MAIN HYDRAULIC SYSTEMS -CENTER SYSTEM RETURN MANIFOLD AND PRESSURE RELIEF VALVE

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777 AIRCRAFT MAINTENANCE MANUAL

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MAIN HYDRAULIC SYSTEMS -HYDRAULIC ACCUMULATORS

Purpose

The hydraulic accumulators absorb pressure changes caused by tail flight control PCU operation. This increases the life of the hydraulic system components.
Physical Description

The right and center hydraulic systems each have a single hydraulic accumulator in the pressure lines to the tail flight controls. The left hydraulic accumulators connect to the tail flight control pressure lines. All accumulators are the same and have a volume of 50 cubic inches.
The accumulators have a hydraulic fluid side and a gas side. A fluid port connects the fluid side to the hydraulic line. You service the gas side of the accumulator with nitrogen.
Location

The hydraulic accumulators are on the forward side of the aft bulkhead of the stabilizer compartment. The hydraulic accumulator service panel is on the aft side of the stabilizer compartment aft bulkhead.
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Training Information Point
A door forward of the APU compartment gives access to the hydraulic accumulator service panel. Gages on the panel show the gas charge for the accumulators in each system. A chart shows the relation of pressure to temperature, and when servicing is necessary. A charging valve for the accumulators in each system lets you connect a nitrogen supply.
WARNING: YOU MUST PREVENT ALL POSSIBLE OPERATION OF THE HORIZONTAL STABILIZER WHEN YOU WORK ON OR NEAR IT. THE HORIZONTAL STABILIZER MOVES QUICKLY AND WITH FORCE. IF THE STABILIZER MOVES WHEN PERSONS ARE IN THE TORSION BOX COMPARTMENT OR NEAR THE STABILIZER, IT CAN CAUSE INJURY TO THEM.
WARNING: DO NOT ENTER THE COMPARTMENT WITHOUT FORCED AIR VENTILATION WHEN YOU SERVICE THE HYDRAULIC ACCUMULATORS. DO NOT RELEASE NITROGEN INTO THE COMPARTMENT WHEN SERVICING THE ACCUMULATORS. NITROGEN WILL DISPLACE AIR AND REDUCE THE OXYGEN LEVEL IN THE COMPARTMENT. LACK OF OXYGEN CAN RESULT IN UNCONSCIOUSNESS OR DEATH. A SECOND MECHANIC WHO CAN OBSERVE THE WORK MUST BE LOCATED OUTSIDE THE COMPARTMENT WHEN NITROGEN IS BEING SERVICED IN A CLOSED COMPARTMENT.
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MAIN HYDRAULIC SYSTEMS -HYDRAULIC ACCUMULATORS

WARNING: DO NOT LOOSEN THE BODY OF THE HYDRAULIC ACCUMULATOR CHARGING VALVE. THE PRESSURE IN THE HYDRAULIC ACCUMULATORS CAN QUICKLY PUSH THE CHARGING VALVE OFF THE MANIFOLD ASSEMBLY. THIS CAN CAUSE INJURY TO PERSONS OR DAMAGE TO EQUIPMENT.
WARNING: DO NOT RELEASE NITROGEN INTO THE COMPARTMENT WHEN DISCONNECTING THE CHARGING ASSEMBLY FROM THE HYDRAULIC ACCUMULATORS. NITROGEN WILL DISPLACE AIR AND REDUCE THE OXYGEN LEVEL IN THE COMPARTMENT. A LACK OF OXYGEN CAN RESULT IN UNCONSCIOUSNESS OR DEATH.
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MAIN HYDRAULIC SYSTEMS -HYDRAULIC ACCUMULATORS

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MAIN HYDRAULIC SYSTEMS -FUNCTIONAL DESCRIPTION -ENGINE DRIVEN PUMP

General
The EDPs operate when the related engine turns.

The EDPs can be depressurized by the EDP switch or by the engine fire switch.
HYDIM L monitors the operation of the left EDP. HYDIM R monitors the operation of the right EDP.
EDP Switch Operation

When the EDP switch is in the ON position, the depressurization solenoid valve is not energized. This permits the pressure output of the EDP to go to the hydraulic system.
When the EDP switch is in the OFF position, power from the left 28v dc main bus (left engine) or the right 28v dc main bus (right engine) goes to EDP solenoid valve. This closes the valve to stop the pump output and depressurize the pump.
The case drain flow continues when the switch is in the OFF position.
Supply Shutoff Valve Operation
When you pull the engine fire switch, the supply shutoff valve moves to the CLOSED position. This stops the supply of hydraulic fluid to the EDP. The engine fire switch also energizes the depressurization solenoid valve to remove pressure from the EDP. There is no case drain flow in this condition.
The supply shutoff valve moves to the OPEN position when you move the engine fire switch to the stowed position.

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MAIN HYDRAULIC SYSTEMS -FUNCTIONAL DESCRIPTION -ENGINE DRIVEN PUMP

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