Steering Angle Protection

Article 16 covered how the NWS works; this article covers the rules for using it — how the angle changes with speed (the angle law), how much the tiller and the pedals each command, a yellow-hydraulic alternate steering many do not know about (ALTN N/W STRG), and the 93° towing oversteer protection and its memos.

The core rule is one sentence: steering authority changes with speed — the faster you go, the smaller the available angle. Per FCOM DSC-32-20-20:

Nosewheel steering control authority depends on aircraft speed. Rudder pedals provide nosewheel steering control when: The speed is below 100 kt at landing, or Up to 150 kt for takeoff. Note: The nosewheel steering angle depends on aircraft speed, and will return to zero if the aircraft is above these speeds.

1. The angle law — faster means less

 angle (° each side)
  72 ┤█ tiller (handwheel)
     │█▉  available below 100 kt, max 72°
     │█ ▉▉   decays above 10 kt per the Steering-Demand/Aircraft-Speed Law
     │█   ▉▉▉▉
   6 ┤█       ▉▉▉▉▉▉── rudder pedal ──┐
     │█  landing: touchdown~100 kt = 0 → 100–40 kt rising → 6° at 40 kt → 40–0 kt holds 6°
     │█  take-off: 0–100 kt max 6° → 100–150 kt decreasing → 0 at 150 kt
   0 ┼█──────┬────────┬────────┬────────┬──► ground speed (kt)
     0       10       40      100      150
       tiller-led (low speed, large angle)   pedal-led (high speed, small angle)

 ALTN N/W STRG (yellow hydraulics): ground only, inhibited above 70 kt — the back-up when green is lost
 oversteer protection: towing beyond 93° → latching red light (cockpit + NLG)

2. The tiller — low speed, large angle

Per FCOM DSC-32-20-20:

The steering handwheels provide nosewheel steering control below 100 kt, and up to 72 degrees left or right.

The tiller is the workhorse for low-speed large angles (vacating, turning round, taxi after pushback). The full 72° is given only at very low speed; as article 16 noted, above 10 kt the angle decays per the speed law — the faster you go, the smaller the tiller angle, preventing a high-speed large-angle turn from rolling the aircraft over or bursting a tyre.

3. The rudder pedal — high speed, small angle

The pedals handle directional trim at higher speed, to a maximum of 6°, with different landing and take-off laws. Per AMM 32-51-00:

(1) Landing: From touch down to 100 kts there is zero degrees... From 100 to 40 kts it increases in proportion up to 6 degrees maximum at 40 kts each side... From 40 down to zero kts the steering movement remains at 6 degrees maximum. (2) Take-off: From zero to 100 kts steering inputs can give a maximum of 6 degrees... When the ground speed is more than 100 kts the steering movement decreases to zero... up to 150 kts.

The design intent:

• Landing: just after touchdown (above 100 kt) the pedals give no steering (to avoid a high-speed input error), and the aerodynamic rudder holds direction; below 100 kt pedal steering is gradually released (rudder authority is fading and the nose wheel takes over), reaching the full 6° below 40 kt.
• Take-off: at low speed (below 100 kt) the pedals give 6° to line up on the runway; as speed builds (above 100 kt) it hands over to the aerodynamic rudder and the nose-wheel steering withdraws (zero at 150 kt), so there is no high-speed nose-wheel steering to go wrong.

The tiller pushbutton (RUDDER PEDAL DISC). Per FCOM DSC-32-20-20: When maintained depressed, the nosewheel steering control by the pedal is disconnected. Used for a ground rudder check, or when a landing needs pure aerodynamic rudder, it decouples the pedals from the nose wheel.

4. ALTN N/W STRG — alternate steering on yellow hydraulics

This is the key point, and it corrects a common belief that "yellow plays no part in the landing gear". It does. Per FCOM DSC-32-20-20:

(2) ALTN N/W STRG pb (guarded pushbutton to be used on ground only) ON: The Yellow hydraulic system supplies the nosewheel steering. This function and the ON light are inhibited when the aircraft speed is above 70 kt.

So:

• Nosewheel steering actually has two hydraulic sources — green (normal, article 16) and yellow (alternate, ALTN N/W STRG) — structurally like braking's green-normal / blue-alternate.
• Ground-only, inhibited above 70 kt: the alternate steering is only a backstop for ground taxi/vacating; it is not allowed at higher speed.
• This completes the three-hydraulic division of labour in the landing gear: green = normal extension/retraction + normal braking + normal steering; blue = alternate braking + parking-brake accumulator; yellow = alternate nosewheel steering (ALTN N/W STRG). Yellow is not absent — it is the steering back-up.

(The AMM does not have a separate description-and-operation section for ALTN N/W STRG; its yellow valve path is not detailed in the source dump, so this rests on the FCOM pilot-level statement.)

5. The A/SKID & N/W STRG switch — one switch, two functions

Per FCOM DSC-32-20-20:

(1) A/SKD & N/W STRG sw: An ON/OFF switch activates or deactivates the Nosewheel Steering and Anti-Skid.

As article 16 stressed, this switch controls both steering and antiskid — turning it OFF loses both the nose-wheel steering and antiskid. An easily under-rated switch.

6. The 93° towing oversteer protection

The tiller maximum is 72°, but a tug during towing could turn the nose wheel further and damage the steering, so a 93° oversteer protection exists. Per AMM 32-53-00:

The NLG Steering Angle Protection System gives a visual warning of excess angular travel of the NLG steering during towing operations... which operates at more than 93 degrees... Two warning lights, one on the NLG and one on the flight deck, come on.

And from the FCOM side, per FCOM DSC-32-20-20:

FAULT: On ground, it comes on red when the nosewheel steering has exceeded 93°. It is associated with the illumination of the red oversteer warning light, located on the nose landing gear. When pressed, the light goes out.

The mechanism: a proximity sensor on each side of the NLG plus a target on the upper torque link; an over-large angle brings the target near a sensor and, through a latching relay, lights the red lights. Two key design points:

• Latching: once triggered it latches until reset — so even if the nose wheel is turned back, the evidence that it was once over-angle (and possibly damaged) remains.
• Reset only with aircraft power (The circuit can only be reset with the a/c power supplies connected) — tug power cannot reset it. Meaning: an over-angle means possible damage, so the warning must be cleared with aircraft power and a maintenance check, not casually by the tug driver. Power is normally from BUS 1; if BUS 1 is lost, the hot bus feeds it through the PARK BRK selector switch contacts — so the protection stays live even with the aircraft powered down.

7. The N/WS DISC and N/W STRG indications

Per FCOM DSC-32-20-20:

N/WS DISC: This memo appears in green if the nose wheel steering lever is in the towing position. N/WS DISC: This memo appears in amber if one engine is running.

The colours: green N/WS DISC = the steering lever is in the towing position (steering disconnected) — a normal ground-towing state, just an advisory. Amber N/WS DISC = steering disconnected and an engine running — a dangerous combination (engine thrust plus an uncontrolled nose wheel could run away), so it is escalated to amber. Towing should have no engine running, and this amber catches the contradictory "disconnected but running" state.

And per FCOM DSC-32-20-20: N/W STRG indication appears amber in case of: Nose wheel steering failure detected by the BSCU / A/SKID & N/W STRG sw is OFF / Failure of both BSCU channels.

[!warning]- Six misconceptions this article corrects (1) Yellow hydraulics is not absent from the landing gear — it drives ALTN N/W STRG (alternate nosewheel steering), ground-only. (2) Full steering angle is not available at any speed — it decays per the speed law (the tiller above 10 kt, the pedals to zero by 150 kt on take-off). (3) Pedal steering is not available the instant you touch down — from touchdown to 100 kt there is zero, released gradually below 100 kt. (4) The tiller does not turn 93° — its maximum is 72°; 93° is the towing oversteer threshold. (5) The oversteer warning can not be reset by the tug — only with aircraft power (so a possible-damage warning is not casually cleared). (6) Green and amber N/WS DISC are not the same — green is a normal towing-position memo; amber is the dangerous "disconnected + an engine running" combination.

Self-test

[!note]- Q1. What are the tiller and pedal maximum angles and speed limits, and why does the angle shrink with speed?

The tiller gives up to 72° below 100 kt (decaying above 10 kt); the pedals give up to 6° (landing below 100 kt, take-off to 150 kt). The angle shrinks with speed to prevent a high-speed large-angle turn from rolling the aircraft or bursting a tyre — at higher speed the aerodynamic rudder holds direction instead.

[!note]- Q2. Which hydraulic system supplies ALTN N/W STRG, and what are its limits?

The yellow hydraulic system. It is for ground use only and is inhibited above 70 kt. This means yellow does take part in the landing gear — as the alternate nosewheel-steering source — completing the green/blue/yellow division (green normal, blue braking back-up, yellow steering back-up).

[!note]- Q3. Give the landing pedal-steering law.

From touchdown to 100 kt, zero degrees; from 100 to 40 kt, increasing in proportion to a maximum of 6° at 40 kt; from 40 to 0 kt, holding 6°. The aerodynamic rudder holds direction above 100 kt; the nose wheel takes over as rudder authority fades below 100 kt.

[!note]- Q4. How is the 93° protection implemented, and why can it only be reset with aircraft power?

A proximity sensor each side of the NLG plus a target on the upper torque link; an over-angle lights latching red lights (cockpit and NLG). It resets only with aircraft power so that a possible-damage warning is not casually cleared by the tug driver — it must be acknowledged with aircraft power and a maintenance check.

[!note]- Q5. What is the difference between green and amber N/WS DISC?

Green N/WS DISC means the steering lever is in the towing position (a normal towing memo). Amber N/WS DISC means steering is disconnected and an engine is running — a dangerous combination (towing should have no engine running), so it is escalated to amber.

Key takeaways

References

A330 specifics per FCOM DSC-32-20-20 (Nose Wheel Steering controls and indicators — speed-dependent authority, tiller 72° below 100 kt, pedals landing below 100 kt / take-off below 150 kt, RUDDER PEDAL DISC, the ALTN N/W STRG yellow source below 70 kt, the A/SKID & N/W STRG switch, the 93° FAULT, the green/amber N/WS DISC, the amber N/W STRG indication), AMM 32-51-00 (the precise pedal-steering laws and the tiller 72° / above-10-kt decay), and AMM 32-53-00 (NLG Steering Angle Protection — 93° oversteer, the proximity sensors and torque-link target, latching, aircraft-power-only reset, BUS 1 / hot-bus supply through the PARK BRK switch). The angle-law diagram is an integrative synthesis of the FCOM and AMM values. The AMM mechanism of the yellow ALTN N/W STRG valve path is not in the source dump, so that rests on the FCOM pilot-level statement; the continuous speed-law function between the given end points is not stated point-by-point and is not extrapolated. NWS fault taxiing 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.