Jonty Says:
The engine has a limit of 5 mins at TOGA (850 deg C)
Well, in effect you're saying that when it's -40 in FAI I can push the engine to 120ish% N1 (red line 105%) and still not be at the engine limit... I don't think so.
FAA Says:
within the engine operating limitations established under Part 33
FAR 33.7(c) applies, and I'm sure similar certification criteria can be found around the world.
Takeoff
thrust is limited to 5/10 minutes, as appropriate;
When commanding takeoff thrust you cannot cross the red line on any limitation (ie. EGT), though, on very cold days, at sea level, etc. some (possibly all) of the red line limits may not be reached when takeoff thrust is achieved. Ever heard the phrases "temperature limited"/"field limited"/"climb limited"/"altitude limited" etc.
When commanding climb thrust you have a set of limits, which again must be followed, though climb thrust may occur before reaching those limits.
Same goes for MCT and GA, etc. etc. etc.
That being said, every manufacturer has built a bit of fudge factor into the engines. I prefer not to test it with all the fans spinning. When pucker factor becomes an issue... I'll start testing.