More details have emerged on the likely configuration of the powerplant for the re-engined version of Boeing's 737, ahead of expected board approval for the programme.
As well as a 167cm (66in) fan on the CFM International Leap-X engine - increased from 157cm on the CFM56-7B, which powers the 737NG - the updated jet is also expected to feature external nacelle chevrons for noise reduction, similar to those featured on the 787 and 747-8.
A fan of the proposed size would remove the need to modify the design of the landing gear, although Boeing declined official comment on the deliberations on fan size.
Detailed assessments are under way to incorporate a revised tail cone, natural laminar flow nacelle and a hybrid laminar flow vertical stabiliser for additional drag and fuel burn reduction.
The airframer's board of directors was due to meet at the end of August to vote on giving a green light to the project.
Boeing is seeking to strike a balance with its design, delivering a 10-12% fuel burn improvement from the updated engine without changing the 737 too significantly and breaking commonality with its current models.
This potentially allows a way in for Airbus to offer its A320neo.
While Boeing's 167cm fan will have a lower bypass ratio and higher specific fuel consumption (SFC) than the 198cm Leap-X and 205cm 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 167cm fan would offer around 13-14% over the 155cm CFM56-7B engine that powers the 737 today. Once integrated on to the aircraft it would deliver a fuel burn benefit of 10-12%.
A design shelved earlier this year, designated the 737RE, featured a 177cm fan, which required a 20cm nose gear extension to meet the required 43cm engine nacelle ground clearance, to avoid hitting taxiway lighting.
The 737 can accommodate up to a 170cm fan before requiring any changes to its 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, requiring Boeing to modify the electrical equipment bay to find new routes for wiring and equipment racks.
The changes would have also likely necessitated widespread modifications to the aircraft's empennage and fuselage.
Boeing is seeking to avoid repeating the trouble 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, which subsequently caused a ripple effect across the jumbo's design.
In turn, this drove up the extent 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 - as it harmonises the range in line with the 747 and 787 - 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 20 July.
Source: Flight International