Boeing has begun assembly of its firstnew-generation 737.Guy Norris/SEATTLE

IT IS UNPRECEDENTED but, by mid-1997, Boeing's Renton site in Seattle, Washington, will be producing six different models of the same jet airliner. The aircraft is the best-selling 737, and the ramp-up represents the phase in its development when production of the present -300/400/500 series begins to be phased out in favour of what the company hopes will be the equally successful 737-600/700/800 series.

When Boeing began studies for its next-generation 100- to 200-seat airliner in the early 1990s, only a few suspected that the answer lay in yet another rebirth of its 30-year-old "baby" twinjet design. "Market requirements drove us to look at five proposals covering everything from a basic derivative to an all-new aircraft," says 737 New Generation assistant chief-engineer Peter Rumsey. "In the end, we recommended a major derivative because of commonality with the existing fleet and because it was a simpler solution."

The timescale for the 737-X, as the programme was first called, also dovetailed neatly with developments at nearby Everett, where entirely new business and production methods were being devised for the 777. The major elements of this new approach - customer involvement, integrated product-teams, and digital design and pre-assembly - would also become foundations for the 737-X effort.

"It's been called a double-derivative programme," says 737-600/-700/-800 programme manager Jack Gucker. "It's a derivative of the 737 and of the processes used in the 777, which we've improved and added to." Digital pre-assembly and design techniques have been taken one step further to include the use of digitally designed production tooling. "Similarly, the integrated product-teams are taking whole volumes of the new aeroplane, like the wing, rather than individual systems such as on the 777," says Gucker.

The aircraft is broken down into three areas of responsibility. Boeing Wichita has the body and empennage and will transport to Seattle fully assembled fuselages up to 39m long (in the case of the -800, the longest version) on specially developed flatbed railcars. Boeing's Renton division has responsibility for the propulsion system, including strut, nacelle and CFM International CFM56-7 engine, and also has the wing.

Nowhere is the "777 derivative" aspect more apparent than in the recently revamped Renton Wing Responsibility Center (WRC). This unit uses production systems developed for the 777, but which have grown to full maturity on the 737, says WRC manager Patrick Day. Automated wing-manufacturing tools, designed using the CATIA computer-aided design system, are installed and work began on the first 737-700 wing main-spar on 1 December.

technical advances

Technical advances within the new-look WRC range from low-voltage electromechanical riveting machines, and environmentally friendly, non-toxic, wing-panel-seal booths, to reconfigurable tooling. "The benefits of digital product-definition flow right through the process to the extent that we can also build different wings on the same tooling. We can change some tools from the 737 wing to the 757 in about 20min," says Day.

Design definition of the new large wing - the most obvious change from the present-generation 737 - is 90% complete, according to Rumsey. The advanced supercritical wing was sized to improve take-off and landing performance at the 737 New Generation's higher gross weights and to enable the aeroplane to cruise faster at higher altitude. Economical cruise speed is expected to be Mach 0.79, compared with M0.745 for today's aircraft. The new wing will also allow a "sprint speed" of M0.82.

The wing holds up to 26,035litres (6,880 US gal) of fuel, allowing the 108-passenger -600, for example, to fly as far as from Boston to Paris, or the 162-passenger -800 to comfortably reach Seoul, South Korea, from Singapore. This 30% increase in fuel capacity trans- lates into an overall range increase of 1,660km (900nm) over the current 737.

"The bigger wing helps us get to 41,000ft [12,500m], so we have a 2,000ft advantage over the competition. This aeroplane has finally grown up," says Rumsey, whose association with the 737 began 30 years ago with the original -100 series. Wing chord is increased by 500mm and the total span by almost 5m. As a result, total wing area is increased by 25% to 125m2.

The revised wing and lengthened fuselage (in the case of the -800) have resulted in increases to tail-surface area. The vertical stabiliser is increased in area over the -300 by 5.5m2, to 26.4m2, while the area of the horizontal stabiliser is enlarged by 1.3m2, to 32.8m2.

Changes to the rudder-control system have also been made following a US Federal Aviation Administration and National Transportation Safety Board audit on the type. The audit was prompted by 35 reports since early 1991 of incidents or accidents involving 737 rudder anomalies, and by continued efforts to find the cause of the crash of USAir Flight 427 at Pittsburgh in September 1994.

The changes include extra limiting on the rudder authority to ensure that lateral control can always overcome the rudder, and changes to the power control unit. The redesigned yaw damper is "utterly and completely changed", says Rumsey, citing nagging problems caused in the past by the coupler between the yaw damper and the rudder.

propulsion system

Much of the early development is focused on the propulsion system, for which Boeing has again followed the example set by work on the 777 and assumed overall responsibility. The CFM International (CFMI) CFM56-7 is due to have its first flight on General Electric's 747 testbed in January 1996. The four engines in the ground-programme have so far accumulated over 500h and will be run for up to 2,500h before certification, in November 1996.

The engine is identical for all three versions, but thrust can be varied - from 82kN (18,500lb) for the -600 to 117.5kN for the -800 - by software changes in the full-authority digital engine-control system. "That will help our production and Boeing's," says CFMI's CFM56-7 programme manager, Bruce Hughes.

The engine has performance benefits, most of them derived from the Snecma-designed wide-chord fan. The combination of the fan, the longer-fan-duct Rohr nacelle and lengthened exhaust duct has also helped noise signature. Successful noise tests have just been completed, says Hughes, who adds that the 737 New Generation will be cumulatively 12dB quieter than the present aircraft.

The price to pay for the improved performance is higher weight. Rotor weight is up about 35%, compared to the CFM56-3. The expected weight increase of the stationary part of the fan module (vanes, guide vanes, casing and frame), however, has been largely offset by changing from a steel fan-frame on the current CFM56-3, to a lighter titanium fan frame for the -7. Stationary-section weight has increased by only about 5% compared to that of the -3, according to Hughes.

The higher energy of the fan, however, has also created extra work on fan/inlet stability and containment and, following tests, two sections of the inlet have been beefed up. Boeing and Rohr are attacking the resulting weight penalty, which has been slightly exacerbated by the need to stiffen the exhaust duct after some cracking was found. "We've had quite a bit of success so far," says Rumsey, who says that there is still some weight to pare off.

In parallel with the engine effort is a major development programme for the AlliedSignal 131-9(B) auxiliary power-unit (APU). Four APUs are involved in the programme, one of which is enduring low-temperature trials in Alaska. To date, the APUs have amassed almost 2,800h and undergone around 6,400 starts.

Another key development, and yet another related to the 777, is the new flightdeck. "We struggled long and hard about what to do in the cockpit because that's where all the commonality comes from," says Rumsey. The decision was made to change to 777-type flat-panel displays, which give the impression of a major change, but computers are used to replicate the exact location, format and size of current instruments on the new displays. Rumsey adds that the system could be sold to an airline, such as Scandinavian Airline Systems, which does not have 737s, with displays programmed to replicate those of the 747, 767 or 777.

New-model operators will therefore decide whether their Honeywell active-matrix liquid-crystal displays will be programmed in either an electronic-flight-instrument-system/map format or a primary-flight-display/navigation-display format. Provision for a head-up-display and the global positioning system (GPS) is also being made standard.

With just under two years to go before the first New Generation 737 is delivered to launch customer Southwest Airlines, the orderbook already exceeds 214, an industry record for a new jet airliner. Boeing Commercial Airplane Group president Ron Woodard comments: "We knew that this aeroplane would be popular, but sales have exceeded our expectations." Gucker adds that "...there are already 2,750 or so out there, and we're anxious to extend this line even further".

european phenomenon

Since Southwest launched the programme in November 1993 with its order for 63 aircraft, every subsequent airline customer has been based in Europe. Attempting to explain the phenomenon, Gucker says that many European airlines missed the chance to buy five or six years ago because their profits were insufficient. Boeing hopes that an intensive sales drive in Asia and with the US majors, where the faster 737-800 in particular is being pushed as a 727 replacement, will result in sales passing 300 early in 1996.

Although sales are swiftly mounting, the process of working out the certification basis for the aircraft is dragging along by comparison. Despite wrangles over the "grandfather rights" issue, Boeing is confident that the certification basis, originally due to be agreed by the third quarter of 1994, will soon be resolved.

"We took advantage of grandfather rights in the past on the 737, but we're not doing that any more," says Rumsey. Instead, Boeing volunteered itself into the fledgling US Federal Aviation Administration/European Joint Aviation Authorities concurrent certification scheme. The ensuing "top-down" certification requirements state compliance with the latest amendments, "...except where compliance is not technically or economically practical, and there is no overriding safety consideration".

Assuming that the certification basis agrees with the digital design now 74% defined, Boeing is poised for a busy three years. The first 737-700 is due to be rolled out in December 1996, followed by a first flight in the first quarter of 1997. Around four months later, the first -800 is due to be rolled out of the Renton factory, followed by the first -600 at the end of 1997.o

New 737s will retain "Flattened" nacelles

Source: Flight International