Boeing 737 successor

Reportedly, Boeing focuses on a replacement of the B737 rather than doing something with the B777 in the nearer future. Mike Bair, who leads the B737RS studies at Boeing, said in the interview with Bloomberg, that "Six or nine months ago, we were leaning toward a bigger airplane sooner" - meaning that Boeing did not expect Airbus to launch the A320NEO and now finds itself captured between Bombardier on the low-end side of the single-aisle market and Airbus trying to steal traditional B737 customers. So they have to react, but reading carefully the Bloomberg interview and also the latest blog entries from Flightblogger Jon Ostrower makes me believe that they are not really sure what to do. Basically, they have two choices:
  • A 6 abreast family sized from 149 seats (to please Southwest Airlines in particular) to around 200 seats. That would mean, that todays -700 would grow a little bit, todays -800 and -900 could stay in cabin lenght were they are today. For sure, cabin diameter has to increase to "speed loading and unloading, with either a wider aisle or possibly two aisles".
  • That brings us to the second option: a 7 abreast in 2-3-2 configuration with two aisles. That configuration would make it very hard to please Southwest with a 149 seater. The aircraft's fuselage would be heavy in comparison with a 6 abreast aircraft, as the extra aisle takes extra space you have to buid around. Then the 149 seater would have 22 seat rows, compared to 23 rows of the  -700 with 137 seats in Southwest configuration today. It would not only look like a "Mini Guppy" (see below), but the problem is that in case of "one engine out" the stabilizer has a small moment arm and thus has to have a relatively large area, comprising weight for the larger family members, which does not need the large stab. Vice versa is the case with the wing: to get exceptable runway performance and range for the largest family member (runway performance is today's weak point of the -900ER), the wing has to be sized accordingly, which means extra weight for the smallest member. Of course that's a problem of today's B737 and A320 families, too. But with the CSeries entering the market, which is optimized around 135 seats and trans-con range, there is an alternative available which does not have to make this heavy compromises.
Another question Boeing and the engine manufacturers have to answer (first for themselves) is: what are the improvements in SFC for an engine with an EIS 2010 compared to EIS 2013 (CSeries) or EIS 2016 (A320NEO, C919, MS-21).
Boeing today dismisses the NEO as the 15% fuel burn improvement would be not enough. Remember: fuel is about 40% of Direct Operating Costs (DOC), even for narrowbodies (the numbers in Jon Ostrowers interview with Mike Bair seem to be too low) and it could rise even further. Historically, SFC gets down by 0.5-1% per year. So the best we can expect for a EIS 2020 engine is 4% better than the NEO engines will be - and they could be upgraded as the V2500 and the CFM56 were upgraded several times in their life.
So where could the big advantage in DOC come from?
  • Aircraft weight: the new aircraft will benefit from new and lighter materials. If you read the interview, it will probably not be a plastic aircraft but a will get a Al-Li fuselage, like the CSeries.
  • Plastic wing with smaller wing area (compared to today's B737) with higher aspect ratio: that leads to lower drag, lower weight and better aerodynamics
  • The first two bullets both lead to a lower thrust requirement, directly leading to lower fuel burn.
  • Maintenance costs of the aircraft: as the B737 today is the airline's darling when it comes to maintenance costs I can't see where this could get dramatically better
  • Maintenance costs of the engines: as these engines will (most probably) be enhanced versions of the LEAP-X and the GTF or something similar from RR (Advantage 2 or 3), the difference in maintenance costs to the NEO engines would be small if at all. They could be even higher, as one way to improve SFC would be to raise OPR and therefore core temperatures.
Conclusion: there will be an cost advantage of the 737RS/797 compared to the NEO: of course. But will it be a step change? I can't see it today. And there are alternatives like the Bombardier CSeries and maybe another 5 abreast model from Embraer - they will decide what to do after Boeing announces their final plans - what could happen in June according to Bair. So in the second half of the year, we should have an announcement from Brasil...watch out!
This is how a 130 seater in a 2-3-2 seat configuration would look like...

Meanwhile there are three more blog entries about the possible 737 successor:
Part II
Part III
Part IV