As news emerges of the rapid change in strategy at Boeing, forced by the American Airlines order, we might pause for a moment to consider the outcome.  The 737RE that many (including us) have been talking about is an airplane born out of a compromise.

The 737 design does not easily allow for the new very high bypass engines to be used. It is low to the ground, and even the CFM engines used currently are squared off at the bottom to prevent scrapes (see image).  The fan diameter was reduced, to fit under the 737 wing, reducing the engine bypass ratio.  Overall thrust was reduced, from 24K to 20K pounds, mostly due to the reduction in bypass ratio. But the CFM56 engine kept the 737 competitive with the A320 for years, indeed one might argue the 737NG we see selling successfully through last year was the first compromise – and it worked very well.

Now we have yet another re-engine program on the 737 and once again there may be an engine compromise.

The CFM LEAP engine has seen a number of refinements.  The part that matters most to the 737 is the fan at the front of the engine.  This is the part that defines the engine’s diameter.  CFM said in an early PR “LEAP-X will feature a 70-inch diameter RTM fan that will be the first of this size in commercial service, which is another industry breakthrough.”  This was for the early test engine. The CFM56 as used on the 737NG has a fan diameter of 61 inches.

To make things really interesting, recently CFM announced that it was increasing the LEAP fan diameter because Airbus’s A320neo stands high enough off the ground to allow this. After all, the P&W GTF, also available on the A320neo, has a fan diameter of 81 inches.   (By the way, P&W tells us they will be competing for American’s neo order, even though the deal has GECAS and therefore CFM written all over it.)

Clearly CFM also wants to make use of a higher bypass ratio that a larger fan allows.  So CFM bumped up its LEAP fan diameter from 76 inches to 78 inches. (This is smaller than  the GTF, but CFM says they will meet or beat GTF numbers.) Note that even as CFM is developing the LEAP, it continues to talk about its next generation being an open rotor with a fan diameter of up to 4.3m. Which should make engine watchers wary – perhaps the LEAP is the end of the line in this market for CFM? A question for a different article perhaps.

Richard Aboulafia recently made an interesting point that the American Airlines order reinforces – it’s all about engines.  Boeing has already been through a re-engine process on 737. It worked with CFM to reduce the first CFM56 fan diameter that meant no new higher landing gear.  The 737 can handle a fan of up to 63 inches without touching the landing gear. There is talk Boeing is considering four fan sizes for the 737RE, one as large as 70 inches.  The A320neo’s CFM LEAP fan will be 78 inches. If CFM is going to clip the LEAP’s fan to make it fit on the 737 as it stands now, it is going to be another compromise to make the 737 work.  For example, allowing for a 63 inch fan on the 737RE means a 19% smaller fan than on the A320neo, for essentially the same CFM engine. That may be good for Boeing – in the short run.  If Boeing goes for a bigger fan it has to spend much more money revising its design. That might catch the FAA’s eye and require more approvals that Boeing managed to avoid when it brought out the NG. But see the 737RE for what it is – a compromise.  Boeing is trying to keep its 737 competitive and CFM has to give a lot of advantage to make it so.  And this time the compromise may not be enough.  Losing that much fan diameter and bypass ratio is not insignificant, regardless of what Boeing or CFM say.  We don’t see how the RE can be as fuel efficient or quiet as neo.

Airlines’ demand for fuel efficiency is at an all-time high.  They won’t compromise because they cannot afford to.  The 737RE cannot hope to offer the same fuel efficiency as the full sized LEAP offers on the A320neo.  Which means it can’t offer the same efficiency as the GTF either.  The 737RE may be the final compromise on the 737 design that could prove to be a short run solution as Boeing frantically works on the NSA.

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