Nosewheel Steering

This is the BSCU's other half — nosewheel steering (NWS). The BSCU is the brain, green hydraulics the muscle. Per AMM 32-51-00:

The Nose Wheel Steering (NWS) System is a computer controlled electro hydraulic system used to change the direction of the aircraft on the ground. The system uses the Green main hydraulic power system... Two hand wheel transmitters in the cockpit supply the primary steering inputs to the BSCU. The rudder pedals and the autopilot supply secondary steering inputs to the BSCU through the FCPC.

This article covers the system itself — the inputs, the green-hydraulic chain to the actuators, anti-shimmy, and castoring versus towing. The full steering-angle laws, input blending, and angle protection are in Steering Angle Protection.

1. Three inputs — tiller primary, pedals and autopilot secondary

Per AMM 32-51-00:

The primary steering inputs are from two hand wheels... Each hand wheel... can be operated independently or together to get a maximum steering angle of 72 degrees each side of the aircraft centerline. When the ground speed of the aircraft is more than 10 knots the angle of steering available decreases... in proportion to the Steering-Demand/Aircraft Speed Law programmed in the BSCU.

The tillers are the primary input, giving up to 72° (for low-speed sharp turns); the rudder pedals and autopilot come in through the FCPC as secondary inputs, giving only small angles (for high-speed directional control — laws in Steering Angle Protection). A key design — a pushbutton on each tiller isolates the pedal input. Per AMM 32-51-00:

Each hand wheel includes a pushbutton that isolates the system from the steering inputs that come from the rudder pedals. This prevents steering movement during the pre flight rudder check when the aircraft is stationary, which decreases NLG stresses and prevents tire wear.

Why it is needed: during a full-travel rudder check with the aircraft stationary, the pedals would drive the nose wheel; turning the wheel in place wears the tyre and torques the NLG, so holding this pushbutton decouples the pedals from the nose wheel.

 Inputs (3)            Brain            Hydraulic chain (green)              Output
 ──────────            ─────            ───────────────────────             ──────
 tillers ×2 ─prim─┐                    swivel selector 5100GC               turning
 (72° each · >10 kt decay) │ ┌────────┐  (green when NLG down / isolated up)   tube
 pedals/AP ─sec─via FCPC─┼─►│ BSCU    │─►selector valve 5113GC (opens control)─► nose
                        │  │ command  │  servo valve 5112GC (torque motor       wheel
 (tiller pushbutton     │  │ +monitor │   + LVDT; meters pressure to a line)     ↑
  isolates pedals)      │  │ dual ch  │  hydraulic block 6GC:                    │
                        │  └────┬─────┘   · 2 anti-shimmy valves                  │
            RVDT feedback ◄─────┘         · bypass / refilling / accumulator      │
            3GC/4GC (command + monitor)   rotating selector valves ──► steering actuators ×2
                                          0–17°: 1 full bore + 1 annulus          5106/5107GC
                                          17°–max: both full bore                 (turn the turning tube)

2. The green hydraulic chain — selector valve, servo valve, actuators

Green enters through the swivel selector valve that follows NLG retraction. Per AMM 32-51-00:

The swivel selector valve 5100GC: isolates the Green hydraulic pressure from the System when the NLG is retracted... connects the Green hydraulic pressure to the system when the NLG is extended.

Then two valves: the selector valve 5113GC (electrically-controlled ball valve) — the BSCU energises it to release the control pressure on the hydraulic block's control valve, which opens to pass green to the servo valve (it also doubles as a 350 bar relief valve); and the servo valve 5112GC (the core). Per AMM 32-51-00:

The servo valve 5112GC isolates the hydraulic pressure from the steering actuators until a steering input is made. When this occurs, the servo valve: connects the hydraulic pressure supply to one end of each steering actuator to get the necessary steering direction / controls the rate of flow to get the necessary rate of turn / connects the opposite end of each steering actuator to the system return line.

Inside the servo valve is a torque motor (two coils) + jet pipe + spool (with an LVDT reading the spool position): the BSCU supplies current, the spool moves to connect pressure, and the LVDT reports the spool position to the monitor channel.

3. The two actuators — 0–17° versus above 17°

Per AMM 32-51-00:

from 0 to 17 degrees, the full bore end of one steering actuator and the annulus of the other / from 17 degrees to the maximum steering angle, the full bore end of the two steering actuators.

Small angles (0–17°) need one actuator driving on its full bore and the other assisting on its annulus (smaller area, less force); large angles (above 17°, up to the 72° sharp turn) need both actuators on their full bores (large area, large force). Switching by hydraulic area matches "small angle wants precision, large angle wants force".

4. Anti-shimmy — suppressing nose-wheel oscillation

At speed the nose wheel tends to "shimmy" (a self-excited oscillation); two anti-shimmy valves in the hydraulic block suppress it. Per AMM 32-51-00:

two anti-shimmy valves... When a steering movement occurs, they let the hydraulic fluid flow between the steering actuators. They also control the rate of fluid return flow to help prevent nose wheel shimmy... At higher pressures they operate as pressure relief valves to increase the rate of return flow.

Shimmy is the nose wheel being twisted rapidly back and forth by an external force; the anti-shimmy valve adds hydraulic damping to that motion by throttling the return flow — like a door damper that stops the door slamming. Above a certain force it becomes a relief valve to vent more fluid, and a bypass valve equalises the two actuators' pressures. An accumulator (nitrogen-charged at 4.5 bar) prevents actuator cavitation when there is no hydraulic supply.

5. Castoring versus towing

Castoring. Per AMM 32-51-00:

The BSCU de-energizes the selector valve when: the A/SKID & N/W STRG switch is set to OFF / the BSCU cannot control the system (because specified failures have occurred). When this occurs, the steering mechanism can castor.

Castoring cuts active steering and the nose wheel becomes a free caster, steered by differential braking / asymmetric thrust (Steering Faults); fluid flows freely between the actuators through the anti-shimmy and bypass valves.

Towing uses the deactivation box lever. Per AMM 32-51-00:

During towing, a maximum nose wheel angle to the left or right of the aircraft centerline is available. The aircraft can be towed when: the parking brake is set to OFF / the lever is set to TOWING and the safety pin 5115GC is installed.

The difference: castoring is a passive state after a steering failure or shutdown (the nose wheel follows); towing is a deliberate ground action — the lever is set to TOWING and the tug turns the nose wheel to a large angle. The towing lever at TOWING hard-isolates steering control so the system cannot move while the aircraft is towed.

6. Availability conditions and fault indications

Per AMM 32-51-00:

The NWS is available when: The L/G selector lever is in the DOWN position / The Green hydraulic power-supply is available / One of the MLGs is in the ground condition / The L/G bay doors are closed / The ANTI-SKID & N/W STRG switch is ON / One of the ENG MASTER switches is ON / The towing lever is safetied in the flight position / The NLG is on the ground and the aircraft speed is less than 100 kts.

This long list ensures hydraulic steering is given only when it makes sense (landing configuration, on the ground, low speed, an engine running). Note that after a gravity extension the NWS is hydraulically isolated (The NWS system is hydraulically isolated after the operation of the L/G Free Fall System) — a gravity extension gives up green hydraulics, and steering goes with it. Before landing the BSCU also runs its pre-land test (the 0.7° self-test in BSCU Architecture and BITE): 10 seconds after NLG-locked-down, the command channel turns the nose wheel 0.7° each side while the monitor channel watches the RVDT — the result fed to the FMGEC to confirm CAT III B.

Fault indications, per AMM 32-51-00:

If the BSCU cannot control the system: the E/WD shows N/W STRG FAULT / the SD shows the WHEEL page with N.W. STRG (amber)... If the A/SKID & N/W STRG switch is set to OFF: the E/WD shows A/SKID & NWS OFF / the WHEEL page shows NW STEER (amber).

The distinction: N/W STRG FAULT = a system fault (not your doing); A/SKID & NWS OFF = you switched it off — and that switch also turns off antiskid.

[!warning]- Six misconceptions this article corrects (1) The pedals are not the primary steering input — the tillers are (72°); pedals/autopilot are secondary (small angles). (2) Full steering angle is not available at any speed — above 10 kt the tiller angle decays per the speed law. (3) The nose wheel following during a stationary rudder check is not something to accept — hold the tiller pushbutton to isolate the pedals, preventing tyre wear and NLG stress. (4) Castoring and towing are not the same — castoring is the passive free-caster state after a failure/shutdown; towing is a deliberate ground action with the lever at TOWING. (5) Nosewheel steering does not survive a gravity extension — the NWS is hydraulically isolated afterwards. (6) A/SKID & N/W STRG OFF does not only switch off steering — it also switches off antiskid (one switch, two functions).

Self-test

[!note]- Q1. Which input is primary, what is the tiller maximum, and at what speed does the angle start to decay?

The tiller is the primary input, up to 72° each side; the pedals and autopilot (via the FCPC) are secondary, small angles. Above 10 kt ground speed the available tiller angle decays per the Steering-Demand/Aircraft-Speed Law in the BSCU.

[!note]- Q2. How do the two actuators divide the work below and above 17°, and why?

From 0 to 17° one actuator drives on its full bore and the other on its annulus; from 17° to maximum both actuators use their full bores. Small angles want precision (less force, one actuator), large angles want force (large area, both full bores) — switching by hydraulic area.

[!note]- Q3. What is anti-shimmy and how does it work?

Shimmy is a self-excited oscillation of the nose wheel. Two anti-shimmy valves add hydraulic damping by throttling the return flow between the actuators, suppressing the rapid back-and-forth (like a door damper); above a certain pressure they act as relief valves to vent more flow, with a bypass valve equalising the actuators.

[!note]- Q4. What is the difference between castoring and towing?

Castoring is the passive free-caster state when steering is lost (A/SKID & N/W STRG OFF or a BSCU fault) — the nose wheel follows and is steered by differential braking. Towing is a deliberate ground action with the lever set to TOWING and a safety pin installed, hard-isolating steering so the tug can turn the nose wheel to a large angle.

[!note]- Q5. What is the difference between the two NWS fault indications?

N/W STRG FAULT means a system fault the BSCU cannot control (not crew-selected). A/SKID & NWS OFF means the A/SKID & N/W STRG switch was set OFF — which also switches off antiskid, so it loses both functions.

Key takeaways

References

A330 specifics per AMM 32-51-00 (Steering — the three inputs, 72° / 10 kt, tiller pushbutton isolation, the green hydraulic chain through the swivel selector / selector valve / servo valve, the torque-motor servo valve, the 0–17° vs above-17° actuator split, anti-shimmy, castoring, towing, the availability conditions, the fault indications, free-fall isolation) and FCOM DSC-32-20-10 (tiller/pedal steering — the angle-law detail is in Steering Angle Protection). The control-chain diagram is an integrative synthesis of the AMM block diagram and hydraulic-block text. Hydraulic-block valve part-level action is maintenance-layer. The full steering-angle laws, input blending, and angle protection — including the yellow alternate steering source — are in Steering Angle Protection; NWS fault handling is in Steering Faults.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.