Introduction:

The intent of this Troubleshooting Tip is to assist in the troubleshooting of the Challenger 300 Nosewheel Steering System (NWSS).

It is not a substitute for the use of the Aircraft Maintenance Manual (AMM). Always refer to the AMM for all work carried out on the aircraft.

System Description:	ATA:
NOSEWHEEL STEERING SYSTEM	32-51-00

The entry point to this Troubleshooting Tip is the NWSS related EICAS message or observed NWSS abnormal condition. Simply go to the appropriate message and follow the troubleshooting instructions.

• NWS FAIL
  (Caution)
• NWS LIMIT EXCEEDED
  (Caution)
• NWS FAULT
  (Advisory)
• Observed Fault: 325OBS002 - AIRCRAFT PULLS TO ONE SIDE

The SCU provides external indication of a fault condition in two ways:

Troubleshooting for the Challenger 350 Nosewheel Steering System (NWSS) is done using the

1. Codes displayed on the Steering Control Unit (SCU) front panel 7-segment display
   OR
2. By accessing the internal fault log via RS-232 Communication Interface.

Each code represents either an SCU status or an NWSS LRU to which a fault has been isolated.

SCU Fault Code	LRU
'0'	No Faults
'1'	Steering Control Unit (SCU)
'2'	Steering Selector Valve (SSV)
'3'	Electro-Hydraulic Servo Valve (EHSV)
'4'	Handwheel (HW)
'5'	Feedback RVT
'6'	Rudder Pedal Transducer (RP)
'7'	Rigging
'8'	Pilot-Initiated Built-In-Test (PBIT) in Progress
'A'	Maintenance/rigging Mode
'U'	Beyond Operational Range

Steering Control Unit (SCU):

The SCU provides electro-hydraulic steering control of the nosewheel landing gear based on pilot input commands. The health of the NWSS LRUs is continuously monitored by the Continuous Built-In-Test (CBIT). The health of the internal fault monitors is tested during the Power-Up Built-In-Test (PBIT).

In the event of a failure condition, the SCU removes hydraulic power from the Nosewheel Steering System allowing the nosewheel to free castor and activates an amber EICAS caution message in the cockpit to alert the pilot of a NWSS failure.

Steering Selector Valve (SSV):

The SCU controls the availability of hydraulic fluid to the NWSS through control of the SSV. When the SSV is energized hydraulic pressure is applied to the NWSS allowing direct pilot control of nosewheel position. When the SSV is de-energized no hydraulic pressure is applied, allowing the nosewheel to free castor.

Electro Hydraulic Servo Valve (EHSV):

Part of the NWS manifold, the EHSV provides controlled flow of hydraulic fluid to the steering actuators. The SCU provides a controlled current (based on the pilot command and the position of the nosewheel) to a torque motor controlling the movement of a spool within the EHSV. A Linear Variable Differential Transformer (LVDT) provides a feedback of the spool position.

Handwheel:

The handwheel provides a mechanical link to two potentiometers that are mounted on a common rotational shaft. The potentiometers provide two electrical signals proportional to the angular displacement of the handwheel. The first electrical handwheel position signal is used by the SCU control loop as a nosewheel steering command. The second handwheel position signal is used for monitoring purposes.

Rudder Pedal Transducer:

The rudder pedals are linked to three potentiometers mounted to a common rotational shaft. The potentiometers provide three electrical signals proportional to the angular displacement of the rudder pedals. The first rudder pedal position signal is used by the SCU control loop as a nosewheel steering command. The second signal si used for monitoring purpose. The third signal is used by the Flight Data Recorder (FDR).

Nosewheel Position  Feedback RVT:

The position feedback transducer is a dual Rotary Variable Transformer (RVT). The dual RVT connects electrically to two channels of the SCU and the common shaft connects to the follow-up and feedback mechanism. The first signal provides input on the action and position taken by the nose wheels. The second signal provides matching information for monitoring purposes.