If it didn’t become immediately apparent when Boeing began alluding to time frames that implied a replacement of the 737 might not materialize until 2020, the company’s recent revelations of a new set of design enhancements certainly erased any doubts that a so-called follow-on will have to wait until designers and engineers squeeze all the efficiency and comfort available from the existing narrowbody family.

The improvements, unveiled in late April during a day of media briefings that included a tour of Boeing’s 737 assembly plant in Renton, Washington, will result in a 2-percent increase in fuel burn and a marked upgrade in cabin comfort and utility, according to Boeing 737 chief engineer John Hamilton. Of course, they won’t come close to the magnitude of gains coveted by the likes of ILFC and Southwest Airlines in an eventual 737 replacement. But for the time being, Hamilton seemed to discount the possibility of any more radical changes such as re-engining with Pratt & Whitney’s new Geared Turbofan.

 “We think the 737 has a lot of life left in it,” said Hamilton. “We’re going to continue to look at how we improve this airplane, but for right now we think we’ve got a dynamite partnership with CFM… and we’re going to stick with them.”

Boeing estimates it will achieve half of the fuel burn improvement through drag-reducing changes to the airframe, while the other half comes from the hardware changes that CFM International plans for the airplanes’ CFM56-7B turbofans under its recently launched Evolution program. Finally, a new interior design inspired by the Dreamliner cabin–called the 737 Boeing Sky Interior–completes the package with not only more striking visual cues, but perhaps more important, added headroom and baggage capacity.

Aerodynamic improvements include “refined” wing control surfaces for less drag over the top of the wings, a wheelwell fairing redesign that results in smoother airflow upon retraction, a reshaped anti-collision light and an environmental control system (ECS) inlet/exhaust modulation.

“It’s the little things that sometimes make a big difference for our customers,” said Hamilton. “[These changes] represent a 16-percent fuel consumption improvement over our Classic 737 airplanes.”

Of course, that 16 percent represents the culmination of more than a decade of product evolution. “The 737 that we delivered in 1999 is not that same airplane that we deliver today,” said Hamilton. “It has evolved and improved over the years and I think that’s one of the legacies of the 737.” Boeing estimates that the latest package of improvements could save an airline up to $1.3 million over 20 years, assuming a fuel price of $2.58 per gallon.

Propulsion improvements include a shortened exhaust nozzle and a longer, recontoured plug, adding to the efficiency of the engines’ exhaust flow and contributing to the performance of a new powerplant package offered by CFM.
According to Hamilton, when Boeing designed the parts in the early 1990s, engineers “optimized” the nozzle for noise rather than fuel burn. “As technology has evolved we’ve been able to find ways to reduce the noise and shorten the nozzle,” said Hamilton. “You make trades when you design the airplane, and we are now optimizing it more for fuel burn and then using acoustic treatments [inside the nozzle] to treat for noise.”

Boeing plans to use one of Continental Airlines’ 737-800s to flight test the engine and drag improvements starting at the end of 2010.

Scheduled to enter service in mid-2011, the CFM56-7B Evolution should deliver a 1-percent improvement in fuel burn over today’s CFM56 Tech Insertion powerplant, introduced in 2007 at an estimated development cost of $100 million. Although CFM’s director of CFM56 Boeing programs Robyn Brands said the company doesn’t usually talk publicly about R&D investment, she did say that the cost of this latest project will rival that of the Tech Insertion undertaking.

“Basically, we’re using advanced computer techniques and 3-D design codes to [improve the efficiency] of the blades and vanes throughout the high-pressure and low-pressure turbines, to help the air flow more smoothly, more efficiently…less losses, which equates to better gas mileage,” said Brands. “The added benefit is by reducing the fuel burn we reduce the carbon footprint by an equal amount.”

The modification includes a change to the outlet guide vanes, the quantity and the shape of the HTP blades in the high-pressure turbine, along with the disc. All told, CFM plans to reduce the number of airfoils in the high- and low-pressure turbines by 9 percent, said Brands, who also noted that the company will change the shape of the airfoils throughout the LP turbine.

The result, expects CFM, will be a reduction in the operating temperatures of the parts, said Brands, resulting in longer time “on wing.” The company estimates that operators will see up to a 4-percent reduction in maintenance costs, depending on thrust rating, in addition to the 4- to 12-percent improvement it already achieved in 2007 with the introduction of the CFM56 Tech Insertion engine. CFM plans no change to the engine’s noise signature.

Ground testing on Boeing’s nozzle improvements began in late April at GE’s test center in Peebles, Ohio, said Brands. CFM plans to start ground testing the actual engine improvements in September, followed by the start of flight testing at GE Aviation facilities in Victorville, California, in February next year. By that time the company expects to have completed 150 hours of block testing, during which time plans call for the engine to undergo five runs of 30 continuous hours at maximum fan speed, core speed and operating temperature, known as triple-redline.

If all goes according to plan, the engine would gain joint certification by the U.S. Federal Aviation Administration and the European Aviation Safety Agency in July 2010. Flight testing on a Boeing 737 would start in October 2010, ahead of expected certification for that airplane in the second quarter of 2011. Once certified, the Evolution engine will carry the nameplate CFM56-7BE.

Meanwhile, seven airlines have agreed to incorporate the new interior design planned for the 737 starting in late 2010. FlyDubai, Continental Airlines, Norwegian Air Shuttle ASA, Malaysia Airlines, the UK’s Tui Travel, Gol Airlines of Brazil and Indonesia’s Lion Air have all committed to the new cabin, characterized by soft blue LCD lighting overhead, sculpted sidewalls and window reveals designed to draw passengers’ eyes to the airplane’s windows. The sidewall design also integrates the air vent in a way that will ease preflight security checks for maintenance staff.

The new interior design includes larger, pivoting overhead stowage bins that add to the openness of the cabin and allow for more roller-bag capacity. Finally, Boeing redesigned reading-light switches so passengers can find them more easily and avoid accidentally pressing the flight-attendant call button.