CFM LEAP-1B enters flight testing

May 7, 2015: The CFM LEAP-1B has entered flight testing on GE’s company-owned Boeing 747. The engine is for the Boeing 737 MAX. Aviation Week has a story and one section in particular caught our eye, as it relates to the controversy over the test results of fuel consumption.

“When we build development engines they are heavily instrumented and built to accomplish extreme test conditions and durability,” he explains. “They are intentionally deteriorated and have open clearances because they are built for the ‘corner point’ in the test effort. We do pre-test predictions and we are within 0.5% of every one, so we are right on track. We fully expect to be right on our commitment as we enter into service.”

Aspire Aviation reported last month that the LEAP-1B was coming up 4%-5% short, a huge number that Boeing immediately denied; CFM declined comment at the time. Airline Economics later reported the same figure. Our information from our sources was conflicting: we were told by one that the shortfall was 2%, a figure we had been hearing for some time and which was characterized as not unusual at this stage; and one other that reported the 4%-5% figure. There it sat. We did a “what-if” analysis of the effect on the MAX at the 2% and 4% numbers.

Last week, we received a clarifying explanation that appears to track with the Aviation Week article and the excerpt above. We were told that the 4%-5% number came from a test stand test in which the tolerances of the engine were much looser than the optimal performance engine. The clearances, we were told, were not to specification–and the result was the 4%-5%.



16 Comments on “CFM LEAP-1B enters flight testing

  1. So, the wisdom that is best is stay tuned and see rather than get too bent.

    • Which is why I immediately called into question Aspire’s (and everyone else’s) numbers. 4-5% seemed way too high and I thought that that was bad gouge.

      However, some “experts” on said the miss was “true” and while the GTF will beat all numbers, the LEAP would miss. Hmmm…..

    • Always a good policy, but it makes for bad news copy.

  2. Glad to hear CFM is within .5% of their own pre-test predictions.


  3. Early test engines, aka development engines, are often not to spec on all tolerances. Especially the FETT (First Engine To Test) is often off by a lot, schedule being prioritized above performance for this engine as its first run is an important project milestone with much press an PR.

    Later engines are gradually better and engine 4-5 is usually close to spec on most parts. The performance test engine (again maybe no. 4 or 5) is closely monitored an few deviations are allowed.

    There may be, however, design differences between the early engines and the serial production version, even on the performance test engine that add to differences in sfc, etc.

    Add all probes for instrumentatio and you have engines that are far from “clean”.

    The compliance test engines are all according to tolerance. For a bew program they can be as late as no 8 or 9.

    Performance shortfalls in early engines are therefore not something to overly concerned about.

  4. Performance shortfalls in early engines are something to be concerned about if they significantly fall short of what could be expected from those early engines at this stage.

    In that case an OEM could try to play down shortfalls as “normal”, “promising”, “beating expectations” and similar free tickets, while scrambling for solutions and hoping customers stay away from the competing supplier.

  5. “The Leap-1B is in development as the exclusive powerplant for the 737 MAX and differs from its stablemates in having a smaller, 69.4-in. diameter fan and two fewer low-pressure turbine stages.”

    If the 1-B has an 8″ smaller fan diameter, two fewer stages and a lower bypass ratio, how does it achieve the same fuel efficiencies of the 1-A?

    • It does not everything else equal. It might run hotter?

    • If you are low on propulsive efficiency due to design constraints you have to raise the bar in the other fields 😉
      forex raise temps and pressure ration ( linked anyway ?)

    • Trooper
      “If the 1-B has an 8″ smaller fan diameter, two fewer stages and a lower bypass ratio, how does it achieve the same fuel efficiencies of the 1-A?”

      I thought they always stated that the LEAP-1B would be about 15% more efficient than the CFM56-7B which is the one on the 737.

      • Oscar I think you are correct. 14%, but also 3% more drag.

        I think it is relevant the Leap-B has no direct competition, while the Leap-A has. Therefor any enhancement on the B will likely be used on the A too, ASAP, to fight the (seemingly promising) PW GTF.

  6. I also believe the 737 is “lighter” than the A320.

    Less thrust for same ops as well as less thrust needed to sustain equal speeds.

    ergo some fuel advantage in that.

    Still to be considered is not just SFC, but cost of the engine to mfg (exotic materials cost more) and the durability on the wing (how often its replaced)

    It my personal take that the GTF has more long term upside as it is a lot easier to increase its efficiency at a lower cost than the LEAP.

    How they compare maint wise I have not heard. CFM has an enviable reputation in that regard and I don’t know if they can repeat with the LEAP not how good P&W quality control is in that regard.

    P&W has had a bit of checkered history and the current F35 engine has not been a good example of cost control (they were supposed to be further down on the cost curve as the sole source and there has been a real fight to get them to reduce engine costs). Certainly the grounding issue with the fan rub is another one.

    We should get some answer as its the first on the A320, so as always, things to wonder about but stay tuned for real facts and data

  7. On a slightly related note, does anyone have any numbers on the SFC of the CFM56-5 of the A320 vs. the CFM56-7 of the 737NG?

    How do they compare in fuel burn?

  8. Despite Boeings claims, there was an assement by airlines a while back that operated both types.

    What they said was they were within 1/2% of each other and neither had the edge, one day one would get that better, the next day the other.

    In other words a wash.

    sorry I did not save the link.

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