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I was reading about slip ratio controlled anti skid system and had this one particular question in my mind.

In addition to the wheel speed data, slip ratio controlled systems need a ground speed reference to estimate and control slip ratio. Going through the relevant literature, I came across multiple methods (like unbraked nose wheel speed/ IRS etc.) for this. Which among these is used in modern aircraft?

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    $\begingroup$ The shortest answer is probably just the Inertial Reference System (IRS) sensors. But that made me think that, for braking, "ground speed" isn't as important as "acceleration/deceleration" forces. $\endgroup$
    – MarkHu
    Apr 13, 2022 at 20:46
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    $\begingroup$ @MarkHu, this is not true. To maximizing braking, you want to prevent skidding. This means that you want to have the wheel rotation equal to the ground speed. In the extreme case of skidding your wheel is not rotating, but you have a ground speed (and there is no effective braking). Comparing the two speeds gives much better control than responding to the measured deceleration. $\endgroup$
    – ROIMaison
    Apr 14, 2022 at 8:28

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On the Airbus A320, the ADIRU (Air Data Inertial Reference Unit) data is used for the reference speed of the anti-skid system:

The system compares the speed of each main gear wheel (given by a tachometer) with the speed of the aircraft (reference speed). When the speed of a wheel drops below 0.87 times the reference speed, the system orders brake releasing in order to maintain the brake slip at that value (best braking efficiency).

In normal operation, the BSCU determines the reference speed from the horizontal acceleration furnished by ADIRU1, ADIRU2, or ADIRU3.

If all three ADIRUs fail, the reference speed equals the greater of either main landing gear wheel speed. Deceleration is limited to 1.7 meters/second² (5.6 feet/second²).

(Airbus A320 FCOM - Landing Gear - Brakes and Anti-Skid - Principle)

The Boeing 737 FCOM does not contain all the details of the antiskid system. However, this presentation confirms that ADIRU data is also used on the 737:

Antiskid System

The antiskid system monitors wheel deceleration and controls the brake metered pressure to prevent skid conditions. These are the antiskid functions:

  • Skid control operates at more than eight knots to control each wheel deceleration during normal antiskid and both wheels on each main landing gear during alternate antiskid.
  • Locked wheel protection compares wheel speeds more than 25 knots difference between the two inboard or the two outboard wheels and releases brake pressure from the slower wheel. [...]
  • Hydroplane protection decreases wheel brake pressure to wheel 1 and 3 when ground speed is more than wheel speed uses the ADIRU ground speed.
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The answers would be incomplete without mentioning the 'original' ABS. It was used very widely on civil and military aircraft alike throughout the world until at least the 1970s.

The system employed a flywheel, connected to the wheel through a one-way clutch. When the wheel turns forward, it spins up the flywheel. If the wheel locks, the flywheel 'overtakes' it, spinning faster than the wheel. This releases the brakes. The system is completely self-contained; each braking wheel could have one.

In this system, the 'reference' is the recent speed of the same wheel. Of course, this assumes that the wheel must spin up before applying brakes. This is sometimes difficult to ensure, esp. on a wet runway.

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Merely complementing @Bianfable's answer:

The question mentioned using a reference speed from the unbraked nose wheel.

This was used by early jetliners (e.g. McDonnell Douglas DC-9). An issue with that, and therefore the move toward inertial measurements, is the aquaplaning (lifting) of the nose wheel, which will require slowing down to bring it in contact with the pavement for the reference speed to be of use. This consequently affects the certified stopping distances.


Horne, Walter B., John L. McCarty, and John A. Tanner. Some effects of adverse weather conditions on performance of airplane antiskid braking systems. No. NASA-TN-D-8202. 1976. p. 4. https://ntrs.nasa.gov/citations/19760019083

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Just for historical reference, the Space Shuttle antiskid system didn't use a ground speed reference, just the speed of the other main gear wheels.

Each main landing gear wheel has two speed sensors that supply wheel rotational velocity information to the skid control circuits in the brake/skid control boxes. The velocity of each wheel is continuously compared to the average wheel velocity of all four wheels. Whenever the wheel velocity of one wheel is 60 percent below the average velocity of the four wheels, skid control removes brake pressure from the slow wheel until the velocity of that wheel increases to an acceptable range.

Source: Shuttle Crew Operations Manual page 2-14.7

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