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.