Boeing appears to be nearing a final decision on the fan size of re-engined 737, settling on a powerplant that will not require any modification to the aircraft's landing gear, confirm company and industry sources.
As the airframer moves closer to a vote by the board of directors later this month, a 66in CFM International Leap-X fan is gaining consensus internally as the final configuration of the re-engined jet which is likely to enter service in 2016 or 2017.
Further, the updated aircraft is expected harmonize the type's brand in line with the 787 and 747, changing the 737-700/800/900ER to the 737-7, -8 and -9.
Boeing declined official comment on fan size deliberation.
The updated jet is also expected to feature external nacelle chevrons for noise reduction, similar to those featured on the 787 and 747-8, and detailed assessments are underway to incorporate a revised tail cone, natural laminar flow nacelle and hybrid laminar flow vertical stabilizer for additional drag and fuel burn reduction.
Boeing is seeking to strike a balance with its design, delivering 10-12% fuel burn improvement with the update engine without changing the 737 too significantly to break fleet commonality with its current models, in turn giving existing customers the flexibility to evaluate the Airbus A320neo without consideration of the switching costs.
Compromise is a central tenet of aircraft design, balancing engineering, manufacturing, certification and cost consideraitons to create a product that best meets customer requirements.
While Boeing's 66in fan will have a lower bypass ratio and higher specific fuel consumption (SFC) than the 78in Leap-X and 81in Pratt & Whitney PW1100G engine options for the A320neo, the smaller engine will weigh less and create less drag on the 737's airframe.
According to one industry assessment of the engine's performance, the SFC improvement of a 66in fan places its SFC improvement around 13-14% over the 61in CFM56-7B engine that powers the 737 today, and once integrated onto the aircraft would deliver a fuel burn benefit of 10-12%.
The narrowing consideration of a 66in fan for the re-negined 737 was first reported by Leeham Co. in an August 10 posting and the study of 65 or 66in fan was reported by Bloomberg in July.
A design shelved earlier this year, designated the 737RE, featured a 70in fan, which required an 8in nose gear extension to meet the required 17in engine nacelle ground clearance to avoid hitting taxiway lighting.
The 737 could have accommodated up to a 67in fan before requiring any changes to the landing gear.
According to that now-defunct plan, the longer nose landing gear would have prompted a redesign in the lower lobe of the forward 41 section would requiring Boeing to modify the electrical equipment (EE) bay to find new routing for wiring and equipment racks.
The changes would have also likely necessitated widespread changes to the aircraft's empennage and fuselage.
Boeing is seeking to avoid repeating the troubles it encountered when developing the 747-8 freighter and Intercontinental, which began its design life as a "simple" re-engine with General Electric GEnx-2B powerplants.
The mounting of the 747's engines and stretching of the fuselage, prompted significant changes to the aircraft's wing and flight control systems that caused a ripple effect across the jumbo's design. In turn, this drove up the extensiveness and cost of the change required to deliver on the jet's performance targets.
Once Boeing receives the go-ahead to offer the 737-7, -8 and -9 to customers, it will be able to begin taking orders for the updated narrowbody, including firming a commitment for 100 of the type from American Airlines, announced on July 20.