777 AIRCRAFT MAINTENANCE MANUAL
MAIN GEAR STEERING SYSTEM -INTRODUCTION
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MAIN GEAR STEERING SYSTEM -NLG STEERING TILLER POSITION TRANSDUCERS
Purpose
Two NLG steering tiller position transducers supply steering tiller data to the MGSCU. The MGSCU uses this data to control the main gear steering.
Physical Description
The two NLG steering tiller position transducers are the same. The transducers are rotary variable differential transformers (RVDTs).
The input shafts of the position transducer turn through a 90 degree arc with the torque shaft from the steering tiller and gearbox.
Location
A transducer is on the bottom of each steering tiller drum mechanism in the forward equipment center.
Training Information Point
Before installation, the transducer input shaft is locked in the zero (neutral) position.
The transducer input shaft is unlocked when you install the three captive mounting screws.
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MAIN GEAR STEERING SYSTEM -NLG STEERING TILLER POSITION TRANSDUCERS
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MAIN GEAR STEERING SYSTEM -MAIN GEAR STEERING CONTROL UNIT
Purpose
The MGSCU gets the steering inputs from the nosegear steering tiller position transducers and controls the main gear steering/locking power control actuators.
The MGSCU also monitors the main gear steering system for faults.
Location
The MGSCU is on the E6-3 shelf in the aft cargo compartment.
Training Information Point
The MGSCU is electrostatic discharge sensitive.
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MAIN GEAR STEERING SYSTEM -MAIN GEAR STEERING CONTROL UNIT
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777 AIRCRAFT MAINTENANCE MANUAL
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MAIN GEAR STEERING SYSTEM -MAIN GEAR TRUCK COMPONENTS
General
These components are on the aft part of each main gear truck assembly:
*
MLG steering/locking power control actuator (PCA)
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Reaction link
*
Steering crank.
Steering/Locking Power Control Actuator
The MLG steering/locking PCA unlocks and turns the aft main gear axle. The PCA locks the aft axle when main gear steering is not used.
The MLG steering/locking PCAs are two-way actuators and have internal locks to keep them in the centered position. An LVDT in the actuator rod supplies position feedback to the MGSCU.
These are the other components in each MLG steering/locking PCA:
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Arming solenoid
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Unlock solenoid
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Electrohydraulic servo valve
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Isolation/power centering valve
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Anti-cavitation check valves
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Compensator
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Manual shutoff valve
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Lock position linear variable differential transformer (LVDT).
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Reaction Link
A reaction link transmits steering forces back to the steering crank. This link connects the left side of the aft axle to the steering crank.
Steering Crank
The steering crank transmits actuator and the reaction link loads to the truck beam.
Training Information Point
A manual shutoff valve permits you to stop the hydraulic flow to the steering locking PCA to disable the main gear steering. You operate the valve with the manual shutoff valve handle. A ball-lock pin keeps the shutoff valve in the selected position.
A valve position sensor sends shutoff valve position data to the MGSCU. The MAIN GEAR STEERING status message show when the handle is in the OFF position.
A silver indicator in the lock indication window on the unlock LVDT shows the lock/unlock condition of the PCA. If you can see the indicator, the PCA is unlocked. If you can not see the indicator, the PCA is locked.
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MAIN GEAR STEERING SYSTEM -MAIN GEAR TRUCK COMPONENTS
WARNING: MAKE SURE THE MAIN LANDING GEAR STEERING SYSTEM IS ISOLATED FROM HYDRAULIC POWER BEFORE YOU DO WORK NEAR THE TIRES, WHEELS, OR BRAKES. IF YOU
DO NOT DO THIS, THE AFT AXLES CAN
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