Anti skid
Joined: Aug 2005
Posts: 1,693
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From: fairly close to the colonial capitol
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.
****Clarification below.
Last edited by vapilot2004; 10th January 2007 at 17:39.
Warning Toxic!
Disgusted of Tunbridge
Disgusted of Tunbridge
Joined: Jan 2005
Posts: 4,011
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From: Hampshire, UK
When a fast reduction in wheel speed is detected, indicating a skid,
Joined: Aug 2005
Posts: 1,693
Likes: 0
From: fairly close to the colonial capitol
Clarification
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.
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.
Joined: Dec 2006
Posts: 236
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From: belgium
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.
That's how I learned it, if you say it doesn't use INS you can be totaly right.
