Boeing 737 MAX: performance if engine has SFC shortfall

By Bjorn Fehrm

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14 April 2015, C. Leeham Co: There have been persistent reports that the CFM LEAP engines should be behind their fuel consumption targets. We commented on these rumors recently. It's normal for engines to be behind final SFC to varying degree during development, this is part of the gradual development and fine-tuning of an engine until its entry into service point.

As we commented before, the key is not where an engine is two thirds through its development but if the engine would fill specification at Entry Into Service (EIS). Gaps to final specifications are normal during development, should there remain any gap at EIS it would also not be the first time this happened. Engines where target specifications are met from day one are historically in the minority. As we are in the unique situation to have a complete airliner performance model, we have modeled how any engine performance gaps would actually affect aircraft performance.


  • We have investigated what any shortfall of LEAP-1B SFC would mean for the aircraft. For situations where there would remain any deficit at EIS we choose to look at 2.5% and the rumored 4.5%.
  • Finally, we compared these two situations with a 737 MAX that would have nominal performance LEAP-1Bs and looked at the improvement in performance for all three compared to today’s 737NG.

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