Boeing is now offering a variant of the 737–its smallest model–designed to carry about as many passengers as the biggest example of its first jetliner, the four-engine 707 of the 1950s. The new Model 737-900ER will carry some 215 passengers in the latest single-aisle twinjet, compared with 219 in the first-generation 707 and 99 in the original 737. And by next June, Boeing plans to have the -900ER entering service with Indonesian launch customer Lion Air.

Here at the Farnborough show this week, Boeing could confirm more orders for the -900ER as some airlines that previously announced plans to acquire generic 737 family models may have resolved their specific choices.

Launched 12 months ago, the extended-range aircraft is to be formally rolled out next month, ahead of a first flight scheduled for September, with airworthiness approval expected before next April. The aircraft was nominally rolled out early last month following the wing/body join-up as it was prepared for the moving assembly line at Boeing’s Renton factory, near Seattle. Before that, the manufacturer had started to install the -900ER’s systems, beginning with hydraulics and electrical equipment.

This month, the U.S. manufacturer is continuing to install instrumentation and completing ground-vibration tests, deputy program manager Patrick Schirmer told Aviation International News. The company has announced orders for 33 examples of the 737-900ER from two customers, including 30 for Lion Air.

The -900ER has 102.4-inch longer fuselage than the standard model which, combined with a new flat rear-pressure bulkhead, provides single-class capacity for up to 215 seats and space for additional fuel storage. To support the heavier loads, Boeing has strengthened the fuselage, wingbox, center section and landing gear.

When the aircraft is configured for passenger loads above the original -900’s legal-maximum of 189, ER operators will have to activate an additional Type II mid-exit door on each side aft of the wing to meet emergency-evacuation airworthiness-approval requirements. Boeing is installing the additional doors as standard to optimize production procedures and is about to conduct an evacuation demonstration test.

In use, the new emergency doors are hinged down from the lower sill, with inflated escape slides canted slightly aft so they can remain parallel to and avoid interference with the wing trailing edge. The nearby cabin interior is blanked off with plain sidewall panels when it is not activated. To facilitate passenger egress, two bins have been removed from the overhead storage area above each of the exits and replaced aft of the cutout by a smaller half-bin.

The nearest passenger seats will comprise two double rather than triple units, with space left adjacent to the sidewall. Space between seat rows also is increased from 13.5 to 20 inches to satisfy U.S. Federal Aviation Administration and European Aviation Safety Agency standards.

The new rear pressure bulkhead–set to become production standard on new-build 737s–is cabin-wide at floor level (rather than dished) and extends the cabin floor aft by 26 inches, compared with earlier 737 variants. This doubles to six the number of full carts that may be stored in a galley aft of the rear doors.

The arrangement retains room for the existing four half-carts and permits elimination of a forward galley to make room for the maximum seating. Alternatively, two full- and two half-carts may be installed at the back with a split half-galley alongside a toilet, again aft of the rear doors.

Low-speed performance enhancements have been introduced, comprising sealed leading-edge slats for takeoff, shorter idle-thrust delay and two-position tailskid for landing, and increased spoiler deflection for use in both phases of flight. The enhancements are aimed at improving aborted takeoff and landing performance.

The slat development involves a new “sealed” position that does not permit airflow between the leading-edge devices and the wing when partially extended at takeoff, regardless of which degree of flap has been selected, while the “gapped” slat position adopted for the approach to landing is retained. On the flight spoiler system, which is used to reduce local lift or ground speed, available deflection has been almost doubled from between 33 and 38 degrees to 60 degrees when used as speed brakes, while available inboard ground spoiler deflection is increased from 52 to 60 degrees for maximum lift dump.

To help reduce landing-distance requirements, Boeing is making ground-idle power available more quickly by reducing the engine spool-down transition delay from approach idle from five seconds to two seconds through changes to the full-authority digital engine controls. Pilots also will be able to use lower Vapp approach speeds through the adoption of (ultimately limited) higher angles of attack.

To protect the rear fuselage in extreme situations, Boeing is introducing a five-inch extension to the tailskid beyond the takeoff position. The slat and tailskid changes are also offered for the smaller 737-600 and -700 models, while all four performance-enhancement elements are options for the -800.

In response to airline range requirements, there is a provision for up to two auxiliary fuel tanks in the -900ER’s underfloor area. With one or two tanks installed, available cargo volume is reduced by 154 and 242 cu ft, respectively, from the -900’s 1,827 cu-ft capacity.

Boeing argues that, when modified with optional blended winglets and carrying standard fuel and 180 passengers under typical mission rules, a 737-900ER offers 2,820-nm range, about 500 nm more than does the Airbus A321. It was only when loyal 737 customer British Midland Airways jumped ship to order this largest member of the Airbus single-aisle family that Boeing agreed to go ahead with the 737-900.

The wingletted -900ER’s claimed range advantage decreases when it and the A321 are both equipped with auxiliary tanks. With one such unit, Boeing says the -900ER’s 3,020-nm range is 435 nm better than that of the A321, while with two tanks it is still 325 nm ahead of its competitor, at 3,200 nm. The U.S. manufacturer points out that with two extra tanks, the A321’s 2,875-nm performance is little more than that of the standard 737-900ER, although Boeing presentations do not specify the comparative ranges for a 737-900ER without the post-production winglet modification.

The 737-900ER is to be powered by two 26,000-pound-thrust CFM International CFM56-7B-26 or 27,000-pound-thrust-27/F or -27B1/F engines. The -27 variants can be operated with 36-degree F higher gas temperatures, but many customers could exchange that for longer periods between removals.

So, is the -900ER the ultimate 737 variant, given that Boeing has appointed a team to consider the elements for a follow-on design–with the smart money betting on a service entry in around 2013-2014? Not necessarily, according to Schirmer. He told AIN that Boeing continues to study possible future variants of the current (now third-generation) 737. It even has looked at extending the landing gear, a necessary characteristic of any stretched development to accommodate rotation requirements at takeoff. Although he acknowledges a “diminishing market demand,” he said any longer variant would be equipped with a taller undercarriage.