Re: Braking

Date:         09 May 97 03:29:00 
From:         stanleyeo@mailhost.net (Eo Khai Chien Stanley)
Organization: Singapore Telecom Internet Service
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On 30 Apr 97 03:19:10 , "Thomas.Enblom"
<thomas.enblom@era.ericsson.se> wrote:

  I'm not an expert in this field but I hope to share my point of
view.  Please correct me if you see there is an error in my statement.

>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.

  Thrust reversal is use so that the aircraft can be brought to rest
within the available length of existing runways and without the
excessive use of wheel brakes.

>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 agree that the plane uses thrust reversal the first time in Kai
Tak to stop within the short runway.  The reason why it isn't used the
second time in Frankfurt is probably not necessary to use the
technique on the long runway.

>I would imagine that air braking is more gentle to the
>aircraft superstructure. That's why it is used whenever
>possible.

  Braking via wheel subject the undercarriage of the aircraft to
stresses.

>I wonder what parts of the aircraft suffer most stress
>when using the thrust reversers?

  I believe the 'door' used to cover the airfoils used to divert the
exhaust towards the front suffered the most stress.  Depending on
engine rating, the load varies between 4 - 13 tons.

>Are there other rules except for runway length that decides which
>braking technique to use?

  The condition of the runway also decides upon the technique used to
halt the aircraft.  It's safer to stop using thrust reversal than
using only wheel brakes when landing on wet, icy or snow-covered
runways.

>How much runway length is needed for an air braking B747-400 compared
>to using thrust reversers?

  ???

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