After almost two decades of relative status quo in the single-aisle sector, the marketing gloves are coming off as the next-generation Airbus A320NEO and Boeing 737 MAX are readied for the fray.

The cutthroat contest is already as intense as anything seen in the long-range battles of the 1990s between the A340 and MD-11, while the contrasting claims over the new A320 and 737 make more recent marketing contests between the A330 and Boeing 777 appear almost good-natured by comparison. The vitriol is all the more intriguing given the length of time before either model will enter service. The A320NEO is not due to begin commercial operations for more than three years, while the 737 MAX's debut is even further off, targeted at 2017.

The high intensity is at least partly linked to the current imbalance between the backlogs of the new Airbus and Boeing designs. Since its launch in late 2010, the NEO has amassed more than 2,400 firm orders, options and commitments, while the MAX—which only officially hit the market in mid-2011—has accumulated orders, options and commitments for around 1,040. In terms of firm orders alone, the NEO is far ahead, with 74% of the announced business. This translates to 1,289 firm orders compared to 451 for the MAX.

Nonetheless, both manufacturers claim to have accurate predictions for perhaps the most crucial parameter of all: fuel burn per seat. Despite the fact that none of the new engines that will deliver the bulk of the improvements has yet run, Airbus and Boeing both stick adamantly to their forecasts. It is the wild disparity in these estimates that remains most striking, particularly since the relative performance gap that each claims is not narrowing as the two designs firm up.

Airbus predicts the A320NEO will deliver roughly double the improvement over the current A320 that the reengined 737 MAX will deliver relative to the present 737 production standard. Boeing, by contrast, says the 737 MAX will have a 17% fuel-burn advantage over the current A320, and a roughly 5% edge over the reengined A320NEO with either CFM International or Pratt & Whitney powerplants.

“It's remarkable how different physics are in Europe versus the U.S.,” says Boeing Commercial Airplanes (BCA) Vice President Mike Bair, who reiterates points made at last year's Paris air show about the lower weight of the 737 versus the A320. Boeing believes this fundamentally underpins performance advantages which will be passed on to the MAX generation. According to published figures, the A319's operating empty weight (OEW) is around 4,900 lb. more than the 737-700's, while the A320's OEW is approximately 1,980 lb. more than that of the 737-800. The comparison for the 737-900 and A321 is more complex because of the various options available, but the OEW of the heaviest 737-900ER variant is more than 7,300 lb. lighter than that of the CFM56-powered A321-200.

While the weight differences between the existing families cannot be debated, Airbus counters that the key to the A320NEO's advantage is its higher wing, with its inherently greater flexibility and ability to maximize available fan size for both engine options, the CFM Leap-1A and Pratt & Whitney PW1100G geared turbofan.

Speaking at the recent International Society of Air Transport Traders meeting in Phoenix, Airbus Senior Vice President for Leasing Markets Andrew Shankland says the A320NEO is expected to produce overall fuel-burn performance benefits of 15% compared to current A320 models. Lower engine-specific fuel consumption (sfc) is expected to contribute 15.3%, and the Sharklet winglets another 2.4%. The extra 4,000 lb. in maximum weight added through the larger engines, strengthened structure and winglets will, however, take 2.7% off the overall improvement, so that it is rounded down to around 15%.

Shankland also presented Airbus's analysis of the 737 MAX which, by contrast, indicates a relatively modest 8% improvement over the baseline 737. The bulk of this, says Airbus, will be around 6% from the improved core performance of the CFM Leap-1B engine, with a further 4% from the 12% increase in fan diameter to 68.4 in. from the CFM56-7B's 61 in. A further 0.5% improvement will come from optimized wing-engine shaping, and an additional 0.5% in airframe drag reduction. However, Airbus predicts a 5,300-lb. weight penalty for the changes, which it says will reduce the fuel-burn improvement by around 3%.

Boeing's predictions for fuel burn per seat, conversely, indicate that the 737-8 version of the MAX will be around 5% better than the A320NEO. This is based on a 162-seat configuration for the 737 and 150-seat arrangement for the A320, however. MAX program officials tell Aviation Week that advances in at least two specific design areas provide encouraging signs that performance will be better than Airbus anticipates. Boeing has developed some unspecified design features to reduce the installation effects of the larger engine. Although Airbus credits the Boeing MAX powerplant integration scheme with 0.5% of the design's overall fuel-burn savings, it also estimates that more drag will contribute to a much larger weight and performance penalty.

The same MAX officials also say studies are underway of a slightly larger Leap-1B fan up to 69 in. in diameter. Any increase in fan area is “free specific fuel consumption” reduction and could be possible through an innovative, low-drag installation which cantilevers the engine further ahead of the wing leading edge, taking more advantage of the minor nose leg extension already planned on the MAX. Firm design for the Leap-1A engine for the A320NEO is due later this year, while the design freeze for the 737 MAX's Leap-1B is not scheduled until around the second quarter of 2013.

To date, Boeing and CFM have defined the Leap-1B fan engine at 68.4 in., a slight increase in diameter which will boost performance without negatively impacting weight and drag. Bair says the size is in the “sweet spot” of a 3-4-in. range of potential fan diameters. “At one end of the 'bucket,' it gets lighter, because a bigger diameter will change the weight,” says Bair. He adds that “it's a trade-off because bigger fans are quieter.”

The company acknowledges that further changes are possible. “However, as detailed design work continues and we incorporate wind-tunnel testing results, we'll continue to work with CFM to refine the engine [including fan size] as we work toward final configuration in 2013,” Boeing says.

The new 737 will also be configured with a digitally controlled pneumatic system in place of the current analog bleed system to give “more precise control of the air bleed from the engines for de-icing and cabin pressurization,” says Bair. For improved maintenance control, the MAX will incorporate onboard servers as well, similar to those on the Boeing 787.

Boeing, meanwhile, has entered the final wind-tunnel test phase for the 737 MAX, with high-speed testing beginning in the company's transonic facility in Seattle on March 19. The low-speed portion of the wind-tunnel work started in February at Qinetiq's facility in Farnborough, England. The tests “will update the model to incorporate the minor changes we're making for the MAX and the larger engine nacelles. These initial test runs will give our engineers a baseline for the current airplane's performance that they can compare to the MAX's optimized design,” says Randy Tinseth, BCA marketing vice president.

Major aerodynamic changes for the MAX compared to the current 737 are focused on the aft fuselage, wing, wing-to-engine integration area and the larger-diameter nacelle of the Leap-1B engine. The revised aft fuselage includes a lower-drag Section 47/48 and tailcone as well as drag reduction changes to the aft pressure-relief port cavity.

“We expect to go back into the tunnel later in the year for some minor work such as additional engine/nacelle-related testing, but we expect to substantiate the forecasted performance of the current MAX design during our high- and low-speed tests ongoing at the Boeing Transonic Wind Tunnel and Qinetiq,” says Boeing. “The baseline model, which will be updated with the design changes on the MAX as we get further into testing, is a 737-800.”