valve, two unidirectional restrictors, the steering
Nosewheel Steering System
actuator, and fluid compensator.
The check valve prevents reverse flow from
As you read this section, refer to figure 4-17.
the unit to the shutoff valve. The servo valve
The nosewheel steering system is an electrically
controls the actuator position by controlling fluid
controlled, hydraulically operated system. It
flow to and from the actuator in response to signal
provides a nonlinear relationship between the
variations from the amplifier. The bypass valve
rudder pedals and the angular position of the
closes off the interconnecting passages between
nosewheel. The system provides the pilot with
both ends of the actuator whenever hydraulic
adequate directional control of the aircraft during
pressure is available to the unit. This permits the
ground operation. It consists of a hydraulic
actuator to act as a steering unit instead of a
steer-damp unit, solenoid-operated shutoff valve,
damping unit. The unidirectional restrictors
command potentiometer, steering feedback
provide a restricted reverse flow to dampen
potentiometer, steering amplifier, and an electrical
nosewheel shimmy. The steering actuator is a
control system. Control of the electrical power to
balanced piston-type hydraulic actuator, which
the system is through either of the ground safety
provides the force to turn the nosewheel. Also,
switches on the left or right main landing gear.
with the restrictors, the steering actuator provides
The landing gear handle switch, a push-button
the shimmy damper action. The fluid com-
switch on the pilot's stick grip, and the rudder
pensator is in the return passage in the unit and
pedals also control electrical power.
traps a quantity of fluid at 40 to 100 PSI. The
When the steering system electrical circuits
compensator supplies fluid to the actuator
energize, the steering system aligns for steering
through the bypass valve and the restrictor when
operation. At this point, the solenoid-operated
the unit is being used as a shimmy damper, and
hydraulic shutoff valve opens to supply fluid to
extra fluid is necessary to prevent cavitation of
the steering actuator. Simultaneously, all related
the actuator. Since the compensator traps fluid
circuitry for controlling the steering servo valve
in the actuator, it includes thermal relief
activates. The electrical section of the steering
provisions to prevent excessive pressure buildup
system is essentially a bridge circuit. One side of
within the steer-damper unit.
the bridge circuit runs through the command
potentiometer; the opposite side runs through
SHUTOFF VALVE. --The steering shutoff
the feedback potentiometer. Output of each
valve is a three-way, two-position, normally
potentiometer goes to the steering system
closed, solenoid-operated valve. The valve
amplifier. Amplifier output currents flow to the
controls hydraulic system pressure to the steer-
hydromechanical servo valve coils, and the net
damper unit. When the valve de-energizes, fluid
signal actuates the servo valve, causing nosewheel
flow is cut off from the steer-damper unit. When
steering action.
During operation, the nosewheel steering
the valve energizes (during normal steering or
system attempts to maintain symmetry of the
arrested landings), pressure flows to the check
valve, bypass valve, and servo valve in the
electrical bridge circuit. The circuit is symmetrical
when both potentiometers supply equal voltages
steer-damper unit.
to the amplifier and the servo valve is at a null
COMMAND POTENTIOMETER. --The
position. The bridge circuit becomes unbalanced
when initiating a turn request by repositioning of
steering system has a rotary-type, pedal-position
the command potentiometer wiper. Current
(command) potentiometer. This potentiometer
differential in the servo valve windings reflects an
mechanically links to, and is driven by, the rudder
pedals. It provides a nonlinear steering response.
unbalanced bridge circuit. Movement of the servo
valve ports hydraulic pressure to the appropriate
The potentiometer sends a signal to the nose
side of the actuator piston to cause the nosewheel
gear steering amplifier showing the degree and
to turn.
direction of turn commanded by the pilot. Moving
the potentiometer, with the steering system
STEER-DAMPER UNIT. --The steer-damper
operating, unbalances a bridge circuit causing the
unit is an electrically controlled, hydraulically
steering amplifier to signal the servo valve to turn
operated package on the nose gear strut assembly.
the nosewheel.
The unit provides both nosewheel steering and the
FEEDBACK POTENTIOMETER. --The
required shimmy damping effect. The package
steering feedback potentiometer assembly consists
consists of a check valve, servo valve, bypass
