Boeing explains confidence on 737 Max promises

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Boeing provided a small glimpse behind the scenes of the 737 Max programme to justify its confidence that it can deliver the aircraft on time in about 4.5 years with a 13% reduction in fuel burn per seat.

The 737 Max has already completed low and high-speed wind tunnel testing at Qinetiq to verify that the aircraft's new engines and aerodynamic changes can perform as designed, says Keith Leverkuhn, vice president and general manager of the 737 Max, who briefed reporters that Boeing invited to Seattle for a pre-Paris air show tour.

Boeing launched the 737 Max in July 2011 with a guarantee to airlines that it could reduce the fuel burn per seat by 11% compared to the next-generation 737-800 on a 500nm stage length.

That estimate of 11% improvement was based primarily on re-engining the CFM56 with the CFM Leap-1B turbofan. Boeing subsequently tweaked the 737NG design to also add advanced technology winglets featuring a distinctive split-tip, a relofted tail cone, fly-by-wire spoilers and an electronic bleed air system. Those changes allowed Boeing to increase the fuel burn reduction estimate to 13%.

"We've gone a long way on the engine testing," Leverkuhn says. CFM is nearly complete with testing the third of the E-Core engine rigs on which the Leap design is based.

"They know a lot about the Leap-1B engine already," Leverkuhn says.

The engine performance is aided by slight tweaks Boeing has made in the materials used within the nacelle. The CFM56 engine, for examples, uses a composite liner on the interior shell of the thrust reverser, with insulating blankets to protect the material from the hot exhaust of the engine.

For the Leap-1B, Boeing is switching to a titanium liner, which is lighter and requires less maintenance than the composite material, Leverkuhn says. The titanium liner also allows Boeing to slim the nacelle design, partly offsetting the weight and drag produced by the 21.3cm (8.4in) increase in fan diameter.

"We made the engine bigger, but the actual profile doesn't have a commensurate increase in cell diameter," he says.

Similarly, Boeing also re-sculpted the fan inlet to improve the laminar flow of the air entering the fan stage of the engine, he says. At the same time, Boeing did not change the distinctive divot of the lower nacelle of the CFM56 on the Leap-1B, Leverkuhn says.

The wind tunnel testing has also bolstered Boeing's confidence on the performance of the advanced technology winglet. In March, Airbus chief operating officer John Leahy said that Airbus engineers were not sure that such a winglet design would offer any aerodynamic improvement. But Leverkuhn pointed to the data Boeing acquired in the wind tunnel testing.

"We we can be confident of is the wind tunnel results we've already got," Leverkuhn says. "We believe we're on very solid footing of what this is actually going to bring."

Boeing revealed that the new winglet design should reduce the 737 Max fuel burn by nearly 1% on a 500nm stage length, Leverkuhn says, adding the fuel burn rate decreases by 1.5% at longer ranges. He did not specify the precise range that produces the 1.5% improvement in fuel efficiency, but he conceded the change is made at a roughly linear scale. If so, it is possible to calculate a 1.5% fuel savings at roughly 700nm range.