD indicators display blank and the OUTBD indicators display REL.
The indications are reversed when the test switch is placed toward OUTBD.
When the parking brake is set, the antiskid valve is deactivated. In this case, a REL indication will not result in the release of the corresponding brakes.
CAUTION: DO NOT OPERATE THE ANTISKID SYSTEM TEST SWITCH DURING TAXI, TAKEOFF, OR LANDING ROLL.