Five years had passed since Boeing added a new member to the 737 family, leaving many wondering whether engineers in Seattle had finally stretched the airframe’s capabilities to its practical limits. Then came the July launch of the 737-900ER, and suddenly it seemed clear that the market hadn’t yet gotten its fill of arguably the most commercially successful civil airplane line ever conceived.

“We see our sales and skylines extended well out there,” 737 program chief engineer Mike Delaney told AIN. “If you think about the fundamental things that would drive a replacement, it would have to be a demonstrable benefit to the airlines in life-cycle costs and operating costs and, obviously, we’re always studying the technologies that are being deployed on the 787 and other families. But right now we think we have the airplane to compete for quite a while.”

In fact, industry pundits project that by the next decade Boeing will introduce a replacement using composite technology born of the 787. But, for the time being, no one at the company wants to turn attention away from its newest product by talking about the possibility of a better alternative some time in the not so distant future.

Scheduled for certification in the second quarter of 2007, the-900ER represents Boeing’s first direct competition to the Airbus A321–now the largest of the two manufacturers’ single-aisle families with a high-density passenger capacity of about 220. By adding a pair of type 2 exit doors to the standard 737-900, Boeing raised allowable capacity in the -900ER from 189 to 215, without the need to stretch the fuselage.

Through what Delaney described as a “general beef-up” of the wing, nose and landing gear structures, Boeing engineers managed to increase the -900’s maximum takeoff weight by 13,500 pounds, to 187,700 pounds in the -900ER, allowing for a range of 3,200 nm with a full passenger load, about 500 nm farther than the standard -900. Other changes include a two-position tailskid and optional blended winglets developed by Aviation Partners Boeing. Up to two auxiliary fuel tanks in the belly displace a negligible amount of cargo volume, according to Delaney.

Central to the design effort was Boeing’s ability to extract more range and payload capacity by rescheduling the wing leading edge devices rather than resorting to higher-thrust engines. The Next Generation 737 family generally uses three slat positions–fully retracted, the intermediate or “sealed” position and the fully extended or “gapped” position. Designers found that the 900ER’s body length and takeoff geometries would allow for a sealed slat position on all takeoff configurations. Delaney estimates that the reduction in slat angle equates to between a 3- and 5-percent thrust increase.

Having passed its critical design review in late summer, the program enters its next phase next month, when machinists start to assemble the first airframe for flight testing. In the meantime, as has become customary when either of the world’s big commercial airframe builders launches a new project, the competition has released the public relations hounds to debunk performance and economy claims.

Weighty Issues

Boeing cites a 7-percent trip/ cost benefit and a 5-percent seat/ cost advantage over the A321, based partially on the point that the -900ER’s lower mtow (roughly 10,000 pounds less than the Airbus) will translate into lower airport fees. Although it concedes the weight disadvantage, Airbus claims that Boeing has based its cost conclusions on unrealistic cabin capacity assumptions.

Rebutting the Airbus contention that the -900ER can fit no more than 204 seats, Delaney pointed to launch customer Lion Air’s specification of 213 seats in the 60 airplanes it ordered and optioned. “At 213 or 215 [seats] we meet all the exit requirements of the [U.S. Federal Aviation Regulations], in terms of door size and aisle width, and we’ve been through that with the American agencies and with [the European Aviation Safety Agency],” he said. “The -900 was really a simple stretch of the -800 for a couple of customers who wanted to extend their family line. All of the -900s in service today are in dual class, so the -900s were actually a very spacious airplane.”

In a two-class layout, the standard -900 now holds 189 passengers–the maximum allowed with the airplane’s four exit doors and four overwing exit hatches. FAA and EASA regulations require two more type 2 exit doors in the -900ER to increase single-class capacity to 215. However, Airbus has insisted that unless Boeing “deactivates” those two doors, a dual-class 737-900ER couldn’t seat as many passengers as a standard -900 because of exit row width requirements. Boeing says it can deactivate the doors by simply covering them with a standard sidewall interior and overhead bin. Airbus has implied that European regulators might take an unfavorable view of those plans.

“The key issue was, historically, the JAA [EASA’s predecessor in Europe, the Joint Aviation Authorities] did not like a deactivated door,” recalled Delaney. “The thinking was that a passenger could come up to something that they thought was a door but it wasn’t activated. In the -900ER, the interior of the airplane when the door is not in use will look like a current -900, so from a passenger’s point of view there will be no door there to mislead them in an emergency or an incident.”

So far Arab national carriers have yet to show a lot of interest in small airliners of any kind, although Saudi Arabian Airlines’ recent order for Embraer 170 regional jets has no doubt caught the attention of Boeing and Airbus. Still, without dismissing this region’s potential, Delaney seems more focused on points farther east. “There’s nothing definitive at this time [in the Middle East], but there are areas where we have much larger proposals out or airlines who have actively taken substitution rights,” he explained.

Although by early last month Indonesia’s Lion Air remained the only firm customer, both Japan Airlines, which earlier this year ordered thirty 737-800s and took options on 10 more, and leasing group SALE, which placed a firm order for twenty -800s and took options for another 20, hold substitution rights and appear likely takers.