Old 21st Mar 2016, 01:56
  #15 (permalink)  
Join Date: May 2005
Location: F370
Posts: 194
Back to the original question:

I believe the answer is that Boeing doesn't do certification testing in that configuration.

Obviously you could have a situation where a full derate provides the same thrust as the higher rating with assumed temperature, and therefore the takeoff would be safe. However, between Boeing and the FAA in the certification process, this (arbitrary) rule is introduced. In practice, I don't find it limiting in any way.
AtoBsafely is offline