Re: aborted takeoff and brake damage

Date:         11 Sep 97 03:35:29 
From:         Steve Lacker <look@the.sig>
Organization: Applied Research Laboratories - The University of Texas at Austin
References:   1
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Peter wrote:
> Questions
> 1: what exactly needs to cool down after such a heavy brake application?
> Is it the hydraulic fluid or the brake discs and pads themselves? I dont
> understand why hot brake surfaces should work less effectively than cold
> ones, unless they melt or something

The coefficient of friction of most materials depends on temperature.
Brakes are less effective when very very hot, but effectiveness will
return when they cool off.  If you read auto magazines, you will find
that a rather abusive test of car brakes is pretty standard fare when
they review an automobile. The testers perform a sequence of high speed
"panic stops". They  then report on how many panic stops the car can
endure before the brake "fade" becomes dangerous. They also typically
publish stopping distances for each of their test stops, and each
successive stop is usually longer than the previous. They typically run
one final test after a complete brake cool-off, and the stopping
distance should be very near the original one indicating that no
permanent damage was done.

> 2: Given the shuddering and groaning of the brakes, what would need to be
> done to them in the way of repairs?

Probably not much. The shuddering and groaning might be due to a
temporary "glazing" of the friction material, which would wear off in a
few normal landings. It could _possibly_ be due to a permanent warping
of the rotors- which would mean they need replacing, but that is less
likely. I

Stephen Lacker
Applied Research Laboratories, The University of Texas at Austin
PO Box 8029, Austin TX 78713-8029