*
Selector valve

*
Bypass valve.

The dual coil solenoids get input from the switches in the landing gear control lever module and from the ELMS to control the solenoid-operated valves.
Off Position
In the OFF position, center system hydraulic pressure goes to both solenoid-operated valves and to both sides of the landing gear selector valve. This keeps the selector valve in the OFF position.
In this position, the landing gear is not pressurized.
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Extension
For a gear extension, the down solenoids get power and ground through the down switches in the landing gear control lever module. This moves the down solenoid-operated valve which releases pressure from one side of the selector valve. Hydraulic pressure and spring force move the selector valve to the DOWN position.
This permits hydraulic pressure to go to the landing gear down lines.
Retraction
For a gear retraction, the up solenoids get power through the up switches in the landing gear control lever module. This power also goes through the ELMS. This moves the up solenoid-operated valve which releases pressure from one side of the selector valve. Hydraulic pressure and spring force move the selector valve to the UP position.
This permits hydraulic pressure to go to the landing gear up lines.
Bypass Valve
The pressure operated bypass valve has a NORMAL and a BYPASS position.
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LANDING GEAR CONTROL SYSTEM -LANDING GEAR SELECTOR/BYPASS VALVES -FUNCTIONAL DESCRIPTION

During normal landing gear operation, the bypass valve is in the NORMAL position.
If the selector valve is in the UP or OFF positions, or if there is no pressure in the center hydraulic system, alternate extend pressure moves the bypass valve to BYPASS.
This makes sure the gear retraction lines connect to return to permit the alternate extension system to extend the gear.
This prevents a selector valve stuck in the UP position from keeping the landing gear retracted.
Normal system gear extend pressure resets the bypass valves to NORMAL.
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LANDING GEAR CONTROL SYSTEM -LANDING GEAR SELECTOR/BYPASS VALVES -FUNCTIONAL DESCRIPTION

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LANDING GEAR CONTROL SYSTEM -FUNCTIONAL DESCRIPTION

General
The landing gear control system controls extension and retraction of the landing gear and the tail skid (if applicable).
Lever Lock
When the airplane is in the air, two air/ground relays in the ELMS supply a ground to energize the lever lock solenoid. This releases the lock on the landing gear control lever and permits you to move the control lever UP.
Retraction
When you put the landing gear control lever UP, two switches in the landing gear control lever module supply power to auto-off relays in ELMS. The auto-off relays supply the power to UP solenoid in each of the landing gear selector/bypass valves.
Hydraulic pressure then goes through the selector/bypass valves to the UP lines for retraction of the landing gear and the tail skid (if applicable).
Another switch in the landing gear control lever module resets the alternate extension system MLG and NLG door release/ safety valve modules.
Auto-Off
After gear retraction, the PSEUs send signals to the HYDIM cards L and R. After a ten second delay, the HYDIM cards energize the auto-off relays in the ELMS.
This removes electrical power from the UP solenoid and the selector valve moves to OFF to remove pressure from the landing gear.
Extension
When you put the landing gear lever DOWN, four switches in the landing gear lever module supply the electrical power and the ground to the DOWN solenoid in each of the landing gear selector valves.
Hydraulic pressure then goes through the landing gear selector/bypass valves to the DOWN lines for extension of the landing gear and the tail skid (if applicable).
Down pressure stays on as long as the landing gear lever is DOWN and the center hydraulic system is pressurized.
Another switch in the landing gear control lever module supplies a signal the weather radar system. See the weather radar system section for more information. (SECTION 34-43)
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LANDING GEAR CONTROL SYSTEM -FUNCTIONAL DESCRIPTION

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777 AIRCRAFT MAINTENANCE MANUAL

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