Tech question derated vs flat rated engines difference
 

Hi all
I am sorry if this question has been discussed already but i couldnt find it.

I d like to know if someone can explain me the difference, if any, between a flat rated engine and a derated engine.

I found this 2 questions in an interview questions database but i cant really get the difference between the two.

Thanks in advance

A flat-rated engine is an engine, which produces the same amount of thrust regardless of temperature from a certain temperature point (around ISA+15 for most engines).

A derated engine is an engine, that has been modified (this modification can be very simple - depends on engine/airframe) to always produce less thrust than what it was designed for. Reasons for this may be better (smaller) noise footprint, reduced maintenance cost, increased EGT margin, reduced VMCG/A, ... On some aircraft you have an option of a fixed (permanent) derate, which requires maintenance action and a "semi-fixed" derate, which can be selected by flight crew to conform the operational needs.