To the question of when Boeing will replace its 737 dynasty, the company remains tight-lipped. But there appears to be no question that Boeing will introduce a new-generation narrowbody airliner and it has suggested that it will likely enter service by the middle of the next decade.

The U.S. manufacturer is studying market requirements for a 737 follow-on (or “Son of 737”) design, with multiple concepts already having been considered. Late last year Boeing Commercial Airplanes chief executive Alan Mulally indicated 2012 to 2015 as the possible service-entry window.

Initially begun as the smallest of several possible future products of all sizes under its generic Yellowstone program–part of the larger Project 20XX–work is continuing as the 737 replacement study (737RS), with Boeing preparing to talk with potential global risk-sharing partners.

Boeing historically has declined to be drawn on the subject. But the engine manufacturers’ willingness, at last year’s Paris Air Show, to discuss 737 replacement prospects–as well as those for an Airbus A320 upgrade or follow-on–clearly indicated the industry is looking beyond the 787 and a possible Airbus A370 to the next likely new market: the 100- to 220-seat single-aisle sector.

The engine makers’ remarks arose from questions about power for the proposed Bombardier C Series jetliner. At the time, OEMs saw that market as insufficiently certain to offer a new powerplant ahead of expected 737/A320-replacement market requirements. And they all would prefer the 737 replacement to be entering service later rather than sooner because of the time required to develop appropriate technology in the context of current and planned programs. For them, 2010 or even 2012, is too early.

The 737RS profile was raised earlier this year when Boeing employees were told after the February rollout of the 5,000th 737 that aircraft programs vice president Mike Cave would lead a project team. Under Cave, 737 engineering director Carolyn Brandsema is studying aircraft specifics, and production marketing vice president Kent Fisher is considering market demand. Finance director Rod Wheeler is overseeing costs, while Don Moon manages the program.

With the 787 scheduled to enter service before 2009, followed by the stretched -9 variant a couple of years later (and Boeing’s continued disinclination to enter the market for a next-generation very large aircraft), the company is free to consider the 737RS. Meanwhile, Airbus remains preoccupied with getting A380 series production under way and resolving twin-aisle A330/A340 development issues ahead of a possible A320 upgrade or replacement in the next decade.

As Boeing analyses perceived airline requirements, studies include at least three cabin sizes matching, respectively, the 120-to-149, 150-to-189 and 190+ seat requirements addressed by the current 737-600, -700, -800 and -900 variants. The manufacturer also must resolve any demand for higher capacity that might overlap with the planned 787-3 twin-aisle design. This is not a new situation given the almost coincident payload/range performance previously offered by certain 757 and 767 models.

Technology now being developed for the new 787 also would be available for a future narrowbody design, which is expected to involve a carbon fiber all-composite structure, fly-by-wire controls, MORE electric systems and integrated avionics. The major consideration, perhaps above all others, is cabin width, since 737 orders faltered after Airbus introduced the wider A320 almost 20 years ago.

Recent aerodynamic developments seen on other Boeing products should be on the menu, including increased span (787), single-slotted flaps (747-8) and raked wingtips (777/747-8), possibly accompanied by Boeing Business Jet-style blended winglets.

None of the major engine manufacturers is expected to go it alone on a 737RS powerplant; Rolls-Royce has made it clear that its involvement would be only as a shareholder with Pratt & Whitney in International Aero Engines, while General Electric would work with Safran (formerly Snecma) through CFM International. The aircraft is expected to require a generic thrust rating of about 26,000 pounds.

Boeing Commercial Airplanes marketing vice president Randy Baseler has suggested that a replacement for the short- to medium-haul 737 would not need the extra 2,000-mile range with which a lighter composites airframe endows the 787. This leaves fuel burn and manufacturing and operating costs as key drivers. Engine and aerodynamic improvements currently available are expected to provide 4 percent lower fuel consumption and 3 percent lower operating costs compared with the 737–not enough to stimulate development costs of perhaps $5 billion, according to observers.

In teasing remarks at last year’s Paris Air Show, Mulally mused on the options available when using composites. “We could do three different fuselages: five- or six-abreast; we could even do a twin-aisle,” he claimed. After all, Boeing has patented a “twin-aisle small airplane” design to carry 90 to 200 passengers.