B737-Flap extension altitude limit

[BMM389EC]

Any suggestions on why you can't take flap on the B737(200) above FL200?

[Skyjob]

Interesting you should ask, this question has just been answered in the Bluecoat internet forum. Unfortunately I deleted most of the postings, but this is what I can remember.

Due to the characteristics of air (compression, behaviour as a fluid, etc) the flight envelope with flaps extended is only tested in those regions where flap extension is likely to happen (ie below 20,000'). Above this level the characteristics of air are such that it could cause changes the behaviour of the aircraft. Someone was saying the extension of flaps at high altitude could have a negative effect on maintain control of the aircraft. (Leading edge devices are said to be inhibited for the same reason above 20,000'.)

[BOAC]

That is what I understand - it is a certification matter i.e. 'not tested above' as with 'demonstrated crosswinds' which then become limits.

2 possible problems are

1) Adverse effect from changed downwash angle (higher TAS)
2) Possible compressibility problems

[Max Angle]

It is simply that Boeing could not think of a flight procedure that required flaps above 20000ft so simply did not bother testing it. It thus, as BOAC says, became an AFM limit. Same on all Boeings I think and certainly on the Airbus range.

[ 20 July 2001: Message edited by: Max Angle ]

[Shore Guy]

Is Hoot Gibson on this forum? Perhaps he has an answer. (Remember the crew who attempted to extend the trailing edge devices at altitude?).

[waypoint5944]

Hi!
Two indications appear wnen climbing thru FL200 are the min manuvering spd increse and disappearance of flap up spd indication.
Good day!!

[Checkboard]

This issue was discussed recently, with some definitive statements by Boeing, on the B737 QRH Questions (http://www.pprune.org/cgibin/ultimatebb.cgi?ubb=get_topic&f=3&t=002164) thread.

Shore Guy, Hoot Gibson was accused of operating a 727-100 with the trailing edge flap out, and the leading edge devices disabled by pulling a circuit breaker. Something he has always denied, by the way. There is significant evidence that this accusation was completely groundless, and that the actuator for the leading edge device may have failed in flight.