Originally Posted by 737zaf
My first thought is that because of increased TAS if you increase lift by extending flaps at that altitude the stress imposed on the wings and flap surfaces can be catastrophic. Please feel free to input in this matter(and correct me!). Thanks
I think the stress on the flaps due to dynamic pressure would be slightly LESS at high altitude than at sea level. This is because Vfe is quoted in calibrated airspeed (CAS) and due to compressibility error EAS becomes less than CAS at high altitude.
Where you could run into a problem could be with Mach effects. Consider Vfe flap 1 = 250 knots CAS. At 20000 ft this is Mach .55 so no Mach problems.
However suppose you were to extend flaps at 250 knots CAS at 39000 ft. This is Mach .81 which is close to Mmo. You might induce unknown critical Mach effects.