Description (AMM 32-42-00)

 

A- The antiskid system is an electro-hydraulic subsystem placed in the hydraulic brake system to prevent manual or automatic braking action that causes skidding. Under any runway condition or selected brake pressure, the antiskid system will monitor wheel deceleration rates and maintain maximum wheel adhesion. The antiskid system is programmed to provide touchdown and locked wheel protection from air-ground and wheel speed sensing’s.

 

A landing gear lever operated switch provides a disable function for landing gear retraction braking. An ON-OFF switch gives the crew control of the antiskid system and a annunciator light gives notice of defective systems. A built-in-test capability provides continuous self test and fault indication. Further manual testing is provided to detect component faults. The auto brake system shares air-ground and wheel speed sensing’s with the antiskid system and requires a fault free antiskid system to operate.

 

B- An alternate antiskid system is automatically scheduled to operate from hydraulic system A pressure when hydraulic system B pressure fails, transferring command by alternate antiskid to the operation of paired wheels of each gear.

 

C- The antiskid system consists of individual wheel speed transducers, an antiskid control unit, six antiskid control valves, a main gear squat switch, a nose gear squat switch and a landing gear lever switch (Fig. 1). The system also includes an ON-OFF control switch and an INOP annunciator light. Built-in-testing is provided on an automatic/continuous basis supplemented.

 

Operation (AMM 32-42-00)

 

A- Antiskid Control Switches

(1) A guarded switch on the P2 flight deck center panel provides both inboard and outboard power for the antiskid unit. If the switch is turned off the antiskid system becomes inoperative, and power from the ANTI-SKID FAIL circuit breaker on the P6 panel through the landing gear lever up switch and fail annunciator logic in the antiskid unit will turn on the ANTISKID INOP light on the P2 panel.

 

B- Normal Antiskid Mode

(1) Wheel speed transducers, one driven by each wheel, produce electrical frequency outputs which are proportional to wheel speed. The signals are processed by individual wheel circuits in the antiskid control unit to control brake pressure to the applicable wheel. The transducer frequency output signal is processed on the wheel circuit card and converted to an analog dc voltage proportional in amplitude to the ac frequency.

 

(2) The analog signal is compared to a reference velocity loop circuit which provides a commanded wheel velocity based on average aircraft deceleration. A velocity comparator provides a velocity error voltage. Maximum braking occurs when this error is minimized.

 

(3) A modulation function is provided to give smooth control for normal brake pressure correction. The rate at which this changes is based on velocity error.

 

(4) A transient signal control reacts to greater than normal wheel speed changes to provide rapid recovery from skids. Included in control response are lead signals to compensate for hydraulic and brake phase lag.

 

(5) A summing amplifier sums the modulation function, transient signal control and lead signal to provide a current output. This current commands the function of the antiskid control valve.

 

C- Touchdown Protection Mode (Fig. 3)

(1) Touchdown protection prevents brake applications prior to wheel spin-up on slick pavements or with light wheel vertical loading. A brake release signal is sent to the inboard wheel pair from the right main gear air-ground sensor and to the outboard brakes from the nose gear ground sensing switch. At ground sensing, the switches send signals through relay R278 (inboard wheels) and relay R321 (outboard wheels) which are located on the E11 shelf.

 

Circuitry within the antiskid control unit delays recognition of ground sensing for three seconds to insure continued touchdown protection immediately following touchdown. Wheel spin up inputs override the air safety brake release inputs to allow brake application when wheel velocity reaches a preset level, even though the air safety relay may still be in the airborne mode.

 

 

D- Locked Wheel Protection Mode

(1) Locked wheel protection is provided by the antiskid control unit from a comparison of left and right inboard and left and right outboard wheel speed signals respectively. The locked wheel error signal will release the brake on the slower wheel when its speed drops to less than 40 percent of the faster wheel.

 

E. Antiskid Control Valve (Fig. 2)

(1) The antiskid control valve torque motor (solenoid) reacts to signals from the antiskid control unit. A flapper pivots between the torque motor and a magnet and covers either a hydraulic return or jet pressure port. The flapper is spring biased to cover the return port so that pressure introduced through the jet pressure port will pressurize the cavity. The cavity is ported to drive a spool valve.

 

Added to this pressure is a spring force normally biasing the valve to allow metered pressure from auto brake or manual application to pass to the brake cylinders. (Should electrical signals fail, the spool valve spring biases the valve to be normally open. Auto brake would be faulted to disarm, but manual metered pressure would pass freely to and from the brakes.)

 

A feedback chamber is opposite the spool valve bias spring and fills with brake pressure, stabilizing the spool valve to a pressure hold position. If there is no change in metered pressure and a skid is sensed, an electrical signal will cause the flapper to restrict the jet pressure port and uncover the hydraulic return port allowing the cavity pressure to release.

 

This causes the feedback chamber pressure to drive the spool valve and port brake pressure to return for release of brakes. At the return of wheel speed, the flapper is electrically caused to cover the return port and cavity pressure covers the spool valve to again port pressure to the brakes. Brake pressure is quickly relieved through a check valve into metered pressure lines at the release of metered pressure.