Re: Thrust reversers

From:         geohull@ditell.com (George Hull)
Organization: DirecTell L.C. - Park City, UT. - 1.801.647.0214
Date:         03 Mar 95 02:27:50 
References:   1
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In article <airliners.1995.207@ohare.Chicago.COM>, lchiluku@ucsd.edu
(Lakshmi N. Chilukuri) wrote:

> Why do we need thrust reversers? The FAA requires aircraft to be able to 
> land safely, without using thrust reversers, even on slippery or icy 
> runways. Common wisdom is that thrust reversers help reduce wear and tear 
> on brakes.
> 
> Does anyone have a feel for the cost of wheel brakes? Is there a move to 
> reduce these costs? Do these costs outweigh the risks associated with 
> potential in-flight deploy of thrust reversers?
> 
> Could a pilot or other expert please enlighten me about an operator's 
> perspective on these matters. This matter should be debate-worthy, since 
> the Lauda Air 767 crash (in Thailand) was initiated by inadvertent thrust 
> reverser deploy.

Airliners are slowed with a combination of techniques, all of which are
typically used during each landing.  Modern aircraft use immediate
deployment of spoilers to add drag and, more importantly, to get the
"weight on the wheels" so anti-skid braking can be immediately effective. 
Thrust reversers are normally used on all landings and their effectiveness
is greatest at the higher speeds immediately following touchdown.  They
must usually be returned to forward thrust at a specified speed to avoid
compressor stalls.  A 757/767 may be landed using auto-spoilers (they
deploy immediately upon wheel spinup), autobrakes (set to varying values
to achieve a smooth deceleration), and reversers deployed manually by the
pilot performing the landing.  Inoperative thrust reversers do not carry a
significant performance penalty, but inop spoilers or anti-skid systems
can potentially make an aircraft unable to "fit" on a given runway.

Wheel brakes are used at the higher speeds upon landing, except when
ambient temperatures make us tend to use them less in order to prevent
overheating of the brakes and wheels.  I think that the manufacturers have
tended to reduce brake wear by tailoring the brake disk compounds rather
than discouraging the use of the brakes.  Brakes and tires are essentially
consumables . . they wear out.

Our company was required to perform the inspections of the reverser
systems which were suspected of causing the Lauda crash.  There was, as I
remember, a problem with a doughnut seal which may have allowed fluid
under pressure to port to the deploy side of the reverser plumbing.  There
are several systems which act to prevent inflight deployment . . we have
an auto-stow system . . but apparently the Lauda accident proved that the
system wasn't fail-safe.