Boeing has narrowed the fan size on the CFM International Leap-1B engines which will power its newly-launched 737 Max family to two options - 168cm or 173cm (66in or 68in).
"There are some advantages to 66 [inches] that we like and there are some advantages to 68 [inches] that we like and we're doing the final trades right now and we'll make a decision in the next several weeks," said Boeing Commercial Airplanes chief executive officer, Jim Albaugh.
"Right now we're looking at two different sizes, a 66in and 68in fan blade. In either case we remain very confident that it's not going to require a lot of modifications to the gear," he added.
At the time of its initial announcement on 20 July, Albaugh said Boeing had narrowed its choices to four options. With this latest move, a 168cm fan allows Boeing to maintain a 43cm (17in) ground clearance beneath the aircraft's nacelles to avoid contacting taxiway lighting.
"Certainly with the 66 [inch] there's no modifications, and I think even with the 68 [inch] there's a very low probability we'll have to touch the front gear.
"What we're trying to do is optimse the integrated design of the airplane and we believe the sweet spot for that integrated design solution is going to be 66 or 68 inches."
Albaugh said the 2017 entry into service allows for both options to be given full consideration.
"We have that built into our reserve for the development of this programme and we have it built into the reserve for the schedule," he said of the 173cm fan.
Albaugh says weight, drag and efficiency are all factors that must be considered when determining the final fan size for the 737 Max.
"Clearly with the heavier engine, you'll have to do some changes to the wing, some changes to the side of body join, some localised stiffening of the airplane," said Albaugh.
"But what we tried to do with our decision on the engine is maximise what this airplane can do. Yeah, with a bigger fan you get more efficiency because of the bypass ratio. But also what you find with the bigger fan is what you find is you get more weight and more drag."
Albaugh said the Leap-1B is 10-12% more efficient than the 156cm (61in) CFM56-7B that powers the Next-Generation 737 family today.
The three model 737 Max family, replacing the Next-Generation 737 with the 737-7, -8 and -9, was launched by Boeing yesterday.
Click here to see images of the new 737 Max aircraft.