N2
Short answer - yes 5 mins
Long answer - In my view there are two aspects that have to be considered whenever it comes to an issue of engine limits:
1 The “legal” side
2 The engineering reality
Taking the first topic first, the manufacturer has to certificate his engine and “guarantee” to the owner (probably not the pilot…!) the performance of the engine, including such things as thrust, sfc, reliability, life and so on. He would not sell many otherwise. Like any such paperwork this has to reflect some agreed build and use standard and included in that lot will be such things as the limits you quote on EGT. Bust those limits and you clearly leave the manufacturer of the engine well placed in any subsequent dispute.
Turning to the engineering reality, if you just exceed the max continuous rating temp, then you are eating into engine life faster than if you did not exceed it. However, as we know the engine is perfectly happy to be run at max rating for some specified time (as per the design and certification criteria) BUT if the max rating was used continuously then engine life would be relatively short (I don’t know say a few hundred hours) So, used only in 5 min chunks this can be spread over a lot of flights and provides a sensible ratio of hours of cruise and minutes of takeoff leading to a nice TBO.
So, to answer your question with a No 1 point hat on, I am certain that once you exceed max continuous you MUST accept the time limit that applies to the next thrust level up even if you never get near the next temp number.
But, with No2 point hat on, the engineering reality is that there is probably a logarithmic scale of increasing damage as the temp exceeds max continuous which means a negligible effect on shortening the engines life until you get pretty nearly all the way to the max temp. So the effect of, say, 20 mins spent 10 deg above max cont (when there is a 25deg jump to the next limit) would probably not be measurable on strip down. But wearing the No 1 point hat again, it is just not practical to offer a whole lot of complex usage options for either certification testing or guarantees to the owner.
The bottom line for a pilot badly needing thrust from an engine that is running normally is that he can overtemp his engine substantially for minutes without any fear of it going boom. But that is No2 hat stuff so don’t expect No1 type people to tell you to go ahead.
(NB the same is not true during start-up or when surged because of the lack of cooling air supply to the hot end under those circumstances)
A premature engine change is nothing to sort out compared to pranging the aircraft for the want of a bit more thrust eh?
As a final thought, in support of my No2 point assertions, we are all used to the fact that airframe design cases/limits have to be factored down for service use, so somebody has demonstrated safety in flight at 110% of your Vne for example. The same is true of engines in respect of both RPM and EGT – margins have been demonstrated. These margins are NOT there for everyday use. But to avoid a prang – don’t think twice, take what you need and then write it up in the book.
JF