Reason for large TO performance decrement with u/s EPR gauge

[TCXCadet]

Dispatched today on a RR 757 with a u/s EPR indication.

IAW MEL full power takeoff, manual thrust and N1 used as reference. All understood so far...

Could anyone explain the reason for the large 7500kg reduction in RTOM when obstacle limited please? I can see there may be a need for a small 'fudge' to allow for pilots being slow to set correct N1, but this reduction seems very large!

Thanks for your thoughts. :ok:

[WindSheer]

Is there any possibility that N1 could give a false/over estimated reading??
If so, would make sure for the safety buffer put in place.

Would make sense for the MEL to require a MAX thrust take off..:bored:

I dont know.....:rolleyes:

[One Outsider]

As RR is using EPR as the primary power setting/indicating parameter, I would imagine that there has been no testing/certification work done without it.

In order to dispatch under the provisions of the MEL, a conservative penalty is thus applied.

[muduckace]

Fear of safety driven. Operators in the past have eliminated EPR on Pratt powered aircraft to simplify thrust to N1. EPR is B/S in my oppinion, set N1 and regulate to EGT limitations.. I do not see how this is is not logical for TO mode, EPR is nice for climb and cruise.

WindSheer..

Once again, N1/EGT are the two limitaions that you do not want to exceed, the N2 and N3 (rolls) are slaves to these factors.

[Golden Rivet]

RR use EPR, because on their triple spool engines, there is a better linear relationship between EPR and engine power.

[dixi188]

I jumpseat on RR powered 757's from time to time and, being a FE, have sometimes noticed quite a split in N1 at T.O.(up to 3%) for matched EPR.

So if EPR is accurate, then without it the thrust could well be reduced. Hence the reduction in TOM.

Have a look at your N1 next time you take off and see if there is a split.(Not neglecting your normal duties of course).

Fly safe!