Re: aborted takeoff and brake damage

Date:         08 Sep 97 02:03:40 
From:         Chris Dahler <dahler@iglobal.net>
Organization: gte.net
References:   1
Followups:    1 2
Next article
View raw article
  or MIME structure

> 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

You should see some of the video footage of Boeing doing aborted takeoff
tests on heavy aircraft for the certification process.  The brakes glow
red hot.  It isn't the hydraulic fluid, it is the brake surfaces that
get so hot.  This can produce two major problems: the brakes are not
nearly as effective when so hot, and the tires can blow due to their
close proximity to this heat.  Brakes lose effectiveness when they get
hot because the friction properties of the metal changes.  The metals
get soft, they can lose their symmetry (causing the shuddering you felt
later), and in effect they get polished to a mirror-like smoothness, and
the two surfaces can't grab each other as well (this causes the loud
groaning sound you hear).  If you want to see the effects of this
yourself, go out in your car, accelerate to 60 mph, and do a panic stop
without locking the wheels.  Do this a few times in a row to heat the
brakes up pretty good, and you'll notice the third or fourth time that
your brakes won't work nearly as well as they did the first time (I *am*
kidding here - this works as advertised, but I'm not seriously telling
you to go out and do this!)

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

A quick brake change solves the problem, but while it doesn't take much
time to do this, it's kind of expensive.  It will depend on whether the
brakes have permanently reduced effectiveness (in which case the brakes
must be changed) or whether the noise is just annoying but the brakes
still work fine (in that case, it's more the airline not wanting to
annoy or frighten passengers than a brake change being required).