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(1)
There are two brake metering valve module assemblies, one in the left wheel well, and one in the right wheel well that provide metered hydraulic actuation of the main landing gear wheel brakes.
(2)
Each module consists of a dual brake metering valve assembly (which includes a wheel brake actuator), valve support structure, quadrant and linkage to transfer input from brake pedal operated cables to the metering valves.
(3)
The dual brake metering valve (Fig. 2) functions to control hydraulic pressure to the brakes as a function of input deflection. There are two metering valves and a brake despin actuator in the assembly. Each of the two metering valves operate in a different hydraulic system. One valve operates in the normal hydraulic system and one in the alternate systems. Only one of the metering valves is pressurised at a time. The valves are spring-loaded to the brakes off position when there is no input.
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NORMAL VALVE
RETURN PORT
ALTERNATE VALVE
RETURN PORT
DE-SPIN
ACTUATOR
MOUNTING
ALTERNATE FACE
METERING VALVE NORMAL VALVE BRAKE PORT
NORMAL NORMAL VALVEMETERING PRESSUREVALVE
LOWER INPUT FACE SHAFT PORT
METERING VALVE ASSEMBLY
RETURN SYSTEM
SPRING INPUT PRESSURE PRESSURE CHAMBER FEEDBACK
HIGH
PRESSURE
STOP
LOW PRESSUREVALVE STOPINPUT FORCE OUTER FEEDBACKVALVE INPUT PISTONDISPLACEMENTINNERMETERING
DASHPOT FEEDBACKSPOOL
FOR PISTONMETERED DAMPING
FEEDBACK FORCE FEEDBACK SPRING METERING VALVE OPERATIONAL SCHEMATIC (NORMAL/ALTERNATE)
PRESSURE
Brake Metering Valve Assembly Figure 2
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(4)
The valves are positioned to meter brake pressure to the normal or alternate system by the brake pedals using a cable system and quadrant on the module to operate the metering valve input shaft through a linkage connection. The valves are spring-loaded to the brakes off position when there is no input. The metering valve spool on both actuators is displaced by individual cranks on the common input shaft to meter system pressure to the brakes. Metered pressure is routed to the brakes and internally to two feedback pistons and a feedback spring in the valve to provide variable metered pressure as a function of pedal travel. The valve operates in the low gain mode for pedal deflections up to approximately 5 degrees from the off position. In this mode, the amount of metered brake pressure increases slowly with each degree of pedal travel for optimal control during taxi maneuvering. At pedal deflections greater than 5 degrees the valve operates in the high gain mode. Here, the metered brake pressure increases rapidly with each degree of pedal travel for swift application of brake force during landing roll.
(5)
The wheel brake actuator, when pressurized by gear retraction operation, will provide input to the alternate metering valve spool to stop the wheels from spinning after gear retraction.
D.
Brake Accumulator (Fig. 1, Detail D)
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(1)
The brake accumulator provides a passive pressure source for approximately 1 to 2 brake applications when left and right hydraulic systems are not available. The accumulator is also used to extend parking brake holding time (Ref 32-44-00) and to stabilize the brake system supply pressure during initial brake application and any subsequent antiskid cycling.
(2)
The accumulator is automatically charged by the right hydraulic system and the charge is maintained by check valve isolation of the supply line to prevent loss of pressure when the right system is depressurized.
(3)
The accumulator is located in the right wheel well on the main fore-aft bulkhead.
E.
Alternate Brake Selector Valve (ABSV) (Fig. 1, Detail E)
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(1)
The ABSV is located on the right wheel well forward bulkhead.
(2)
The ABSV isolates left hydraulic system source from the alternate brake system. When the right hydraulic (normal) system source is depressurized, the ABSV cycles to allow the left hydraulic (alternate) system source to provide brake pressurization. ABSV value cycling takes place when control pressure drops to approximately 48 percent or less in the normal pressurized system.
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