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MAINTENANCE MANUAL
123
TO LH & RH TO LH TO RH
ACTUATORS ACTUATOR ACTUATOR
(RETRACT) (EXTEND) (EXTEND)
Hydraulic Control Module Schematic
Figure 3
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1 ARM CMD INHIBITSMSGS
Body Gear Steering Fault Indication
Figure 4
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747-400
MAINTENANCE MANUAL
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C21348
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747-400
MAINTENANCE MANUAL
5. Steering Actuator (Fig. 1)
_________________
A. The actuators can steer each body gear truck to approximately a 13-degree position. Each pair of actuators is attached to a machined yoke and lower torsion link of each body gear. The yoke is bolted to the body gear outer cylinder while the lower torsion link is attached to the truck beam.
B. Each actuator contains an axial feedback transducer and a positive, hydraulic operated mechanical lock. The exact position of the mechanical lock is indicated by the integrally mounted switches. Closed position of these switches positively shows that the lock is in the disengaged position.
6. Arming (Fig. 1)
______
A. The arming switch is actuated by a cam on the rudder interconnect quadrant which is cable-driven by the nose wheel steering tiller.
B. To arm the steering system (control module solenoid valves energized) for the body landing gear, the system must receive the inputs that follow:
(1)
The nose landing gear must be turned at an angle more than 20 degrees.
(2)
One of the left body landing gear or wing landing gear and one of the right body landing gear or wing landing gear must be in the not tilted position.
(3)
The wheel speed must be less than 15 knots.
(4)
The BSCU must show the ground speed is less than 40 knots.
(5)
The BSCU must show the airplane is in the ground mode for 10 seconds or more.
C.
Tilt sensors on wing and body gear monitor tilted condition of gear (Fig. 1).
7. Operation
_________
A. Functional Description
(1)
Steering of body gear trucks is available after the nose wheel has been steered to approximately 21 degrees. At this angle the command transducer begins sending an electrical signal to move the hydraulic actuators. This electrical signal is amplified in the electronic control unit which directly actuates the servo valve in the control module. The servo valve is capable of porting hydraulic fluid to hydraulic actuators at 3000 psi.
(2)
The sensor switches are closed (tilt sensors deactuated) every time the truck is level, i.e., not tilted. Deactuation of the tilt switches provides ground mode condition through the brake system control unit. This enables arming of body gear steering system when the arming switch on the rudder interconnect quadrant is actuated. The arming switch energizes the solenoid valve at approximately 21 degrees of nose wheel steering. Further movement of the command transducer causes the inboard truck to move to a 13-degree maximum turn while the outboard truck steers approximately 11.5 degrees. The maximum turn of body gear trucks occurs at approximately 70 degrees of nose wheel steering. At this point, with the nose gear wheels turned fully left, the body gear wheels are turned right, to provide the tightest turning angle possible.
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