Re: aborted takeoff and brake damage

Date:         06 Sep 97 02:55:07 
From:         "matt weber" <mweber@cyberltd.com.au>
Organization: Customer of Access One Pty Ltd, Melbourne, Australia
References:   1
Next article
View raw article
  or MIME structure


> We had to wait at the terminal "for the brakes to cool down"
> for about 20 minutes. I suppose this was to ensure good braking
> efficiency for the next attempt.
> We took off again and the flight proceeded uneventfully to Haneda until
> on taxiing, I heard loud moaning and felt heavy shudders whenever we were
> braking.
>
> 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 application of braking to bring the aircraft to a halt at speed close
to takeoff causes the brakes to dissipate enormous amounts of energy. In
rejected takeoff tests, the brake  disks may be literally 'white hot'. The
regulatory requirement is that they not actually catch fire for 5 minutes.
A simple calculation of the energy in the aircraft if you have to stop
from say 120km/hr, which isn't very fast for takeoff, the energy to be
dissipated is 1/2 the mass of the aircraft times the speed squared. It is
HUGE. The tires have fuse plugs to allow them to go flat if they overheat
from the energy radiated onto them from the brake disks. If the tires were
not fitted with fuse plugs, they would explode. The fuse plugs always
release the air, and tires are almost always destroyed in the FAA mandated
rejected takeoff tests for new airliner certification.

The temperatures are not merely hot, they are extreme. An effort to sustain
a further aborted takeoff without having brought the brakes back to near
normal temperatures would result in catastrophic failure of the brake
disks, and quite probably the wheels as well.  Very simply, if the brakes
are very hot, it is not possible to dissipate the energy into them.

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

Aircraft bakes don't have especially long lives. Depending upon how badly
they have been abused, it may be necessary to replace the disks and other
components. Traditionally brake disks have been steel, but many now use
carbon fiber instead because you can get it as hot, and it weighs a lot
less. Generally one good rejected takeoff operation of the brakes, and the
disks will have to be replaced.