747-400 AMM 飞机维护手册 起落架 LANDING GEAR 7(178)

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 A. Functional Description
 (1)  Nose wheel steering is available when the nose gear is in the down
 position and compressed by weight of the airplane. Positioning
 the landing gear control lever to down makes system hydraulic
 pressure available from the landing gear down line to the steering
 metering valve for steering. The nose gear must be compressed more
 than 8 inches before steering is attempted, to avoid damage to the
 centering cams.
 (2)  When the airplane is on the ground (shock strut compressed), the
 switch actuator is away from the squat switch sensor. With the
 squat switch not activated, the nose gear squat relays remain
 de-energized (Fig. 4). This provides 28-volt dc power to the
 interconnect actuators. The No. 1 interconnect actuator retracts as
 the No. 2 interconnect actuator extends, engaging the steering
 mechanism with the rudder pedals.
 NOTE: Pedal steering engagement occurs if one or both interconnect____
 actuators operate. If only one actuator operates, the status
 message PEDAL STEERING will come into view on the EICAS.
 (3)  Any movement caused by the rudder pedals is then transmitted from
 the steering crank to the rudder pedal steering quadrant. This
 quadrant is connected to the nose wheel steering cables. The
 quadrant is free to move with the cables whenever the steering
 tiller is used, or drive the cables when positioned by rudder pedal
 steering mechanism.
 (4)  When the shock strut is extended (gear retracted) the actuator is in
 proximity with the squat switch sensor and the switch is activated.
 With the squat switch activated, the nose gear squat relays are
 energized. This provides 28-volt dc power through the nose gear
 squat relays to energize the interconnect actuators. The No. 1
 interconnect actuator extends as the No. 2 interconnect actuator
 retracts, disengaging the steering mechanism from the rudder pedals.
 NOTE: Pedal steering disengagement occurs only if both actuators____
 operate.
 Disengagement of steering prevents the nose gear from turning when
 rudder pedals are moved during flight.
 (5)  The PEDAL STEERING status message will come into view on the EICAS
 to indicate failure of either one of the electrical interconnect
 actuators. Failure of one of the actuators reduces the movement of
 the actuator lever, but the actuator lever moves sufficiently for
 operation of the interconnect mechanism.

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 ALL  ú ú 01 Page 7 ú Jun 10/88
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

Nose Gear Steering Circuit Diagram
Figure 4

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 32-51-00
 ALL  ú ú 01 Page 8 ú Jun 10/88
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (6)  
Movement of the steering tiller puts one cable under tension. The bulkhead drum rotates, turning the pivot links. This causes rotation of the trunnion drum, mounted on the aft side of the shock strut trunnion. The corresponding cable, NWS2A or NWS2B, is then under tension, causing movement of the control piston in the steering metering valve. Movement of the control piston directs hydraulic fluid under pressure to the correct ports of the left and right steering actuators (Fig. 5). The steering collar is moved to the desired direction of turn. As the amount of turn desired is reached, the tension in cable is reduced by follow-up action and the control piston is returned to centered position. The tiller must be held throughout the turn or the system will return to center due to action of the centering spring in the rudder pedal steering mechanism.

 (7)  
During steering operation hydraulic fluid is directed by the metering valve to one side of the right actuator piston and to the opposite side of the left actuator piston, producing a push-pull action on the steering collar as shown on Fig. 5. This push-pull action continues as long as the valve piston is displaced, or until approximately 54 degrees of steering. At this point the pulling actuator has reached the end of its stroke and the turning motion of the actuator has closed off the hydraulic pressure and return lines in the steering actuator swivel valve. Continued pushing by the other actuator moves the pulling actuator past the null point. Above approximately 55 degrees of steering, hydraulic pressure for the steering cylinder that is overcenter has been opened in both ports, rod end and head end. Further turning from about 55 degrees to the maximum is provided primarily by the pushing actuator and aided to a limited amount by the overcentered actuator.
　
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