An assumed/ flex temp is a performance calculation technique used to find the takeoff EPR/N1 setting for aircraft's take off weight .
Assumed /flex temp takeoff thrust is limited to a 25% reduction (you can used only in certain condition eg. not with rwy contaminated)of takeoff thrust or selected climb thrust whichever is the greater value .
Derated value or D-TO is a fixed percentage ,usually 90% or 80% of takeoff available what you can obtain in certain airplane type selecting
D-TO 1 or D-TO 2 .
There are two two main reasons for using a reduced or assumed/flex takeoff thrust:
1 To protect engine life and improve engine reliability
2 to reduce the noise generated by the aircraft
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