Re: 757 flap restriction

Date:         01 Jul 98 02:42:43 
From:         Kees de LezenneCoulander <>
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"Yves E. Hoebeke" <> wrote:

>Anyway, the aircraft manual states not to use flaps above FL200. I
>believe the B737 has the same restriction.
>Unfortunately, I can not get anyone to give me a clear answer on why the
>restriction exists.

     Sometimes a limitation is caused by nothing more sinister than the
following reasoning: flaps are only needed for take-off and landing =>
flaps have only been tested below 20000 ft => use of flaps is only
permitted below 20000 ft.
     In this case, there are some fundamental reasons for not using flaps
at high altitude. It is not so much the altitude itself which is the
problem, but rather the higher Mach numbers that go with it. For the same
indicated airspeed, the corresponding Mach number increases with altitude.
I do not know what the actual flap limiting speeds are for the aircraft you
mention, but let us use 200 kts as an example. At Sea-level this equates to
Mach 0.302, at 10000 ft to Mach 0.365, at 20000 ft to Mach 0.446 and at
30000 ft to 0.555.
     As flaps increase wing camber and therefore cause higher local
airspeeds around the profile, you definitely do not want to have flaps down
at the higher Mach numbers because of compressibility effects.
               Kees de Lezenne Coulander
  C.M. de Lezenne Coulander
  The Netherlands                    E-mail:

  Aircraft Development and Systems Engineering B.V.
  Schiphol-Rijk, The Netherlands