Antiskid System

The antiskid system consists of 3 basic elements

  • Antiskid Detectors.
    Wheel speed sensors/transducers in each
    main wheel axle.
  • Antiskid Control Shield.
    A transistorized computer in the left main
    wheel well.
  •  4 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. So the supplied signal is 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 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 inflight arming circuit through both LH and RH Safety Switches. By that all brakes are released.
If the landing gear is left down after takeoff, normally release indications 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 to the antiskid (hydraulic pressure control valves. Brake pressure is corrected in 2 different ways:

  1. SKID (deceleration rate control if a sudden deceleration occurs due to a skidding wheel, a signal is sent to the antiskid valve. By that the hydraulic brake pressure to that skidding wheel is reduced in proportion to the appeared skid. As the wheel speed recovers, partial up to full metered pressure is applied to the brake until another skid causes a pressure reduction. Hereby optimum hydraulic brake pressure is attained. There will be no REL indication as skid occurs.
  2. Locked Wheel Control All inboard wheel detectors record to the inboard memory circuit, all outboard wheel detectors to the outboard memory circuit. Only if at least one wheel from a memory circuit speeds ~ 17,5 mph, the locked wheel control will be active. If one or more wheels tend to lock, a full strength signal is applied to the antiskid valve(s), fully releasing the brake until wheel spin up. If all wheels from a memory circuit (inboard resp. outboard) report a wheel speed below 17,5 mph there will be no brake release due to locked wheel control (for instance: no aircraft movement). However, the skid control is still 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, so that they share common pressure – and return lines, but operate independently. Valves of this type have a 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 towards INBD an AC-Signal will be introduced into the antiskid control system, simulating the inboard wheels are rotating. Simultaneously the inboard memory circuit is connected with the outboard memory circuit.
Comparison in the control shield results in sensing “locked” outboard wheels. So both OUTBD-indicators display REL on ground. In flight with gear down and locked the same action will make brakes available on the “rotating” inboard wheels, resulting in same indication namely: INBD’s display blank and OUTBD’s display REL. Displays reciprocate with test switch towards OUTBD.

When the parking brake is set, the antiskid valve will be deactivated. In this case a REL indication will not result in a release of the corresponding brakes.
CAUTION: DO NOT OPERATE THE ANTISKID SYSTEM TEST SWITCH DURING TAXI, TAKEOFF AND LANDING ROLL.