Anti skid

[Bubesi]

How do modern anti-skid operate? Do they compare a rapid, wheel, slowdown to another wheel? or do they compare braking to a computer model?

Thanks

[vapilot2004]

Aircraft anti-skid systems use sensors to measure spinnng landing gear wheel speeds. When a fast reduction in wheel speed is detected, indicating a skid, the system interrupts the hydraulic brake pressure to that wheel allowing it to regain traction with the ground. The interruption of pressure occurs at a rate of several times a second and results in a reduction of applied pressure to the associated disk calipers. ****The wheels speeds are individually evaluated.

****Clarification below.

[Bubesi]

So there is no comparison, just a fast slowdown detected.
Merci;)

[Rainboe]

When a fast reduction in wheel speed is detected, indicating a skid,
The way the system knows there is an incipient skid is to actually compare wheel speed, or pair of wheels speed in some systems, with other wheels on the bogie or other undercarriage leg, and repeatedly back off braking until the speed difference is zero. No fancy computer decceleration models are used, or input from INS groundspeed.

[vapilot2004]

The way the system knows there is an incipient skid is to actually compare wheel speed, or pair of wheels speed in some systems, with other wheels on the bogie or other undercarriage leg, and repeatedly back off braking until the speed difference is zero. No fancy computer decceleration models are used, or input from INS groundspeed.

I over simplified the function of anti-skid and there is more to it than a locked wheel function and systems vary by manufacturer. Rainboe, you are correctly referring to paired wheel monitoring which can also be used as an control criteria.

Some Airbus systems do use an ADIRU input to calculate a target wheel speed and will keep the wheels spinning above a certain percentage of that rate. (Computer model method) Deceleration is limited to a predetermined value. :ok:

[Piper19]

mmm, I tend to remember from the 757 and a300 course that these aircraft DID use aircraft groundspeed. The anti-skid computer is the main thing in the system. It gets 8 input signals from all the main wheels. This signal is detected by a 30-40cm long probe in the center of the wheel axle, and the probe measures the wheel speed in revolutions per time frame. The anti skid computer calculates this into velocity. A 9th signal that goes into the computer is aircraft ground speed (from GPS, INS, computer...). When wheel speed is slower than ground speed, that means this wheel is skidding. Then the computer sends a signal to an anti skid valve that shuts off hydraulic supply to the brake/wheel skidding, even when the pilot presses the brakes. When wheel speed is the same as ground speed again, braking is normal.
That's how I learned it, if you say it doesn't use INS you can be totaly right.