The main hydraulic components of the nosewheel steering system are the nosewheel steering power unit and selector valve. See figure 12-14.

NOSEWHEEL STEERING POWER UNIT. The nose wheel steering power unit incorporates a rotary, vane-type motor that is powered hydraulically and is electrically controlled through various system components to provide the nosewheel steering function. When not in the steering mode of operation, the nosewheel steering power unit serves as a nosewheel shimmy damper. 

The nosewheel steering power unit is mounted to the nose landing gear cylinder, and the output drive gear is meshed with the ring gear of the nose landing gear torque collar. The torque collar deflects the nosewheel as selected by rudder pedal positioning. Hydraulic fluid displaced by the rotating vane during the steering mode is directed back to the hydraulic return system. 

When in the damping mode, fluid displaced by a rotating vane is directed through an orifice restrictor inside the nosewheel steering power unit to the opposite side of the vane to provide the dampening feature.

NOSEWHEEL STEERING SOLENOID SELECTOR VALVE. The nosewheel steering solenoid selector valve is an electrically controlled and hydraulically operated valve. The valve provides pressure and return fluid porting during the steering mode of operation.

Nosewheel steering electrical components vary greatly. The system uses three basic components. These components are the feedback potentiometer, the command potentiometer, and the steering amplifier.

FEEDBACK POTENTIOMETER. The feed-back potentiometer is mounted to the nosewheel steering power unit, and is mechanically linked or geared to the vane motor shaft. See figure 12-14. During the steering mode of operation, vane motor rotation drives the feedback potentiometer. When driven, the position transmitter provides a feedback signal to the steering amplifier that is proportional to the amount of vane motor rotation.

COMMAND POTENTIOMETER. The com-mand potentiometer is attached to the rudder pedal linkage. When the rudder pedals are moved, the command potentiometer generates an electrical signal proportional to the amount of rudder pedal deflection.

STEERING AMPLIFIER. The steering ampli-fier sums the signals received from the feedback potentiometer and the command potentiometer. This summation is converted to a modulating signal that is directed to the nosewheel steering power units servo valve for nosewheel steering response. With the signals from the command and feedback potentiometer balanced, the servo is returned to a neutral condition, and the nosewheel steering power unit stops at the selected position.

Maintenance of an electrically controlled nose gear steering system consists of operational checks, trouble-shooting, system bleeding, and parts adjustment. These maintenance functions normally require a joint effort on the part of the AM and the AE personnel. See figure 12-15.