From the FCOM:
RUDDER
RUDDER CONTROL SYSTEM
The rudder provides directional control about the yaw axis. The system is mechanically controlled and hydraulically pow- ered. The mechanical part of the system has control pedals, levers, pulleys and cables which connect to two hydraulic power control units (PCUs) in the vertical stabilizer. The PCUs cause the rudder to move in proportion to the flight-crew’s control pedal input. A rudder-control feel unit supplies a simulated pedal load to the flight crew.
The pilot and copilot rudder pedal assemblies are both connected to the forward quadrant assembly, with no means to dis- connect one from the other. Two cable systems beginning at the forward quadrants follow the same path in the fuselage until they reach the engine rotor burst zone, where they are routed separately towards the rear fuselage. In the event of a severed single cable in the rotor burst zone, the aft quadrant still receives input from the unaffected cables, thus retaining rudder system functionality. On the ground, trapped hydraulic fluid provides rudder control surface gust lock damping when the hydraulic systems are depressurized.
Rudder position is displayed on the EICAS FLIGHT CONTROL synoptic page. A full-scale deflection of the rudder posi- tion indicator corresponds to maximum rudder travel.
Sep 13/2004 Flight Crew Operating Manual Volume 2
REV 1 CSP 100-6
10-01-03
FO1001002_014 PRIMARY FLIGHT CONTROLS (Cont)
RUDDER POWER CONTROL UNIT (PCU)
There are two hydraulic rudder PCUs attached to the rear spar of the vertical stabilizer. The left hydraulic system energizes the upper rudder PCU and the right hydraulic system energizes the lower rudder PCU. If the right hydraulic system loses pressure, the lower rudder PCU is powered by the auxiliary hydraulic system.
The rudder PCUs also give protection to the rudder control surface against gust loads while the aircraft is on the ground.
RUDDER TRAVEL LIMITER
The rudder travel limiter actuator puts a limit on the movement of the rudder control surface, as a function of airspeed. This insures that the aerodynamic force on the rudder is not more than the structural limit of the vertical stabilizer. The horizontal-stabilizer-trim electronic-control unit (HST ECU) controls the movement of the rudder travel limiter-actuator.
NOSEWHEEL STEERING DESCRIPTION
The nosewheel steering is designed to provide safe and reliable directional control of the aircraft during all phases of taxi, takeoff and landing. The nosewheel steering provides electrically controlled and hydraulically powered actuation of the nose landing gear steering actuator mounted on the nose gear assembly. When the nosewheel steering is not armed, or under failure con- ditions, the system continues to provide effective damping to ensure dynamic stability of the nose landing gear.
The Nosewheel Steering System (NWS) is armed by selecting the NWS switch on. The NWS switch is located on the LANDING GEAR control panel and is illuminated when it is in the OFF position. Steering commands are input to the steer- ing control unit (SCU) through the pilot handwheel (full authority, ±65° at the nose wheel) and/or through the rudder pedals (limited authority, ±7° at the nose wheel). Position feedback is provided to the SCU from a position transducer on the NLG. There is no mechanical connection between the flight deck controls and the steering actuators. Nosewheel steering com- mands are transmitted electronically using “steer-by-wire” technology.
COMPONENTS AND OPERATION
STEERING CONTROL UNIT
The steering control unit (SCU) controls the electro-hydraulic steering valve to port hydraulic fluid to either end of the steering actuator to turn the nose gear. Hydraulic pressure will move the rack, which in turn engages and rotates the pinion gear on the steering cuff. Mechanical torque links transmit the movement of the steering cuff to the nose landing gear axle. Nosewheel feedback is sent to the SCU from transducers mounted on the steering actuator. The SCU continuously monitors the NWS. Fault detection results in steering system shutdown. Any faults detected are annunciated as CAS messages. When the steering system is shutdown or disarmed, the SCU reverts the nosewheel steering to free-castering mode.