Antiskid

Antiskid is the most real-time of the BSCU's four functions. Its core idea is one sentence: a wheel about to be braked to a stop — about to skid — is given a little less brake, allowed to spin back up, then braked again — so the tyre sits near its maximum-friction point throughout: braking hardest while never locking up and bursting the tyre. Per FCOM DSC-32-30-10:

The antiskid system provides maximum braking efficiency by maintaining the wheels at the limit of an impending skid.

This article builds on the servovalve states from Normal Braking — antiskid is the maximum-current-releases state at work.

1. The A/SKID principle — wheel speed against a reference

Per FCOM DSC-32-30-10:

The A/SKID system compares the speed of each MLG wheel... given by a tachometer with the aircraft speed called reference speed.

So each main wheel has a tachometer, and the BSCU compares that wheel's speed against the reference speed (the aircraft speed). A wheel turning much slower than the reference is being braked into a skid. The trigger is a precise fraction. Per FCOM DSC-32-30-10:

When the speed of a wheel decreases below 0.88 time the reference speed, brake release orders are given to maintain the wheel speed at the value (best braking efficiency).

So below 0.88 × the reference speed, the brake on that wheel is released to bring its speed back up — holding each wheel at the slip that gives the best braking, the "limit of an impending skid".

2. Where wheel speed comes from, and the ADIRU-failed reference

The wheel speed is the tachometer in each main-gear axle (the hollow bogie axle of Main Landing Gear houses it), one per main wheel, so the BSCU knows the speed of each main wheel individually. The reference speed is normally the aircraft speed; only on a total inertial-reference loss does it fall back to the wheels. Per FCOM DSC-32-30-10:

In the case all ADIRUs are failed, reference speed equals the maximum of either landing gear wheel speeds.

Why the maximum wheel speed as the fallback: a skidding wheel slows down, so the fastest wheel is the closest remaining estimate of true ground speed. This is the gear's dependence on the ADIRS (the normal reference) made visible — covered further in the System Interface Map.

3. How it releases — back to the servovalve states

The release is the third state in the Normal Braking servovalve table: driving the servovalve current to its maximum flips the slide valve to connect the brake line to return, so pressure drops to zero (released). Per FCOM DSC-32-30-10, at skid start the brake-release orders are sent to:

Normal servovalves / Alternate servovalves / ECAM system which displays the released brakes.

So the same release principle applies whether braking is normal or alternate, and the ECAM shows which brakes are released. Counter-intuitively, increasing the current is what releases the brake — on detecting a skid the BSCU slams the current to maximum to release that wheel, the wheel spins back up out of the skid, then the current is restored to re-apply. Each wheel independently, in a high-frequency loop, keeps every wheel near its maximum-friction point.

4. Low-speed deactivation

Per FCOM DSC-32-30-10:

The antiskid system is deactivated below 20 kt (ground speed).

Below 20 kt the tachometer signal weakens, the reference-speed estimate is less accurate, and the risk and consequence of a momentary lock are both small. Below this speed there is no antiskid protection — so brake gently at low speed; the pilot is now responsible for avoiding a lock. The antiskid is enabled or disabled by the A/SKID & N/W STRG switch (per FCOM DSC-32-30-10, An ON/OFF switch activates or deactivates the antiskid system) — and note that switch also controls nosewheel steering, so OFF loses both.

Without antiskid (the switch OFF, or a failure), braking drops to the alternate-without-antiskid mode (see Alternate Braking), ANTI SKID shows amber, and the pilot must prevent a lock manually — cadence braking, gentle, watching the brake-pressure gauge. On a wet/contaminated runway the antiskid cycles frequently and the deceleration can feel weak; that is the antiskid working normally, not a failure (an important point for Brake Failure and Degradation).

[!warning]- Five misconceptions this article corrects (1) Antiskid does not judge a skid from a measured "ground speed sensor on a wheel" — it compares each wheel's tachometer speed against the reference (aircraft) speed, releasing below 0.88 × the reference. (2) Releasing the brake is not done by reducing current — it is done by driving the servovalve current to its maximum, flipping it to return (the third state). (3) The reference speed is not "the highest wheel speed" in normal operation — it is the aircraft speed; only if all ADIRUs fail does it fall back to the maximum landing-gear wheel speed. (4) Antiskid is not active at any speed — it is deactivated below 20 kt ground speed, after which a manual lock must be avoided. (5) Antiskid cycling on a wet runway is not a brake fault — it is normal work (brief releases to avoid a lock), and applying LOSS OF BRAKING there would worsen performance.

Self-test

[!note]- Q1. State the A/SKID principle and the release threshold.

The A/SKID system compares each MLG wheel's tachometer speed against the aircraft speed (the reference speed). When a wheel's speed falls below 0.88 × the reference speed, brake-release orders are given to bring it back up — maintaining each wheel at the limit of an impending skid for best braking efficiency.

[!note]- Q2. What is the reference speed normally, and what is the fallback if all ADIRUs fail?

Normally the reference speed is the aircraft speed. If all ADIRUs are failed, the reference speed equals the maximum of the landing-gear wheel speeds — the fastest wheel being the closest remaining estimate of true ground speed (a skidding wheel slows down).

[!note]- Q3. Where do the brake-release orders go at skid start, and why does increasing the servovalve current release the brake?

To the normal servovalves, the alternate servovalves, and the ECAM (which displays the released brakes). Increasing the current to its maximum drives the slide valve to connect the brake line to return, so the pressure drops to zero — exactly what is needed to release a skidding wheel.

[!note]- Q4. At what speed is antiskid deactivated, and what protects against a lock below it?

Below 20 kt ground speed. Below it there is no antiskid, so the pilot brakes gently to avoid a lock — there is no automatic backstop at low speed.

[!note]- Q5. On a wet runway the deceleration feels weak and the brakes seem to cycle. Is this a fault?

No — that is the antiskid working normally, briefly releasing the brakes to avoid a lock on the slippery surface. Applying LOSS OF BRAKING here (switching antiskid off) would make a lock more likely and worsen the landing performance.

Key takeaways

References

A330 specifics per FCOM DSC-32-30-10 (the A/SKID principle — maximum braking efficiency at the limit of an impending skid, each MLG wheel's tachometer speed compared against the aircraft reference speed, brake release below 0.88 × the reference speed, release orders to the normal/alternate servovalves and the ECAM, deactivation below 20 kt ground speed, the all-ADIRU-failed fallback to maximum wheel speed) and AMM 32-40-00 (the BSCU controlling antiskid by comparing wheel speeds with aircraft speed and releasing a skidding wheel). The servovalve maximum-current-releases mechanism is verbatim in AMM 32-42-00 (see Normal Braking). The detailed slip-control algorithm and the tachometer signal-conditioning are maintenance-layer and not stated. Antiskid under alternate braking (blue hydraulics, same release principle) is in Alternate Braking.

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.