Learn how an aircraft anti-skid braking system detects wheel slip, modulates hydraulic pressure, differs from autobrake and behaves on wet runways.
An aircraft anti-skid braking system monitors wheel speed and automatically reduces hydraulic brake pressure when a tyre is about to skid. Once the wheel accelerates towards the aircraft’s ground speed, pressure is reapplied. This rapid modulation preserves steering, protects tyres and usually produces the shortest controllable stop.
In Aviation & Real-World Flying, anti-skid is best understood as a pressure limiter between the pilot’s brake demand and the wheel brakes. It cannot create grip; it extracts as much useful braking as the tyre and runway can provide. Our explanation of how wheels, brakes and ground-sensing systems fit into the landing gear provides the wider mechanical context.
What happens when the pilot applies the brakes?
The system runs a continuous feedback loop for each controlled wheel or brake channel.
- A braking demand is made. Pressure is requested through the toe brakes, an autobrake system or, on some aircraft, another automatic braking mode.
- Brake pressure reaches the wheels. Conventional systems use hydraulic metering and anti-skid valves, while brake-by-wire designs have an electronic controller command the valves directly. The underlying pressure supply is covered in our guide to aircraft hydraulic power and distribution.
- Wheel-speed transducers report rotation. Sensors at the wheels send electrical signals to an anti-skid control unit.
- The controller detects excessive slip. It looks for a wheel decelerating too quickly or rotating significantly slower than a calculated reference speed.
- Pressure is reduced. The appropriate anti-skid valve restricts or releases brake pressure before the tyre reaches a sustained lock.
- Pressure is reapplied. As the wheel recovers speed, the controller restores pressure and repeats the cycle as required.
The reference is not simply indicated airspeed, which is affected by wind and is not a reliable measurement of ground speed. Depending on the design, the controller derives its reference from wheel-speed trends, the fastest valid wheel, acceleration models and other aircraft data. The aim is controlled tyre slip near the point of maximum friction, not a perfectly free-rolling wheel.
What prevents the wheels locking at touchdown?
Touchdown and locked-wheel protection stop brake pressure being applied when a wheel is stationary in the air or has not yet spun up properly.
- Touchdown protection blocks or limits braking until wheel spin-up, valid ground logic or another design-specific condition confirms that the aircraft has landed. This protects a tyre if the pilot is already pressing the pedals.
- Locked-wheel protection releases pressure from a wheel rotating far more slowly than its reference or companion wheel.
- Low-speed cut-out disables anti-skid modulation near walking or taxi speed on many aircraft because wheel-speed signals become too small for reliable control.
The exact logic varies considerably. Some systems use wheel spin-up as the main touchdown cue; others combine it with weight-on-wheels, timing or inertial data. Not every light aircraft has anti-skid, and simpler installations may lack the protections found on transport-category jets.
Is anti-skid the same as autobrake or ABS?
No: anti-skid prevents wheel lock, while autobrake commands a target level of braking or aircraft deceleration.
| System | Main job | Relationship to brake pressure |
|---|---|---|
| Anti-skid | Prevent excessive tyre slip and wheel lock | Overrides or reduces commanded pressure when grip is being exceeded |
| Autobrake | Produce a selected deceleration after landing or during a rejected take-off | Commands braking automatically, but remains subject to anti-skid protection |
| Automotive ABS | Prevent road-wheel lock | Uses the same broad principle, although aircraft logic must also handle touchdown, wheel spin-up and very high braking energy |
An autobrake selection does not guarantee the selected deceleration. If anti-skid repeatedly reduces pressure on a wet or icy runway, the aircraft may decelerate more slowly because available friction is the limiting factor.
When can anti-skid not provide maximum braking?
Anti-skid cannot create tyre-to-runway friction where little or none exists.
- Standing water: A hydroplaning tyre can lose most of its contact with the pavement. Anti-skid may prevent a sustained lock, but braking and steering can still be poor.
- Ice, slush or contamination: The wheels may approach a skid with very little brake torque, forcing the system to keep pressure low.
- Wheel unloading: Bumps, runway undulations or failed ground-spoiler deployment can reduce wheel loading and cause repeated pressure releases.
- Very low speed: Anti-skid effectiveness ends below the system’s low-speed threshold, so a wheel can still scrub or lock during a tight taxi turn.
- Damaged components: A failed wheel-speed sensor, control channel or valve may remove protection from one wheel, a wheel pair or the entire system.
Ground spoilers increase braking capability by removing lift and placing more aircraft weight on the wheels; reverse thrust contributes separately and is not controlled by anti-skid. For a practical example, see how wheel braking, spoilers and reverse thrust affect A320 landing distance.
Should a pilot pump anti-skid brakes?
On an anti-skid-equipped aircraft using normal braking, the pilot generally applies firm, continuous pressure and lets the system perform the modulation.
Pumping the pedals can release pressure just as the controller is trying to restore it, increasing stopping distance. Maximum-effort procedures remain aircraft-specific, however. If anti-skid is unavailable or the aircraft has unprotected brakes, the flight manual may require progressive braking and an immediate reduction in pressure if a skid develops.
What happens if the anti-skid system fails?
An anti-skid fault usually leaves some form of conventional braking available, but protection against locked wheels may be reduced or lost.
A cockpit caution or status indication identifies the fault on suitably equipped aircraft. Depending on the design, the crew may need to limit brake pressure, allow more stopping distance or use a specified alternate-braking procedure. Parking, emergency and alternate brake systems can bypass anti-skid on some types, so functioning brakes do not necessarily mean functioning skid protection.
How is anti-skid represented in flight simulators?
Flight simulators range from a simple wheel-slip calculation to detailed modelling of individual sensors, hydraulic channels, brake temperatures and failure modes.
The mistake we see most often is blaming anti-skid for an input problem. If the brakes activate while moving the yoke, joystick or rudder, check duplicate assignments, noisy axes and combined pedal mappings; our guide to diagnosing brakes linked to flight-control movement covers that fault directly.
For realistic operation, use proportional toe-brake axes where possible, verify that the aircraft’s anti-skid switch is armed or on, and hold steady brake pressure rather than tapping a keyboard command. Simplified aircraft may modulate braking even without a functional cockpit switch, while detailed add-ons may model touchdown protection, channel failures and alternate braking separately.