Boeing's customers may have given a tepid reaction to a 737 re-engining project, but behind the scenes the airframer has not slowed the pace on work to refine the design of an updated narrowbody.
A previously unreleased photo of the re-engined 737 being tested in a Qinetiq wind tunnel was presented in a public Boeing document on the US Federal Aviation Administration's website detailing the company's efforts to develop new quiet technology for its jetliners.
The image is the first visual clue to Boeing's thinking about how it might re-engine the 737. A key challenge is to gain enough ground clearance under the wings to accommodate the next-generation CFM International Leap-X or Pratt & Whitney PW1000G engines, which have a larger exterior diameter than the CFM56-7B powerplant currently available on 737s.
The wind tunnel model suggests Boeing may be able to buy extra ground clearance by moving the engines further forward, to tuck them up tighter to the wings, and also by lengthening the nose landing gear by some 200mm (8in) - a nose blister fairing to accommodate longer gear is clearly visible.
That approach would be similar to one taken by Airbus earlier this year when it needed to lift the nose - to level the floor - of its A330-200 when it developed the freighter variant.
However, Boeing sources indicate that new configurations undergoing wind-tunnel testing have done away with the blister fairing and its associated drag penalty, adding that only Leap-X configurations have been tested to date.
At the Zhuhai air show in November, 737 chief engineer John Hamilton said his team was considering moving some components from the forward electronics bay to the aft to help accommodate longer nose gear. Critically, he said, any design must maintain 430mm ground clearance to avoid contact with taxiway lighting.
The exact benefit of new engines is as yet unknown, but while some estimates envision a double-digit improvement in fuel burn, the overall operating cost reduction may end up in single figures.