Stretching a stretch

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Guy Norris/SEATTLE

Boeing's newest version of its 767, the -400ER, is unusual for two main reasons. It is the first widebody airliner to be stretched for a second time, and it is the first Boeing commercial jet design ever to directly involve Douglas Aircraft engineering input, thereby marking a milestone in the history of US civil aircraft development.

"It was amazing," recalls 767-400ER chief project engineer Hank Queen. "We had such a shortage of people at Boeing that, even before the merger talks began, we signed an inter-technical assistance agreement with Douglas." The original deal, dating from late October 1996, stemmed from crushing pressure on Boeing's resources from its efforts with Next Generation 737, 747-500X/600X, 757-300 and Joint Strike Fighter development. The build-up at Boeing contrasted sharply with a sudden hiatus in design work at Douglas caused by cancellation of the MD-XX programme by the McDonnell Douglas (MDC) board earlier that month. Combined with MDC's loss of key defence contracts, the MD-XX decision was a big factor in pushing both sides into merger talks.

When Boeing and Douglas teams set about the -400ER design, no-one on either side at the project level even suspecting the possibility of a merger, such events were still in the future, however. Douglas, which made up 60% of the strength of the original team, was given work on the empennage (Section 48) and wingtip area. The two companies also pioneered the "virtual teaming" concept, which Boeing now employs regularly for long-distance design efforts with subcontractors and partners on several programmes. Following the formal launch of the 767-400ER in January 1997, the project team was concentrated in Seattle and the Douglas engineers subsequently "transitioned back" to Long Beach, says Queen.

UNDERSTANDING THE PROGRAMME

Now, almost 18 months later, the first parts of the big twin are in production. "We have 50% release on all our engineering at this stage," says Queen, who adds that "-at least half of the work is behind us now". Major assembly is due to begin in January 1999, and "-our focus is making sure we understand the build plan, flight test and tooling". Boeing believes that its "understanding" of the -400ER is already better at this stage. This is mainly because of strict adherence to a requirements management process which Queen says has been vital in controlling costs. These increased as the -400ER evolved into "more than a simple stretch". The process helps to reduce costs by imposing rigid control over all design aspects. "We don't have any piece of work anywhere that is not a valid requirement," says Queen, who adds that the process controls 25 documents containing 16,000 requirements. It also provides a method for double checking exactly what is being done, and when.

Boeing also hopes to keep costs at bay by establishing the certification basis much earlier than usual in the development cycle. The harsh lessons of the Next Generation 737, and to a lesser extent the MD-90, are being brought to bear on the -400ER, which is expected to benefit from an unprecedented level of harmonisation between US and European regulatory authorities. Other than last-minute differences over issues such as smoke detection and fire suppression in the lower cargo hold, both the agencies are close to agreement. "We're driving towards closing the certification basis by the end of August. We want to close this early because it really helps stabilise the design and the test programme," says Queen.

Establishing the certification base has been made harder because of the changing nature of the -400ER. From its beginnings as a niche-market, simple stretch of the -300, the aircraft has evolved into a substantially different derivative. The result is a product aimed at the transcontinental growth/replacement market to supersede early Airbus A300s and A310s, as well as 767-200s, and as an intercontinental replacement for McDonnell Douglas DC-10-30s and Lockheed L-1011 TriStar 500s. In this respect, the -400ER is uncannily similar to the original 767-X concepts which eventually evolved into the 777-200.

DERIVATIVE APPROACH

This time, Boeing has stayed firmly focused on the derivative approach for several reasons. Primarily, the -400 addresses a gap in Boeing's product range which Airbus is attacking with the A330-200. It also gives the 767 family a new lease of life as it enters its third decade.

To develop what is almost a new model, yet to retain maximum commonality with its forebears, the -400ER design incorporates tricks learned on the New Generation 737, 777 and 717. As a result, configuration objectives included not only the use of existing 767 structures, fuselage design, engine nacelles and struts, but also the introduction of dramatically new features such as a 777-style flightdeck.

As the objectives also included keeping the same type rating as the 767-200/300 and maintaining a common type rating with the 757, the original flightdeck was unchanged from those of the current models. The new flightdeck was adopted late in the programme and "-wasn't a part of the aircraft when we launched", says Queen. "Both Delta and Continental bought it with the existing cockpit. But, after 20 years, we've pretty much maximised what we could do. A lot of operators are very happy with what they've got, so we had to figure out how to develop something with commonality."

The solution is based on six Honeywell 200 x 200mm liquid-crystal displays that can mimic either the 757/767 electronic flight instrument system format or the primary flight and navigation displays of the New Generation 737, 747-400 and 777. Other changes include a new main instrument panel and glareshield, centre console, two-position landing-gear selection lever and a new air-data/inertial-reference system which integrates the air-data computer and inertial reference units.

Other internal changes include an all-new interior, featuring streamlined architecture and pivot-type overhead bins similar to those of the 777. Another feature borrowed from the bigger twin is the 120kVA electrical power system and its attendant integrated-drive generators. The higher power and air-conditioning requirements of the larger cabin also drove the development of a bigger auxiliary power unit. The resulting AlliedSignal 331-400 is 40% more powerful than the GTCP332 found on current -300s and was developed from the 331-400 used in the A330/A340.

STRETCHED BODY

Naturally, the most obvious external difference is in the longer fuselage, which, at 61.4m (201ft), is 6.4m longer than that of the -300. The stretch is made up of an aft plug of 3m and a forward plug of 3.4m, which combine to provide about a 15% increase in available seats. In a typical tri-class configuration this increases capacity from 229 to 245 seats. Range is pegged at around 10,360km (5,600nm), with a maximum take-off weight (MTOW) of just over 204,000kg (450,000lb). Although Boeing says that this satisfies 99% of all current -300ER operations, it is studying a further weight increase, to 208,840kg. This would involve using the horizontal tail to store fuel, thereby stretching the aircraft's range to around 11,100km.

Queen says the jump to the higher weight is unlikely to come quickly, however, as it will almost certainly require the development of a 68,000lb (302kN)-thrust engine. The two engine suppliers for the programme, General Electric and Pratt & Whitney, currently are developing engines of 62,000-63,400lb thrust. Although these are derivatives of the CF6-80C2 (B7F1 and B8F) and PW4000 (4062), respectively, both companies recognise that a more powerful engine - in the 67,500lb thrust-class - could also power the proposed 747-400X and therefore they have suggested development programmes to Boeing. "We've got some preliminary product development studies going, but we are going to follow customer demand," cautions Queen.

The increased body length also requires a taller main gear, to achieve acceptable rotation angles, take-off speeds and distances. "We'd have had to do something anyway because we were increasing MTOW from 410,000lb to 450,000lb," says Queen. The larger gear was the subject of a complex trade study, which evaluated a three-cylinder "shrink link" gear and larger, strengthened conventional gear with a relocated wing-mounted trunnion. The shrink link gear, which effectively telescoped to allow for more compact stowage, was similar to that used on the A340. "It did not receive a warm reception from the airlines because of maintenance concerns," says Queen, adding that he found the final choice surprising.

The resulting gear is 460mm taller than the current unit. The wing-mounted trunnion is moved outward by 50mm and downward by 100mm. The gear is fitted with the same wheels, brakes and radial tyres as used on the 777. "We have 46in [1.17m] tyres on the -300, whereas 48in was our baseline and the 777 has 50in tyres," says Queen, who adds that space limitations meant the design team was "-not sure we would be able to use the 777 wheels, but by cleaning up the wheel-well space, we were able to use them - which was a big win for us and the customers."

The strengthened main gear and larger wheel assemblies resulted in an overall weight increase of 1,360kg, which, in turn, required a higher-capacity air-drive unit to speed up retraction. The improvement was enough to allow obstacle clearance limits to be raised, resulting in an increase in payload capacity of up to 1,270kg. A further 455kg was created by adding a shorter, crushable, tail skid, allowing a slightly higher rotation angle for take-off.

RAKED WINGTIP

The wing is also significantly modified, with increased-gauge skins and thicker ribs and spars. As a result, the -400ER wing is quite different from that of the -300ER and it has few common parts. The most notable external change to the wing is the tips, which sport Boeing's newly patented raked extension. These improve the overall aerodynamic efficiency of the wing, while reducing take-off field length and increasing climb performance and range, as well as helping to reduce fuel consumption.

The raked wingtips represent a compromise between a relatively straightforward span extension and winglets, the latter having being considered until quite late in the -400ER design evolution. The design was adopted when studies revealed that the 767 wing would be aerodynamically and structurally better suited to the new device than to a winglet.

The overall advantage of the design is its simplicity, although the "-structural join to the wing is more complex because of the bending loads, which are higher than a winglet's", says Queen. The tips are 2.34m long and are swept back at a 56.9¼ angle. They are of composite construction, with an aluminium leading edge to protect against erosion, and are structurally reinforced with an aluminium spar. Other than a revision to the outboard leading-edge slat, the raked tip requires no significant changes to the basic 767 wing design, and the -400ER will even be capable of dispatch with the tips removed.

Another advantage, says Queen, is gate access. "The baseline configuration had a span of 181ft [55.2m] with winglets, which was chosen before we really began the wingtip studies," he says. Not only did the raked wingtip allow Boeing to meet the mission goals with a reduced span of 51.9m (compared to 57.6m for the -300), it allowed the -400ER to use the same gates as the DC-10, MD-11 and L-1011.

Meanwhile, manufacture of the first parts of the -400ER is now under way. Initial landing-gear forgings were produced in July and the first wing-rib posts are in production at Boeing's Auburn site in Washington. The first aircraft is due for final body join in June 1999, with roll-out scheduled for about the end of August that year. The first flight is tentatively set for 1 October, 1999, with initial delivery to launch customer Delta scheduled for May 2000.

By this month, sales had reached 52 aircraft, with 26 orders for Continental, 21 for Delta and five for International Lease Finance. Although it has been static for some time, the orderbook is widely tipped to grow before the end of 1998. Queen says that there is "significant interest in the programme", and he adds that "-we still expect it to be a best seller". If the last two decades of 767 history are anything to go by, he may be proved right.