320 IAE EPR rating limit changes during T/O

[Airmann]

With regard to the EPR rating limit on the E/WD during T/O on the 320 series V2500. I've noticed that the engines quickly race up to the initial flex EPR rating limit but once there the limit then increases further before finally settling down at one number. It's quite tricky when trying to judge whether the engines are stable and when T/O thrust has been set.

Two questions:

1) Why does this happen? Is it to ensure engine parameters are stable at a relatively high setting (but not entirely at full T/O power) before FADEC sets final power? Saving engine life? Or could it be that at the calculated EPR rating limit the desired thrust has not been obtained and the EPR must increase to meet required thrust.

2) When is "thrust set"? As PNF, and having to make this call before 80kts, I find it hard to judge. If thrust is set when the engines have fully stabilised at max EPR rating limit then with the changes it's very close to 80kts if not after.

[darkbarly]

1. Usually on turbofan, FADEC controlled engines, this shift in EPR or N1 ensures the EGT/ITT margin between that required for the calculated thrust and MAX egt is maintained, all the way from static application of power with no ram air effect all the way to V2. Helps extend engine life without compromising the thrust you need for takeoff on X length runway.

2. All engines EPR/N1 have reached thrust command bug. Anything after the associated thrust set call is scheduled by FADEC or a failure/stall/surge. Use the time between 'set' and V1 to monitor everything else!!

In other words, has the engine reached the commanded thrust by setting the thrust lever in the gate?