Boeing plans to have a final configuration solidified for its re-engined 737 within three to four weeks, with final decisions focusing on the size of the CFM International Leap-X engine that will exclusively power the new variant, says Boeing Commercial Airplanes CEO Jim Albaugh.

"We've got an extremely firm configuration on the airplane," said Albaugh on the sidelines of the American Airlines press conference announcing the purchase of 460 aircraft split between Boeing and Airbus, including 100 of the re-engined 737s, which aim to be 12-15% more fuel efficient than today's Next Generation 737.

"There's one critical decision that we have to make and we should be able to make that over the next three or four weeks. We need to look at the fan size, we need to look at the engine, we've got four different options that we've looked at, all of them are good."

Albaugh declined to disclose the fan sizes being entertained or how much larger they could be than today's 1.5m (61in) CFM56-7BE fan diameter.

737 chief engineer John Hamilton said in a June interview: "We've looked at fan sizes of 1.8m (70in) and we know it can fit under the airplane. With the larger fan sizes we have to extend the nose gear to create a little more space.

"We're still trying to find that sweetspot. As you go up in fan size you get diminishing returns on fuel burn" with a higher engine weight and drag, added Hamilton.

Industry sources say an 20cm (8in) increase in nose gear would be required to accommodate a 1.8m fan.

Boeing must maintain a 43cm (17in) clearance underneath each nacelle in order to clear taxiway lights.

Albaugh added that Boeing aims for a mid-decade first delivery, with American's first Leap-X-powered 737 to arrive in the carrier's fleet by 2018.

While Albaugh did not elaborate on the mid-decade timing, comments from the company's May investor conference pointed to a 2016 or 2017 entry into service.

Following the establishment of its firm configuration, Albaugh said the airframer will seek the authority to offer the new 737 variant, adding: "I think the probability of board approval is high, but I'm not going to presuppose what the board is going to do."

Source: Air Transport Intelligence news