OPERATEDDOWN
SEQUENCE VALVE UNLOCK DOWN
UPC C
OPEN LOCK UNLOCK
TEE UP
UPLOCK DOWNLOCK
ACTUATOR TIME
ENGAGED PRESSURE
DISENGAGED OPERATED RESTRICTOR CHECK VALVE
RETURN INT DN UP DOWN
UP TO AUTOBRAKE ACTUATOR GEAR ACTUATOR
LOCK UNLOCK
LEGEND:
UPLOCK OPER CLOSE OPEN

UP SEQ VALVE HOOK PRESSURECLOSED DOWNLOCKOPEN RETURN
OPEN
ACTUATOR DOOR
DOWN SAFETY
DOOR PRESSURE VALVE
ACTUATOR SNUBBER
VALVE
CLOSE Wing Gear Hydraulic System Figure 8 (Sheet 3)
U23250
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

HYDRAULIC
PRESSURE
RETURN
CONDITION:
SYSTEM 4
GEAR UP SELECTED
GEAR UP AND LOCKED

LANDING GEARDOORS CLOSING SELECTOR VALVE
NEUTRAL DOWN RETURN
TO
DOWNLOCK DOWN PRESSURE SECOND
OPERATED WING
SEQUENCE UP PRESSURE GEAR
VALVE
DOOR CLOSED UNLOCK PRESS.DN DOOR OPEN
RETURN UP RET
DOOR
INT.

OPERATEDDOWN
SEQUENCE VALVE UNLOCK DOWN
UPC C
OPEN LOCK UNLOCK
TEE UP
UPLOCK DOWNLOCK
ACTUATOR TIME
ENGAGED PRESSURE
DISENGAGED OPERATED RESTRICTOR CHECK VALVE
RETURN INT DN UP DOWN UP
TO
AUTOBRAKE
ACTUATOR
GEAR ACTUATOR
LOCK UNLOCK
LEGEND:
UPLOCK OPER CLOSE OPEN

UP SEQ VALVE HOOK PRESSURECLOSED DOWNLOCKOPEN RETURN
OPEN
ACTUATOR DOOR
DOWN SAFETY
DOOR PRESSURE VALVE
ACTUATOR SNUBBER
VALVE
CLOSE Wing Gear Hydraulic System Figure 8 (Sheet 4)
U23252
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (8)  
When the gear is up and locked and DN is selected, the uplock hook is closed. This positions the uplock operated sequence valve so that fluid is directed to the open side of the door actuator. As the uplock actuator opens the gear up hook, allowing the gear to extend, the sequence valve is mechanically positioned to block fluid entering from the down port. However, the door actuator still remains in the open position due to fluid from the downlock sequence valve. When the gear is down and locked, fluid from the downlock-operated sequence valve is directed through the interconnecting line and the uplock sequence valve to the close side of the door actuator and doors close.

 (9)  
When gear UP is selected, the uplock hook is still in the open position which positions the valve so that fluid is directed to the open side of the door actuator, where it remains until the gear is retracted and locked up. When the gear is locked up, the hook closes. This mechanically repositions the valve so that fluid is directed to the close side of the door actuator.

 (10)
 When the landing gear extends from the up and locked position, the wing gear actuator is pressurized to retract. Retraction of the actuator piston causes the trunnion to rotate, extending the gear. The wing gear downlock actuator is also pressurized pulling the jury strut into the overcenter (locked) position preventing the side strut from folding.

 (11)
 When hydraulic pressure first enters the actuator to retract the landing gear, the piston rod applies force to the trunnion lug tending to rotate the trunnion in the direction for gear retraction. The opposite reaction of the extending actuator exerts an outboard force on the walking beam. The beam is attached to the upper lugs on the trunnion, and the outboard force on the beam transmitted to the beam trunnion lugs results in an increased force tending to rotate the trunnion in the direction for gear retraction.

 (12)
 When the gear is selected DN, hydraulic pressure causes the door actuating cylinder to unlock and retract. This opens the door. The internal lock in the actuator may be manually unlocked by means of a rotating shaft on the cylinder end of the actuator. Rotation of this shaft unlocks the internal lock mechanism when the alternate landing gear release system is operated, or when the ground door release handle is operated. Simultaneously the door safety valve releases pressure to return. The doors then free-fall by gravity.
　
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