Posts: 111
Joined: Mon May 22, 2000 7:01 am


Thu Aug 31, 2000 4:03 am

This might be a stupid question but. How does the Pilot know when to retract the flaps after takeoff,Is it speed or altitude?

Posts: 3094
Joined: Tue May 18, 1999 6:40 am

RE: Flaps

Thu Aug 31, 2000 4:36 am

Both speed and height above the ground determine when the flaps will be retracted.

After rotation on a 727 for example the pilot will fly at V2+10k until he reaches acceleration height which varies according to company policy but is usually 1000 feet AGL unless there are second segement obstuctions which may dictate this altitude to be higher. This info can be found in the runway analysis.

Once the acceleration height has been reached the airplane is then accelerated to the minimum manouvering speed for the next flap setting to be selected which varies with aircraft weight.


Posts: 6409
Joined: Tue Mar 21, 2000 6:25 am

RE: Flaps

Thu Aug 31, 2000 4:45 am

Oh no, not stupid question at all. But a lot of factors are involved.
For most efficient flight the plane would be accellerated as fast as possible so the flaps and slots can be cleaned up as soon as possible. When? it depends on weight - pax- and fuel load.
But very often that is rather irrelevant because the flight profile is dictated by ATC - getting out of the way for the next take-off. Or by passing over "noise sensitive areas". Sometimes a plane will climb fast at slow speed in the beginning, then continue slowly at reduced power over a noise sensitive area, then climb steeply again, continue slowly over another sensitive area, and then finally speed up and clean the wings. That's one of the reasons why flight deck crews must train on simms and read a lot of "books".
But the easy answer to your question is that at any given take off weight he will know an "indicated minimum airspeed" at which he will retract flaps. Indicated airspeed is at sea level identical to true airspeed, but at any higher altitude it is a higher true airspeed compensating exactly for the reduced reaction on the wing of the thinner air. The indicated airspeed can be read on the instrument panel. It's of course also a very important figure when programming the autopilot.
Hope that helped.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs
Posts: 111
Joined: Mon May 22, 2000 7:01 am

RE: Flaps

Fri Sep 01, 2000 3:41 am

Thanks for your answers guys it helped alot  


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