ATA 32 LEARNING PATH
Read the chapter in sequence
01 · STEER-BY-WIRE ARCHITECTURE
WSCS converts steering demand into a controlled wheel angle
The A350 Nose Wheel Steering (NWS) system is steer-by-wire. Cockpit steering demands are electrical inputs, not direct mechanical links to the nose wheels. The Wheel Steering Control System (WSCS) application hosted in CPIOMs calculates the demanded steering angle and monitors the resulting wheel position.
The source describes redundant WSCS sides, each with command and monitoring functions. Side responsibility alternates by flight cycle or changes if the active side becomes unserviceable. This follows the same design philosophy seen elsewhere in the A350: active/standby or command/monitoring partitions are arranged so that faults can be detected rather than remaining dormant.
The NWS control chain can be studied as five blocks: steering input, WSCS computation, hydraulic enable/metering, actuator movement, and RVDT feedback. When troubleshooting, keep those blocks separate.
02 · STEERING INPUTS
Handwheels provide primary ground steering; pedals and autopilot add limited authority
The two cockpit steering handwheels are the primary NWS inputs. Turning a handwheel commands nose-wheel movement in the same direction. The WSCS also receives rudder-pedal and autopilot steering demands through the primary flight and guidance computer path.
The source describes the input demands as being combined and converted into a nose-wheel demand angle. Handwheel authority is much larger than rudder-pedal or autopilot authority because the operational tasks are different: handwheels are used for low-speed taxi maneuvering, while pedal/autopilot steering supports smaller directional corrections.
Each handwheel includes a PEDAL DISC pushbutton. When selected, the WSCS ignores rudder-pedal steering inputs. This is useful during ground checks because the rudder can be exercised without causing unwanted nose-wheel tire scrubbing.
For dispatch configurations covered by the MMEL, the training source also refers to electrical disconnection of the steering handwheel or Steering Disconnect Panel-related maintenance actions. Actual dispatch procedures must always come from current operator-approved data.
03 · LOGIC & CONDITIONS
Steering is available only when the aircraft state supports it
The source identifies several enabling conditions for powered NWS: the aircraft must be on the ground, at least one engine master lever must be ON, the Yellow hydraulic system must be pressurized, and the towing/disconnect condition must not inhibit steering.
Steering authority decreases as ground speed increases. At low speed, handwheel demand can reach the large steering angles required for tight taxi turns, while rudder-pedal/autopilot demand is limited to a much smaller angle. As speed rises, both are progressively reduced.
This speed scheduling is an important safety concept. A steering angle that is useful at walking or taxi speed would be structurally and dynamically inappropriate at high ground speed. Therefore the commanded wheel angle is not a simple fixed ratio to handwheel displacement.
04 · HYDRAULIC ACTUATION
Yellow hydraulic power is enabled, metered and routed through the NWS hydraulic block
Normal NWS operation uses the Yellow hydraulic system. The NWS isolation valve determines whether hydraulic pressure is supplied to the steering system. Downstream, a mechanical shutoff/swivel function and the NWS hydraulic block route pressure to the steering actuators.
The hydraulic block meters flow to the actuator chambers through the steering servo-valve function. The actuator pistons then move through push/pull kinematics to rotate the NLG wheel assembly. The same hydraulic block also provides shimmy damping.
The mechanical shutoff function is linked to NLG extension/retraction state so that the steering circuit is isolated when the gear is not in the operating ground configuration. This is a good example of hydraulic and mechanical interlock being used together.
CRDCs and CPIOM monitoring functions participate in valve electrical control. If the electrical command is lost, the isolation strategy drives the system toward a safe non-powered state rather than leaving uncontrolled steering pressure applied.
05 · CLOSED-LOOP FEEDBACK
Two RVDTs tell WSCS where the nose wheels actually are
The actual nose-wheel angle is measured by two Rotary Variable Differential Transducers (RVDTs). Their signals are acquired through the NWS control architecture and used as steering feedback. The source describes an averaged wheel-angle value being used in the control loop.
This feedback allows WSCS to compare demand with response. If the wheel angle does not follow the command, the control system can identify a disagreement rather than assuming the hydraulic actuator moved correctly.
For engineers, that creates a powerful diagnostic split. If demand changes and hydraulic pressure/servo command are present but RVDT feedback does not move, the problem could be mechanical, hydraulic or feedback-related. If the wheel physically moves but indicated angle is incorrect, attention shifts toward RVDTs, rigging, wiring or signal processing.
06 · CASTORING MODE
When powered steering is unavailable, the nose wheels can follow aircraft motion
If NWS is unavailable because of system failure, hydraulic failure or a towing-disconnect condition, the system can operate in free-castoring mode. The nose wheels then align according to aircraft movement rather than an active steering command.
The source describes residual hydraulic pressure being maintained by an accumulator and return-check arrangement to prevent cavitation in the steering-actuator chambers. Fluid can transfer between actuator sides as the wheels castor.
Shimmy damping remains available in this non-powered condition. That distinction is important: “no powered steering” does not mean the hydraulic block becomes irrelevant. Damping and fluid-management functions still protect the mechanical system.
07 · TOWING & PUSHBACK
The steering system must be deliberately disconnected before external ground equipment controls the nose gear
The Steering Disconnect Panel (SDP) is installed on the NLG and includes the towing switch/pin arrangement used to inhibit powered NWS for towing or pushback. When the towing condition is selected, the actuators are configured for bypass/free-castoring operation and the isolation valve is commanded closed.
A green indication on the SDP confirms the towing-disconnect condition. The cockpit also receives a steering-disconnect status message. The key objective is to prevent the powered steering system from opposing the towbar or towbarless tractor.
The source emphasizes that steering disconnect and reconnect operations must be performed with the aircraft fully stopped; reconnection also requires the correct parking-brake condition. These are operational safety details and should be performed only in accordance with current approved procedures.
Centering cams and towing geometry
The NLG shock absorber includes centering cams used during extension/retraction. If shock-strut extension becomes excessive during towing, the cams can engage at the wrong time and be damaged. The source therefore identifies a towing/pushback shock-strut dimension limit that must be respected.
If centering cams are damaged, the gear may not be able to establish the centered condition needed for retraction. This is a clear example of a ground-handling event creating a later airborne/gear-cycle consequence.
08 · OVERSTEER DETECTION
Excess towing angle is treated as a detectable maintenance event
During towing or pushback, the NLG can approach its mechanical steering limit. The source describes an oversteering-detection system using proximity sensors and deformable brackets to identify excessive angular travel.
An oversteer event can generate cockpit/warning indications, ground-crew indication and ACMS reporting. The mechanical bracket concept also allows evidence of an oversteer event to remain available even when the normal WSCS is not powered.
The maintenance significance is straightforward: an oversteer indication is not merely a message to clear. The NLG requires the applicable inspection and subsequent system checks in accordance with approved data.
09 · MAINTENANCE & RIGGING
Mechanical zero must be established before electrical zero means anything
The training source gives special attention to NWS RVDT zero checks and rigging. Before the electrical output can be judged, the NLG itself must be physically centered within the specified tolerance.
Mechanical centering can be confirmed using NLG zero marks, steering-actuator stroke measurement or the centering-cam condition. Once mechanical zero is established, the OMT/aircraft monitoring functions can be used to read the RVDT output values.
The RVDTs are mechanically adjustable. This means a steering-angle indication problem can be a rigging problem even when the sensor itself is electrically healthy. Correct troubleshooting therefore needs both physical geometry and electrical data.
- Establish mechanical zero. Confirm actual NLG alignment using the applicable approved method.
- Read feedback values. Compare RVDT outputs using the maintenance monitoring function.
- Assess symmetry and tolerance. Determine whether one or both channels disagree with the mechanical position.
- Rig only per approved procedure. Adjustment changes the relationship between physical wheel angle and electrical feedback.
- Re-test the control loop. Verify commanded steering and feedback through the specified maintenance test.
10 · CONTROL & INDICATION
The WHEEL page shows steering availability, not a live nose-wheel angle display
The source notes that the WHEEL System Display page does not show continuous NLG steering movement. Instead, it provides steering-availability/fault status near the NLG representation. A normal available condition is shown in green; an NWS fault condition is shown in amber.
The towing-disconnect condition is also annunciated to the crew, while the SDP provides a ground-level local indication. Oversteer generates its own alerting path. These different indications correspond to different system questions: is powered steering available, is it intentionally disconnected, or has an excessive mechanical angle occurred?
That distinction should be preserved during fault analysis. One indication does not substitute for the others.
END OF ATA 32 SERIES
This completes the AvioScope A350 ATA 32 article sequence
Part 1 established the landing-gear structure, doors, normal extension/retraction, monitoring, GDO and alternate extension. Part 2 followed the wheel and braking systems through normal, degraded and monitoring modes. Part 3 completed the chapter with NWS command, hydraulic actuation, feedback, towing and maintenance logic.
For course development and future long-form video production, these three parts can now be expanded independently without losing the sequence of the full ATA 32 chapter. The recommended learning order remains mechanical structure and movement → braking and condition management → steering and ground handling.
No further “next part” is shown here because this is the final article in the ATA 32 sequence.