Re: Braking

Date:         17 May 97 15:15:48 
From:         "DaveHam" <DaveHam@OnAustralia.com.au>
References:   1
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Thomas.Enblom <thomas.enblom@era.ericsson.se> wrote in article
<airliners.1997.1062@ohare.Chicago.COM>...
>
> I noticed when I landed at Kai Tak in a CX B747-400 that the pilot
> used the thrust reversers to slow down the aircraft.
>
> When continuing with CX to Frankfurt the same aircraft this time only
> used air brakes (and probably wheel brakes) in order to slow down.
>
> I believe that the thrust reversers were utilized in Kai Tak due
> to the relatively short runway while Frankfurt's runway is
> long enough for just air braking.
>
> I would imagine that air braking is more gentle to the
> aircraft superstructure. That's why it is used whenever
> possible.
>
> I wonder what parts of the aircraft suffer most stress
> when using the thrust reversers?
>
> Are there other rules except for runway length that decides which
> braking technique to use?
>
> How much runway length is needed for an air braking B747-400 compared
> to using thrust reversers?

It is normal for all landings to use automatic wheel brakes.  These are set
to provide a particular deceleration rate depending on how quickly the
aircraft is to be stopped.  They automatically engage when all main wheels
are firmly on the ground.  Air brakes (or more correctly speed brakes or
spoilers) automatically deploy on all landings and serve the main purpose
of reducing lift from the wings to put weight on the main wheels to assist
braking.  Should the Auto-spoiler system be inoperative the landing
distance increases by 160 m in the 747-400.

Assuming a set deceleration rate is required and modulated by the
auto-brakes, the use of reverse thrust simply relieves some of the work
done by the wheel brakes.  Due to the modern carbon braking systems fitted
on the 747-400 some airlines opt to use only idle-reverse thrust when
possible because normal landing brake loads are negligible compared to what
the brake units are capable of.  Reverse thrust does cause some wear and
tear on the engines.  Some reasons airlines who have this policy would
stipulate the use of reverse thrust would be for short runways (with high
braking loads required), wet or contaminated runways (with reduced wheel
braking potential), fast turn arounds (where high brake temperatures would
not have time to dissipate) and difficult runways.  Kai Tak runway 13 is
still a reasonably long runway (2786m) but because of the unusual approach
the aircraft is harder to setup for landing and more runway may be used
than normal so best play it safe and always use reverse thrust.

The 747-400 is certified for rejected take-offs and all landings without
the use of reverse thrust.  The brakes are so powerful, the aircraft can
accelerate to V1 (the takeoff decision speed where if before it you have an
engine failure you stop) which is usually just short of liftoff speed and
stop on the remaining runway with wheel brakes and spoilers alone.  However
aviation authorities require a major speed retarding device be available in
reserve or the planned landing distance required is to be increased by 15%.
 The reverse thrust is the reserve speed retarding device.  Obviously full
wheel brakes and full reverse thrust will pull the aircraft up much quicker
than brakes alone but operations are not predicated on this.   Normal
landing distance required for 747-400 at an average landing weight (240,000
Kg) on a dry runway at sea level with no wind and all brakes operational is
1780m.  If one engine reverser is inoperative (allowable on the 747) the
distance must be increased by 200m as a precaution but will not necessarily
be used.  This landing distance has a safety factor built in so the
aircraft should pull up in 60% of this distance with full braking and no
reverse thrust.

You will notice on older aircraft with non-carbon brakes most operators
always use reverse thrust regardless.  This is because the cost of brake
maintenance will always outweigh the wear and tear of reverse thrust.

DaveHam