Originally Posted by
Ray_Y
Yes, please
So it took 20 years or more, on the 747 fleet? Was that the reason why MTU was involved regarding compressor?
Couldn't find that one. Any small hint?
You guys are going to make my head swell - if I was still racing I'd need to get a new helmet
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The problem with the PW4000/94" compressor was that the case didn't stay round as it got hot - which then rubbed the blade tips and increased clearances, which in turn reduced surge margin. The problem mainly affected higher time engines - or more specifically higher time compressors as the blade clearances gradually increased. Pratt tried something called "Cutback Stator" - where they changed the stators to reduce the load on the compressor stages that were prone to stall - but that just moved the problem to the front of the compressor instead of the back and in fact made the surge problem considerably worse. Somehow they got Cutback Stator certified and into service but it make the surge problem so bad it was quickly removed from service (I wasn't directly involved in Cutback Stator, but several of my friends were and they were amazed it was ever certified - they got numerous surges during the flight test program that should have alerted people to the problems).
I came back to the 747 program after Cutback Stator - after Pratt had finally admitted the problem was fundamental to the compressor design and went back to the drawing board. The result became known as "Ring Case Compressor" or RCC - by getting rid of the split case they were able to eliminate the joint structure that didn't expand at the same rate during a thermal transient (the tradeoff was that it was significantly harder to assemble). The FAA had learned from the Cutback Stator experience, and the flight test really put the engine through the ringer but RCC (which was based on the PW4000/112" compressor) worked like a charm. It eliminated the compressor stability issues, and the more stable compressor structure gave a meaningful fuel burn improvement and better performance retention (~1% fuel burn IIRC - that may not sound like much but engine designers will sell their mothers for 1% fuel burn). The RCC mod was AD'd, with a quite aggressive implementation schedule given it required the engine to be rebuilt - at every tech review we had with Pratt for the next several years, the RCC implementation status was one of the first agenda items.
I think the duel surge event was an A310, but I'm not sure (which is why I didn't specify in the original post) - maybe around year 2000? After that Pratt had to come out with a special test to measure compressor stability on higher time in-service engines. They'd run the engine at (IIRC) 1.60 EPR, hold it there for about two minutes, then a specially modified engine control would give it a big extra shot of fuel to see if it surged. If it didn't surge, then it was good for another xxx cycles - then they had to run the test again. Ideally this test was run in a test cell, but since that wasn't always practical we came up with a procedure to do it on-wing
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