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MAIN GEAR STEERING SYSTEM -MAIN GEAR TRUCK COMPONENTS
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777 AIRCRAFT MAINTENANCE MANUAL
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MAIN GEAR STEERING SYSTEM -ELECTRIC FUNCTIONAL DESCRIPTION
General
The MGSCU gets the inputs from the NLG steering tiller position transducers and sends signals to operate the left and right steering/locking PCAs. Both PCAs move the same amount in the same direction.
The MGSCU has these functions:
*
Arm/unlock logic
*
Steering control law
*
Left and right PCA control
*
BITE/Monitor.
The left 28v dc main bus supplies the electrical power to operate the main gear steering system.
Control Inputs
The captain’s and first officer’s NLG steering tiller position transducers supply tiller position data to the MGSCU. The MGSCU uses the captain’s position transducer to control the steering/locking PCAs. The BITE/Monitor in the MGSCU compares the first officer’s position transducer to the captain’s position transducer to find failures.
Arm/Unlock Logic and Steering Control Law
The arm/unlock logic energizes the arm solenoid and the unlock solenoid for both PCAs when the NLG steering input is more than 13 degrees. This unlocks the PCAs.
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The steering control law sends signals to the EHSVs for both PCAs to turn the main gear in proportion to the nose gear position. These signals start when the nose gear position is more than 10 degrees. A maximum of 8 degrees of aft axle command occurs when the nose gear command angle is at 70 degrees.
The steering position LVDTs in the PCAs supply feedback signals to the MGSCU to null out the command when the PCAs are at the commanded position.
The arm/unlock logic removes the arm signal when the NLG steering input is less than 9 degrees.
BITE/Monitor
The BITE/Monitor function gets inputs from:
*
The steering position LVDT
*
The lock/unlock position LVDT
*
The shutoff valve position switch
*
The NLG steering tiller position transducers.
The BITE/Monitor function uses these inputs to find failure conditions and to monitor the shutoff valve position. The MGSCU sends fault and position data to the AIMS for flight deck indication.
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MAIN GEAR STEERING SYSTEM -ELECTRIC FUNCTIONAL DESCRIPTION
The MGSCU also sends main gear steering lock status to the left and right warning electronics units. A CONFIG GEAR STEERING warning message shows if a steering/locking PCA is not locked during takeoff.
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MAIN GEAR STEERING SYSTEM -ELECTRIC FUNCTIONAL DESCRIPTION
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MAIN GEAR STEERING SYSTEM -HYDRAULIC FUNCTIONAL DESCRIPTION
General
The MGSCU gets input from the two NLG steering tiller position transducers and sends signals to the steering/locking PCAs. LVDTs on the PCAs send signals back to the MGSCU.
PCA Initial Condition
Each steering/locking PCA is centered and locked. Center system hydraulic pressure is on both sides of the PCA piston. The lock collar is in the locked position.
Arm -Unlock
When steering tiller input is more than 13 degrees of NLG steering angle, the MGSCU sends a signal to energize the unlock solenoid. This moves the unlock solenoid valve to the unlock position.
The MGSCU then sends a signal to energize the arm solenoid. Hydraulic pressure now goes through the unlock solenoid valve to unlock the lock collar on the actuator. The unlock LVDT sends an actuator unlock feedback signal to the MGSCU.
Pressure through the arm solenoid also moves the isolation/ power centering valve to the ON position. This permits pressure to go to the electrohydraulic servo valve (EHSV).
Main gear steering is now armed and unlocked.
Control
The MGSCU sends signals to the EHSV to extend or retract the actuator. The aft axles turn in proportion to tiller position.
Maximum aft axle angle of 8.0 degrees occurs at a nose gear angle of 70 degrees.
The actuator position LVDT sends actuator position data to the MGSCU.
Disarm
When tiller angle is less than 9 degrees, the MGSCU removes power from the arm solenoid and the unlock solenoid. This stops hydraulic pressure to the lock collar and to the isolation/ power centering valve.
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