(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 spinup 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. Locked wheel drop out will occur at approximately 20 knots.

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 ALL  ú ú 01 Page 10 ú Jun 25/85
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 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 autobrake or manual application to pass to the brake cylinders. (Should electrical signals fail, the spool valve spring biases the valve to be normally open. Autobrake 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.
 10. Alternate Antiskid (Fig. 3)__________________
 A.  The alternate source select valve automatically transfers hydraulic system A pressure into the alternate brake system lines when hydraulic system B pressure fails. Switch S811, located on the alternate source select valve, will sense A system hydraulic pressure when it is applied to the alternate brake metering valves and will send a signal to the antiskid control unit to enable the indication of alternate antiskid faults should any occur.
 B.  Circuits to each of the two alternate antiskid control valves compare wheel speeds of each main gear paired wheels. The higher of the two signals is issued to the alternate antiskid control valve to control antiskid action of both wheels.
 C.  In the alternate mode the ANTISKID INOP light signals failure of the alternate antiskid system.
 11. Parking Brake Circuit (Fig. 3)_____________________
　
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