A few years ago, my airline had leased an additional aircraft, a 747-238B to supplement our fleet of 747s...
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The airplane was reported to have JT9D-7J engines (there was a little "7J" label on top of each row of engine instruments), and we often used maximum power for takeoff, as that aircraft was used as back-up for our 747 fleet which is normally powered by JT9D-7Q engines...
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If I remember well, there was an engine (nš 3) constantly showing higher EGT when we went to maximum EPR equal with the other engines... None of the crews flew that airplane often, so we did not mind. Was probably for a year or even more time... Until one day...
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Maintenance found out that engine nš 3 was actually a JT9D-7AH... So that engine got "overboosted" (if you use that vocabulary) for hundreds of takeoffs at -7J EPR settings... Eventually, we got a -7J on that position, but it demonstrates that some engines can take a lot of punishment of the "overboosting" nature.
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Makes me smile, I know that a JT9D-7AH is actually a JT9D-3A that has been upgraded to JT9D-7A standards. Must have been a very "old" engine near retirement, yet it survived all these years, giving 50,000 lbs of thrust for takeoff, when only rated for 46,150 lbs thrust...
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Happy contrails