CFM LEAP-1B missing SFC

The revelation by Aspire Aviation about the performance shortfalls of the CFM LEAP engine family did not came totally unexpected. Rumours have been around for a long time. But the severity of the shortfall in the LEAP-1B engine is somewhat shocking, at least for Boeing customers. The shortfall of 2% in the case of the LEAP-1A is relatively unspectacular: the shortfalls of recent engines at EIS like the GEnx and the Trent 1000 were higher. A shortfall of 2% can be corrected with a suitable performance improvement program (PIP), but a
shortfall of 5% in the case of the LEAP-1B cannot be corrected without a complete redesign of the engine, probably not without changing the whole engine architecture. GE needed two PIP’s to get the GEnx close to spec. One PIP touched the high pressure spool, effectively redesigning the whole HPC, the other touched the low pressure spool. Here the whole LPT was redesigned, getting more blades and vanes into the flowpath, as the original low number of blades and vanes proved to be too low. One should think that these "lessons learnt" were incorporated in the LEAP engines, so the current shortfall (and of course we don’t know from which components they stem from) should have different reasons than in the case of the GEnx.

PW made their own experience with such a shortfall with the PW6000, when their own 5 stage HPC was replaced by a 6 stage compressor designed by MTU. In the case of the LEAP-1B adding an LPT stage as Daniel Tsang suggested would of course lead to a higher component efficiency and thus a better SFC, but the added weight and increase of maintenance costs would eat up most if not all of that gain. Also the center of gravity of the engine relative to the wing would move away from the wing, possibly leading to a lower flutter margin that would have to be countered with more mass in the wing. Redesign of the engine and the wing would then lead to a later EIS, at least a year.
The 2% shortfall of the LEAP-1A could hurt CFM financially long term, as many of the A320neo engine contracts include a guarantee that the selected engine (either the LEAP-1A or the PW1100G) is better in fuel burn than the other one. Typically the guarantee is a 2% difference. With the PW1100G fuel burn being on spec and the CFM being 2% worse than spec and with Airbus saying that the A320neo saves 15% or 1.4 million liters of fuel per year, CFM would have to pay for a 4% fuelburn miss relative to guarantee – and 4% would equal almost 100.000 gallon per year and aircraft. Even with a long term low fuel price of $2/gallon this would be $200.000 per aircraft and year that needs to be paid by CFM to the customer. Large customers like American Airlines or easyjet which ordered 100 A320neo with the LEAP-1A would get $20 million per year, let alone AirAsia, now having more than 300 of them on order. If similar competitive fuel burn clauses (in that case relative to the A320neo) are in place in LEAP-1B contracts, then the problem for CFM would be even larger.
Another consequence of a 5% SFC miss could be reduced life, especially in the high pressure turbine. As more kerosene has to be burnt to get a certain thrust, temperatures will be higher than planned. A few degrees higher temperature reduce life of the turbine blades, vanes and disks significantly. As most engines are sold with an aftermarket agreement attached (Flight Hour Agreement), the costs for the airlines will not be directly affected. But shop visits for the engines will be more frequently and this is a hazard for the customer at most.
If this is all true, then the most significant advantage the CFM56 always had, namely on-wing time, would be - for now- gone with the LEAP engine.

I am sure CFM, respectively the two partners GE and Safran have all the abilities to solve the issues, but it may be costly and take some time.


  1. When Aspire starts to hand out negative rumors on their "love relationship" there is a good chance that this is a strategic rumor.
    For Boeing a dud engine on the MAX pressures an NSA or MOM developement to the right.
    This in turn could push Airbus into preserving development resources for a complementing product.
    What then might come under pressure is A380 upgrades of any significance.
    ( At least that is the Rube Goldberg Maschine I could envision in this scope ;-)

    1. These rumours not only came from Aspire - I heard them before through different sources. So I can rule out that this is a "strategic rumor" from Aspire itself.

    2. Aspire doesn't have the creativity for generating their very own rumor.

      They are on occasion a "mouthpiece" or just the "postman".

      As a rumor I expect this to have substance.
      In the creation/offer phase perfomance was "ladled on"
      in quite a generous but haphazard way.
      ... home to roost or something in that direction ..

  2. It is almost impossible for CFM Leap-1A to be as good in SFC as a PW PW1100G, why, because of the Bypass Ratio which is bigger at PW.
    In a Turbofan engine we have the maximum Specific Thrust at a Compressor Pressure Ratio between 20-30. The Specific Thrust is higher with higher Turbine inlet temperetures(material limited). The SFC is lower for higher Compressure Pressure ratios. Taking into account that the compressor pressure ratio and turbine inlet temperatures of both engines should be more or less the same, we have that for an overall pressure ratio fixed (OPR at around 50:1 for both) the engine with the higher bypass ratio will have the lowest SFC and the lowest Specific Thrust((KN.s)/Kg_air)). This is supported by the fact that for example a turboprop(very high BPR) even with a smaller OPR can have a lower fuel consumption than it's equivalent Turbofan!
    Now solutions for the Leap, or they increase the bypass ratio by having a slightly smaller and more efficient core which is unthinkable, or they deliver more power to the compressor by redesigning the HPT (better and more efficient colling with less bleed and other little tweaks probably can deliver 1-2% better SFC) and LPT (adding one stage in the case of Leap 1B and another tweaks can deliver an additional 1% better SFC in my opinion). So, i think they can easily manage to get the Leap 1A within spec, but they will strugle to with the Leap 1B (2% shortfall at best).

  3. I'm a little confused with the "bigger bypass" been always more fuel efficient. Although that sounds like it is true, and for the most part it is. Isn't the IAE V2500 engine on the B320 family said to be about 3% better than the CFM56-5A engine on the same airplane, even though the fan on the V2500 is more than 4 inches smaller so more likely smaller bypass ratio? So what gives? Could it be that one engine having different design could just be better than another one in the same class? And if that is the case, it could never be done again?

    1. The V2500 has a 2 stage HPT and thus a better turbine efficiency. Also the fan efficiency should be better as it is a wide chord fan whhere the CFM56 still has an "old fashioned" fan with snubbers.

    2. And the V2500 has a mixed nozzle, improving propulsive efficiency

  4. How does a 2% worse than spec turn into a 4% fuelburn miss? What's the calculation here or is this a mistake?

    "Typically the guarantee is a 2% difference. With the PW1100G fuel burn being on spec and the CFM being 2% worse than spec and with Airbus saying that the A320neo saves 15% or 1.4 million liters of fuel per year, CFM would have to pay for a 4% fuelburn miss relative to guarantee – and 4% would equal almost 100.000 gallon per year and aircraft."

    1. This comment has been removed by the author.

    2. Spec is Spec+/- 2%! And PW is already talking about raising prices on their engines so they must have good results, better than Spec. That raises the gap between 737 max and A320, and pushes those who choose the A320 to the PW engine.

    3. 1. Assumption: Spec for both engines is the same
      2. Assumption: PW1100G is on spec
      3. Assumption: LEAP-1A is 2% worse than spec
      4. If now CFM promised to be 2% better than PW1100G they have to pay the customer 4%, as they are not 2% better but 2% worse than the competitor.

  5. Bypass ratio is one thing, fan diameter is another. For example Trent 900 has a bypass ratio of around 8.5:1 and a fan diameter of 116 inch with 78000 lbf. A trent 1000 has 112 inch fan (so less 4 inch) and has a bypass ratio of 11:1 for the same 78000 lbf! The IAE V2500 and CFM-56-5A were developed 20 years ago, were materials were not as good and things were not optimised to the limit. Today they are, and because of that they are very similar. The IAE has a slightly higher Pressure ratio with a slightly lower bypass ratio, so more or else the same TSFC as the CFM56-5A. But having the same TSFC does not mean it consumes the same fuel, since there are other factor's like the nacelle, wing/engine integration which is better for the IAE specially for the long runs . But here we are discussing the engine, not the engine in the airplane!

  6. Testbench (wind-tunnel) TSFC data differ from on-wing (in-flight) trip fuel data, the latter sanctioned by contractual Mission (payload-range) guarantees. Boeing have excellent records for engine-nacelle-pylon-wing structural integration, the 737 NG (ceo) itself being the archetype of proficient powerplant structural integration. MAX maître-couple + CoG re-positioning in the x-z plan may successfully contribute to mission fuel recovery of up to 3 % of the announced -1B shortfall of 3-4 %, leaving 1-2 % for recovery (PIP + tweaks ?) by CFMI. Conclusion : there is no major crisis yet with the MAX, except if Boeing's design engineers make a direct blunder themselves out of the MAX engine-to-wing structural integration, which is not their style from records ?

    1. Repositioning of the engine can account for 3% fuel recovery? You will have to explain that one please. And even if true, why would GE not include that in its previous estimates? These guys do not leave even the slightest fuel burn benefit on the table. Witness P&W raising prices to accommodate better than expected fuel burn. PIP can bring some fuel recovery but they only do those if they have to - they are expensive and usually have penalties elsewhere (ie, more blades). Plus, you tie up engineers that you need to work other programs. Repositioning the engine also may require airframe design changes, no? Please clarify.

    2. We have the 737 NG and we shall have the 737 MAX. CFMI are in charge of the MAX. They deliver (testbench) TSFC performance of NG vs MAX powerplants, Boeing are in charge of engine-to-wing integration, involving nacelle and pylon + fairings design. The -1B Leap engine is heavier/has its CoG shifted fwd vs the engine it replaces, plus has a significantly larger Maître-Couple. Mission guarantees (where Boeing may be exposed if the MAX perfo is failing) are incorporating take-off/climb as well as descent/approach flight phases, which combined with cruise forms the complete "mission" flight profile. Particularly sensitive to correct powerplant-to-wing integration is (powerplant contribution to) trim-induced drag. The relative impact thereof in the balance when measuring overall trip fuel exceeds - 2/3 % (clever powerplant integration) to + 2/3 % (messy powerplant integration) vs mission fuel nominal, the remaining installed perfo depends on TSFC. The control of the (engine) trim-induced drag parameter is in the hands of Boeing, not of CFMI. Summary : it is within reach for Boeing engineers to recoup up to 2- 3 % of a disfavourable TSFC ... but again : this requires the best-case scenario accomplishment. If blundered, integration could add on to TSFC short-fall, aggravating the overall picture ?

    3. According to this story below, Boeing has already done those optimizations and could only deliver a claimed 1% improvement. So one less area where SFC improvements can be made.


      Boeing today confirmed an additional 1% fuel-efficiency
      improvement for the 737 MAX family of aircraft. This addition
      brings the total fuel savings from 13% to 14% over the current
      generation 737NG models. The additional 1% derives from
      improved aerodynamics, particularly associated with reducing drag
      created by vortices between the engine and wing, as well as
      continuing improvements to the winglets.

  7. There is something that I fail to comprehend from the big shortfall in fuel efficiency for the Leap-1B engine.
    According to CFM, the new composite fan and fan case by themselves being lighter and more efficient layout should bring a potential 2% improvement in SFC. Since the fan of the Leap-1B is close to 9 inches bigger than the CFM56-7B, then a potential 5% SFC (as the saying that every inch of fan increase can bring a half percent improvement). The Leap-1B was supposed to come in at about 14% better than today CFM56-7B.
    So, if we were to put this new composite fan and fan case on the CFM56 engine core of today, we can most likely get a 7% improvement in SFC. So, after all this year and testing of new technology (ceramic lining, pressure ratio, new lean burn and efficient combustors, etc.) that's the best that CFM can come up with the new LEAP core, a mere 2% better (judging by the 7% improvement on the composite fan and its size increase and the fact that they might be missing SFC by up to 5% on the LEAP-1B)? I know that there are a few other things that might come into play, but if the new core of the LEAP-1B can only provide this little improvement, wouldn't just be better to use the CFM56 core and improve it a bit? After all the CFM56-1A already uses a fan size similar to the one for the LEAP-1B, so not much changes there to be made. This news if true will be weird and discouraging of new technology. So much study and testing it and a meager 2% improvement? Definitely something weird.

  8. Are we missing the obvious? Leap-1B SFC shortfall could all be down to its fan size. And due to geometric constraints, Boeing can't just add a bigger fan. It probably explains why the PurePower engine on the NEO is already performing beyond its SFC requirements - due, no doubt, to its 81 inch fan. Looks to me like Boeing and CFM really do have their work cut out.

    As Airbus claims below:


    One key element in the NEO vs MAX debate is fan size. Airbus says that each half-inch change in the
    fan diameter equals 0.5%, exclusive of other factors. The LEAP on the A320neo is 78 inches; the
    MAX is expected to be 68 inches. By this standard, this puts the 737 at a 5% disadvantage before
    any other factors are calculated.