Quite possibly the last member of the best selling family of airliners in the history of the industry, the recently certified Boeing 737-900ER has at once filled a void in the 200-seat-class market left by the production retirement of the 757, presented Airbus with its first direct competition to the A321 and provided the platform on which CFM International launched its Tech Insertion upgrade for the CFM56-7 turbofan. Of course, the industry’s insatiable thirst for more capability and cost efficiency won’t allow the 737’s engineering team to reflect for long on their accomplishment, nor will the competition.

“We are going to continue to look at what the market wants,” Boeing 737 chief engineer John Hamilton told AIN during a pre-show briefing. “I think we’ve got a long road ahead on the 737. It’s selling quite well and we’re continuing to look at how we can invest in the airplane to provide features the airlines value…We’re constantly working with CFM, especially along the lines of fuel burn.”

For now, CFM’s Tech Insertion has given the CFM56-7 a 10-degree C higher exhaust-gas temperature margin, helping it burn cleaner and cooler and cutting emissions to meet the latest ICAO standards. Since Boeing delivered the first of sixty 900ERs ordered by Indonesian discount-fare carrier Lion Air in April, every newly built 737 has rolled off the Renton, Washington assembly line with the Tech Insertion upgrade.

By the end of this year, Boeing expects to test fly the first set of carbon brakes ever installed on a 737–on a 900ER, in fact–in the next major effort to reduce weight in the airplanes. According to Hamilton, the switch will result in a weight savings of about 700 pounds.

“Carbon technology has really improved from what it was even just a few years ago,” he said. “They used to degrade especially with runway de-icing agents, but the companies that make the carbon technology today have actually introduced new agents to improve that.” Messier Bugatti would supplant steel-brake supplier Honeywell for the supply of carbon brakes, while Boeing’s other steel brake supplier–Goodrich Aerospace–will also provide carbon brakes.

The weight savings of carbon brakes would coincidentally just about offset the weight disadvantage of replacing the domed rear pressure bulkhead with a flat design in the 900ER. Similar in design to the bulkhead used in Boeing 757, the 900ER’s flat bulkhead allowed Boeing to “stretch” the interior by 26 inches, or nearly the equivalent of another row of passenger seats, without changing the fuselage length.
However, the need for thicker-gauge support ribs than even Boeing originally anticipated increased the weight by between 600 and 700 pounds–quite a bit more than first thought.

Notwithstanding the weight penalty, Japan Air Lines accepted Boeing’s offer to use the same flat bulkhead on its 737-800s, the first of which the company delivered last year. However, Boeing has encountered difficulty making a case for the option in the 800, precisely because of the weight issue. “The 800’s our best selling model; it provides great economics and the operators are basically saying they don’t need that extra stretch to get better economics on that airplane,” said Hamilton.

Boeing has “kicked around” the idea of using a different material in the bulkhead to extract some weight savings, he added. “We haven’t made any decisions to do that but we’re constantly looking at investing in this model of this airplane, and looking at we can do to further lighten the weight overall,” said Hamilton.

Still, the 737 design team met its weight goal with 190 pounds to spare, said the chief engineer, enough to account for another revenue passenger. Certified to carry 220 passengers compared with 189 in the standard 737-900, the 900ER derives most of its added capacity from the addition of two Type II emergency exit doors behind the wings. Aside from the extra doors, the fuselage otherwise looks identical to that used for the standard 900. At 187,700 pounds, maximum takeoff weight increases by some 13,000 pounds, but still totals some 10,000 pounds less than that of the Airbus A321.

Boeing accommodated the extra weight through traditional methods–by using heavier-gauge wing skins, stringers, cords and webs along with stronger landing gear, supports and higher capacity wheels, tires and brakes. Other changes include a two-position tailskid and optional blended winglets. Up to two auxiliary fuel tanks in the belly displace a negligible amount of cargo volume and increase range in a typical two-class, 180-passenger configuration to 3,200 nm, about 500 nm farther than the standard -900 and roughly the same as the 737-800. Although the engine core remains the same, Boeing now offers both 26,000- and 27,000-pound-thrust options.

Ultimately, the success of the design hinged on 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, resulting in an aerodynamic advantage that equates to at least a 3-percent thrust increase.

Of course, the goal of the entire exercise centered on increasing capacity beyond the 189 seats regulatory authorities would allow with the standard -900’s four exit doors and four overwing exit hatches. All but one of the 900ER’s customers, including Lion Air, have opted for more than 189, which generally means a single-class, high-density configuration. Although it managed to secure authority to offer a 220-seat layout, such a configuration would leave a somewhat tight 28-inch seat pitch. In fact, Lion Air’s airplanes come with 213 seats and Boeing originally sought to go only as high as 215, but, according to Hamilton, some potential customers expressed an interest in still more seats, prompting the company to successfully certify it to 220 absent any structural changes.

Continental Airlines, the 900ER’s second-biggest customer with a firm order for 30 airplanes, stood as the one exception, opting for a dual-class configuration that will result in a capacity of somewhere near 160 seats, said Hamilton. Typically, to allow enough room to seat as many passengers in a two-class configuration as it can seat in a standard -900, Boeing offers to deactivate the two new exit doors and decrease the width of what otherwise would serve as exit rows. Although deactivating the doors actually saves weight due to the lack of need for certain related hardware and mechanisms, in the interest of further weight savings Continental asked for a fuselage plug instead. Although by early last month the sides hadn’t yet worked out all the details, Boeing expects to deliver Continental’s airplanes, scheduled for first deliveries some time next year, with plugs rather than deactivated exit doors. “That’s the plan right now,” said Hamilton.

Airbus had predicted a resistance to Boeing’s deactivated door plan when it came to light at product launch in 2005. The European consortium also claimed that Boeing based its operating cost conclusions on unrealistic cabin capacity assumptions. Boeing promotional literature cites a 9-percent trip-cost benefit and a 7-percent seat-cost advantage over the A321, based partially on the point that the -900ER’s lower mtow will translate into lower airport fees. Granted, even a 215-seat cabin layout would result in a tight squeeze for the passengers, but the FAA’s endorsement appears to have validated Boeing’s claims, at least in an academic if not a practical sense.

According to Hamilton, the 220-seat approval gives the 900ER a seat-cost advantage over the A321 “on the order of eight percent,” and, he said, leasing companies in particular like the flexibility of the deactivated door option. In fact, GE Capital Aviation Services has ordered four of the airplanes ahead of lease deals with European tour operators.

“It makes a great airplane for a tour company because if you want to increase it above 189 passengers and fly the same routes as the [737]-800 you can. Also, if somebody wanted to combine it with existing 757 airplanes or other 737 airplanes, it makes a great match,” said Hamilton, who wouldn’t discount the possibility of a commercial reaction from Airbus. “It’s had its own issues that have been brought out in the media, but I do not underestimate Airbus by any means,” he said. “It is a very good company and I am sure that it is looking at what it can do to further improve the competitiveness of its airplane going forward to compete with us.”