The CFM LEAP-1B discussion

Last week, after the first flight of the LEAP-1B on the GE Flying Testbed, Aviation Week had a story about the LEAP engine family and the alleged performance shortfall, in particular the -1B for the B737MAX. Leeham then also had a short article, stating that the SFC shortfall, first reported by Aspire Aviation of 4-5% was only measured in one engine, where “tolerances” (i.e. gaps and, in particular radial tip clearances) where intentionally opened to test a deteriorated engine.

This kind of testing is new to me, I never heard that these kind of tests were done. In the end it is very expensive to build a test engine solely to measure the performance with so called “end of life ”clearances”. Normally the clearances in the engine are constantly measured so you get a relation between the change of the clearances due to wear and rub ins and the change of the performance (both fuel burn and compressor stability).

To build in intentionally larger tip clearances makes sense only for the compressors to test compressor stability. And if these tests showed a SFC shortfall of 4-5% (I now heard 5%) then either the performance retention of the LEAP engine is very bad or there indeed is a performance shortfall as the effect of larger tip clearance should not lead to a loss in SFC larger than 2%.

Now CFM always mad big  noise about the good performance retention the LEAP engine family would have, in particular compared to the P&W GTF Family. One could ask: how do they want to know that? And I think, neither CFM nor P&W nor anybody else can know today and will know until real life data is available. With “real life data” I mean data that will come out of the field once airlines really fly the aircraft and engines for some time.

So we don’t know for sure how much of the (obviously measured) 4-5% are attributable to a new and with production tip clearances build engine. But CFM’s argumentation does not add up to the point where they would meet not only their own expectations. To meet the own pretest prediction within 0.5% does not mean a whole lot to the outside world until you know where the prediction was relative to specification, i.e. relative to the target SFC where the B737MAX would meet 14% fuel burn reduction vs. the B737NG.