PARIS: Boeing explains confidence on 737 Max promises

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Boeing confident of Max fuel-burn cut

Re-engined 737 on course to deliver planned 13% efficiency improvement through aerodynamic and engine enhancements

Boeing has 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 four and a half years - with a 13% reduction in fuel burn per seat.

The 737 Max has already completed low- and high-speed windtunnel testing at Qinetiq's facility in the UK 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 on a pre-Paris air show media tour in Seattle.

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 with the next generation 737-800 on a 500nm (926km) stage length.

That estimated improvement was based primarily on the re-engining of the CFM International CFM56 with the Leap-1B turbofan. Boeing subsequently tweaked the design to also add advanced technology winglets that feature 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 has nearly completed testing of the third of its E-Core engine rigs, which the Leap design is based on. "They know a lot about the Leap-1B engine already," Leverkuhn says.

MATERIAL DIFFERENCE

The engine performance is aided by the slight tweaks that Boeing has made in the materials used within the nacelle. The CFM56 engine, for example, 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 resculpted 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 CFM56's lower nacelle on the Leap-1B, Leverkuhn says.

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

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

The new winglet design should reduce the 737 Max fuel burn by nearly 1% on a 500nm stage length, Leverkuhn says, adding that 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 that the change is made at a roughly linear scale. If so, it is possible to calculate the 1.5% fuel savings at roughly 700nm range.

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