*
NLG doors -CLOSED
*
NLG gear-operated sequence valve -GEAR UP
*
NLG door-operated sequence valve -CLOSED.
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NLG EXTENSION-RETRACTION -RETRACTION SEQUENCE
The NLG selector/bypass valve automatically removes pressure from the NLG retract lines approximately ten seconds after the gear is up and locked.
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Initial Positions
These are the initial positions for the nose gear components:
*
NLG -UP AND LOCKED
*
NLG doors -CLOSED
*
NLG-operated sequence valve -GEAR UP
*
NLG door-operated sequence valve -CLOSED.
Control
When you select DOWN with the landing gear control lever, the nose landing gear selector/bypass valve moves to the DOWN position. Hydraulic pressure then goes to the extend lines.
Doors Open
Hydraulic pressure goes through the NLG-operated sequence valve to the NLG door actuator. The pressure unlocks the actuator and opens the forward nose landing gear doors.
Gear Unlocks -Gear Extends
When the nose landing gear doors are almost at the full open position, the NLG door-operated sequence valve moves to OPEN.
Pressure then goes to the extend side of the NLG lock actuator to unlock the uplock.
The NLG extends by the force of its own weight and by airloads. As the NLG starts to extend, the NLG-operated sequence valve moves to the GEAR-IN-TRANSIT position.
Doors Close
When the NLG is almost down, the NLG-operated sequence valve moves to the DOWN position. Pressure now goes to the retract side of the NLG door actuator and the doors start to close.
The NLG lock actuator continues to extend and locks the NLG when the gear is fully down.
The lock inside the NLG door actuator locks when the doors are in the CLOSED position.
Final Positions
The final positions for NLG components are:
*
NLG -DOWN AND LOCKED
*
NLG doors -CLOSED
*
NLG-operated sequence valve -GEAR DOWN
*
NLG door-operated sequence valve -CLOSED.
Pressure stays in the NLG lock actuator and the NLG door actuator when the gear is down and the center hydraulic system is pressurized.
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777 AIRCRAFT MAINTENANCE MANUAL
THIS PAGE IS INTENTIONALLY LEFT BLANK
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BOEING PROPRIETARY -Copyright # -Unpublished Work -See title page for details.
LANDING GEAR ALTERNATE EXTENSION SYSTEM -INTRODUCTION
Purpose
The alternate extension system permits you to extend the landing gear when there is no center hydraulic system pressure or if normal extension fails.
You also use the alternate extension system to open the landing gear doors on the ground.
Components
The alternate extension system components include:
*
The alternate extend power pack
*
The main landing gear door release/safety valve modules
*
The nose landing gear door release/safety valve module
*
The main landing gear alternate uplock release actuators
*
The nose landing gear alternate uplock release actuator
*
The alternate extend hydraulic pressure switch.
General Description
The alternate extension system operates independently of the normal extension and retraction system.
The hot battery bus supplies power to control and operate the alternate extension system. The ELMS contains relays that control the power to the alternate extend power pack.
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