I thought it is logical to understand that it is not the extension process (e.g. flaps going for example from 0 to 5) that is the issue above 20.000ft, it's the flaps extended, since aircraft hasn't been tested with flaps extended at higher Mach numbers. Therefore, Boeing has only provided maximum flap speeds only in KIAS - do you think it's really safe to fly at FL370 at M.75 with 240 KIAS and flaps 5 (limitation for F5 in -400 is 250 KIAS)?
AFM limitation:
Quote:
Maximum flap extension altitude is 20,000 feet pressure altitude.
FCOM:
Quote:
To prevent excessive structural loads from increased Mach at higher altitude, flap
extension above 20,000 feet should not be attempted.
Sometimes a bit of logic helps, but as it appears not with everyone...
That's what I think too, as said, I'd never fly above 20000 feet flaps extended; it was just a discussion with a colleague that prompted me to find something printed clearly.