Boeing has finalized the all-important fan diameter for the 737 MAX derivative program at 68 in., answering a growing number of questions from would-be operators and bolstering the twin jet’s credibility in continuing campaigns against Airbus’s successful A320NEO.

At the same time, Boeing announced that order commitments for the MAX have exceeded 600 from eight airlines, up from the 496 from five airlines previously disclosed when the program was officially launched in August. “The customers are responding quite well; we’re expecting several hundred more commitments quite soon,” says 737 Chief Program Engineer John Hamilton. Aside from American Airlines, no other customers have yet been identified.

The choice of a 68-in. diameter fan is “the sweet spot” for optimum bypass ratio and performance against increased drag, and is “the right decision for us” says Boeing Commercial Airplanes President and CEO Jim Albaugh. Speaking at the Goldman Sachs Global Industrials Conference in Boston yesterday, Albaugh said the MAX project “allows us to keep operating margins over the A320NEO and the money we didn’t spend on New Small Airplane we can use to improve the 777 and develop the 787-10, or stretched 787. We really have de-risked this decade,” he added.

The 68-in. fan size is the largest possible that can be accommodated beneath the wing without extensive modifications to the landing gear. Although the 68-in. fan does not strictly require a nose leg extension, Hamilton says, additional design space will enable a more optimized design. “We’ve allowed our designers to remove that restraint, and the nose gear will float up a little bit," he said, adding that a nose gear dimension has not been selected.

Overall, Boeing continues to play down the extent of the modifications that will be made as it moves to MAX, although it does concede that specific technologies could be introduced if they "earn" their way on to the program. The redesigned aft fuselage Section 48, for example, will be based on the current aluminum structural materials of the existing aircraft, rather than composite. The wings, while requiring discrete strengthening for the higher weights of the Leap engine, will be essentially unchanged. Even the planned change to fly-by-wire spoilers, confirmed by Hamilton on Thursday, is being made in a less disruptive way by adapting a tried and trusted architecture based on the 757. Such a system does not require the same levels of redundancy as a primary flight control system, “so the weight impact is not the same,” says Hamilton.

Boeing completed an internal review of the MAX program in late October and is “on schedule for service entry in 2017,” he adds. However, reflecting on comments made earlier in the day by Albaugh, Hamilton concedes that “he will continue to push the team to pull that forward to the left.” Firm configuration is targeted for 2013. Although precise details of the remaining schedule remain under wraps, Boeing is expected to complete design loads and critical design reviews in 2014, with assembly of the initial MAX aircraft in 2015. Boeing says first flight will be in 2016. Depending on how early in the year this is achieved could provide sufficient time for test and certification before the end of 2016, thereby bringing forward potential entry-into-service.