BRAKE HYDRO-MECHANICAL CONTROL -ALTERNATE SOURCE SELECTOR AND ACCUMULATOR ISOLATION VALVES
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BRAKE HYDRO-MECHANICAL CONTROL -BRAKE SOURCE SELECTOR VALVES -FUNCTIONAL DESCRIPTION
General
The alternate source selector valve and the accumulator isolation valve operate together to control pressure to the normal and alternate brake systems.
Valve Operation
These valves are slide-and-sleeve type valves. Control pressure on the control piston closes the valve to stop the pressure source from going through the valve.
When the control pressure decreases to 48 percent of the pressure source, the valve opens and permits the hydraulic fluid to go through the valve.
Brake Alternate Source Selector Valve (ASSV)
The right hydraulic system pressure is the control pressure for the ASSV.
When the right and center/reserve hydraulic system pressures are equal, the ASSV closes to depressurize the alternate brake system. In this position, the ASSV allows landing gear retract pressure to pressurize the alternate brake system during gear retraction.
When right hydraulic system pressure is low, the ASSV opens and permits center/reserve hydraulic system pressure to pressurize the alternate brake system.
Brake Accumulator Isolation Valve (AIV)
Center hydraulic system pressure downstream of the ASSV is the control pressure for the AIV.
When right hydraulic system pressure is low, center hydraulic system pressure through the ASSV closes the AIV to isolate brake accumulator pressure from the normal brake system.
When right hydraulic system and center hydraulic system pressures are low, brake accumulator pressure opens the AIV to pressurize the normal brake system with accumulator pressure.
Training Information Point
A pressure switch on the ASSV sends pressure signals to control the brake source light on the flight deck.
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BRAKE HYDRO-MECHANICAL CONTROL -BRAKE SOURCE SELECTOR VALVES -FUNCTIONAL DESCRIPTION
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BRAKE HYDRO-MECHANICAL CONTROL -BRAKE SOURCE PRESSURE SWITCHES
Purpose
These two brake source pressure switches control the brake source light and send pressure signals to the BSCU and AIMS for brake control and indication:
*
The normal brake hydraulic system pressure switch
*
The alternate brake hydraulic system pressure switch.
The brake source light comes on when there is low right hydraulic system pressure and there is low alternate brake system pressure.
Location
The brake source light is on the P1 left forward panel above the brake accumulator pressure gage on the flight deck.
The normal brake hydraulic system pressure switch is on the keel beam in the right main landing gear wheel well inboard and aft of the brake accumulator.
The alternate brake hydraulic system pressure switch is on the alternate source selector valve in the left main landing gear wheel well.
Physical Description
These switches are interchangeable pressure-operated switches that open at 1700 psi and close at 1200 psi.
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BRAKE HYDRO-MECHANICAL CONTROL -BRAKE SOURCE PRESSURE SWITCHES
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777 AIRCRAFT MAINTENANCE MANUAL
THIS PAGE IS INTENTIONALLY LEFT BLANK
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BRAKE HYDRO-MECHANICAL CONTROL -FUNCTIONAL DESCRIPTION
General
The source selection operation supplies pressure for:
*
Normal brakes
*
Alternate brakes
*
Reserve brakes
*
Accumulator brakes
*
Gear retract braking.
When the pressure in one hydraulic brake source decreases, the ASSV and AIV valves automatically change position to select the next available source and send the pressure to the brake metering valves.
Brake pedal input transmits through the brake pedal bus mechanism and cables to the left and right brake metering valves. These metering valves use this mechanical input to control the selected pressure source and pressurize the brakes.
Normal Brakes
When the right hydraulic system pressurizes, the ASSV operates to depressurize the alternate brake system. The brakes can then get metered right system hydraulic pressure from the normal brake metering valves.
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