Every Boeing commercial airliner since the 707 has been extended at some stage, with two exceptions: the 747 and 757. It now seems that, after many years of study and debate, the 747 is about to be elongated into the -500 and -600 series and the 757is finally set to grow some 18 years after the programme was launched.
In December 1997, Boeing will begin major assembly of the first stretched 757-300. The project was given the go-ahead at the 1996 Farnborough air show, and in mid-November its design was settled. This also set the clock ticking on a fast-paced, 27-month, effort to deliver the first aircraft in January 1999 to launch customer Condor of Germany. This timetable makes the -300 the shortest design-to-production and then delivery-cycle time of any Boeing derivative-aircraft programme.
Boeing is tackling this challenge in two major ways. Firstly, it is keeping the stretch as simple as possible, with very few customised features for the longer version. Secondly, a variety of design and production initiatives from several other programmes, including the 777 and the next-generation 737, are being brought to bear on the 757-300, to help reduce the complexity and cost of its development.
"It's really just a 'fly-more-people' aircraft," says Bruce Nicoletti of 757-300 product marketing. "We've increased the maximum take-off weight capability to 270,000lb [122,600kg - from a maximum of 115,800kg on the -200], and increased seating capacity by 20%, or about 40 passengers. This aircraft will have a 3,500nm [6,500km]-range capability with that load. That's the main design goal," Nicoletti adds. "The extra seating capacity, plus a staggering 48% increase in available cargo volume, lowers operating costs dramatically," he says.
Direct operating costs (DOC) in cents per available seat kilometre over typical European short/medium routes are expected to be around 9% lower for a 240-seat 757-300 than those of the baseline 201-seat 757-200, and some 13% lower than those of a 183-seat Airbus A321-200. Based on a dollars-per-kilometre percentage scale, Boeing believes that the -300's DOC will undercut those of a 232-seat 767-200 by around 10% and those of an identically configured A310-300 by up to 19%.
Structural changes
The basic 757 will be stretched either side of the centre of gravity, with a 4.06m extension of the forward fuselage and a 3.05m extension aft. Overall length is therefore increased by 7.1m, to 54.5m, just 0.4m shorter than the 767-300. Importantly, the extra length is not being added in the form of fuselage plugs. "We're not adding splices, we are extending the existing fuselage sections," explains 757-300 chief project engineer Dan Mooney. The body extension is therefore achieved by extending section 43 (forward fuselage) aft by 4.06m and by stretching the aft body section 46 forward by 3.05m. "We're going to take the overwing exit body section [44], and combine it with a strengthened, four-door, section 46. This will allow us to meet the evacuation requirements without adding any new doors," he adds.
The revised structure produces some large components. "Some of the fuselage panels end up as long as 400in [10.2m]. That's single panels, which may present some handling challenges," says Mooney. Most of the body strengthening will be made to the overwing section 44, although some increases in skin and frame gauges will be made to sections 43 and 46. "The horizontal stabiliser will also be beefed up a bit," adds Mooney. The landing gear will be strengthened by removing less metal from the forgings, and the wheels will be fitted with 26-ply tyres to cope with the heavier landing weight and associated higher landing speeds. Boeing estimates that touchdown speed will be around 140kt (260km/h) at maximum landing weight, or around 5kt higher for the larger aircraft. The nose gear will also be fitted with a spray deflector to ensure that the extended spray pattern of the longer airliner does not affect the engines.
Local strengthening of the wing and engine pylon attachments is also planned. The pylon changes will be based on the strut modification now being implemented on the 757/767 line, to counter the discovery of some fatigue cracks in the current fleet. "The changes will incorporate this and will take account of the slightly higher strut loads that we expect. The strut will be basically strengthened to keep the fatigue life of the present unit," says Mooney.
The 757-300 has been launched by Condor, with the 192kN (43,100lb)-thrust Rolls-Royce RB.211-535E4B. At a maximum take-off weight of 122,470kg, including 43,490litres of fuel and 279 passengers, the charter-configured -300 will be just capable of flying from London to Bahrain in its design range of 5,090km. Popular tourist destinations in the Atlantic islands, the Mediterranean and North African areas are all within range. The -535E4B-powered aircraft will also be able to be flown from New York to Los Angeles, or as far south as Quito, in this configuration. In a mixed-class configuration, seating around 240, the design range increases to 6,000km, increasing range capability to include New York to Paris, or London to Lagos.
The Pratt & Whitney PW2043 engine, rated at 195kN, is being offered for the first time on the 757, says Mooney. The higher-thrust engine has been certificated (as the F117-100) for the McDonnell Douglas (MDC) C-17, and provides slightly greater range capability with a design range of 5,470km. This results from a slightly lower empty weight, because of the PW2000's lower installed weight, and reduced fuel consumption. Boeing says that the P&W-powered 757-300, in charter configuration, will burn 18kg/passenger on a typical 920km mission, compared to 19kg/ passenger for the identical R-R-powered aircraft. The extra range enables the PW2043-charter version to reach the UK from New York and destinations as far east as Tashkent from London. In a mixed-class role, the PW-powered aircraft has a design range of 6,500km.
Systems and Software
Despite the simple-stretch philosophy, the sheer length of the -300 has forced Boeing into certain systems changes. Others, such as the cabin interior and vacuum-lavatory system, have been introduced as state-of-the-art technical advances which have earned their way on to Boeing's other new aircraft.
The -300 will be fitted with a version of the retractable tail-skid developed for the 767-300 and 777 to offset the potential for tail-strikes, which have affected the standard version. "Tail-strike is a concern," says Mooney, who adds that tail-strike events on the -200 prompted some of the initiative. The tail-skid will be augmented by a body-contact sensor, which will alert the crew to a more serious contact on rotation or touchdown. The tail-strike indicator, as it will be called, will consist of a small, frangible, foil extending down from the fuselage. If the foil makes contact with the ground, a message is flashed up on the engine-indicating and crew-alerting system.
"As long as there's no indication, the crew won't have to perform an air turn-back," says Mooney. The design is also intended to make it harder for the crew accidentally to scrape the tail on landing, by linking the pitch-attitude sensor to the spoiler-deployment system. "If the pilot is abusing the landing and coming in nose-high and too slow, the sensors will delay spoiler deployment," says Mooney. Spoilers induce a significant nose pitch moment on the 757 in landing configuration and, by modifying the spoiler-control schedule, Boeing plans to expand the touchdown protection envelope.
The aircraft will also have a revised environmental control system. "The air-conditioning packs are actually 767 systems, as are the generators, but the pre-cooler does not have the capacity, so we're planning to use a bigger one and increase the flow. We're not changing the architecture at all, but we aim to maintain the equivalent flow as in a -200," says Mooney. An extra temperature-control zone has also been added, to maintain closer environmental control in the longer cabin. The other major cabin-system change is the introduction of the centrally plumbed vacuum-lavatory system. "Here, we are simply taking advantage of lessons learned," says Mooney, who adds that the maintenance and operating cost benefits of the new lavatory system make up for its weight.
Boeing has taken the opportunity of the -300 development to introduce the new cabin interior style developed originally for the 777 and then tailored to the next-generation 737. The cabin will therefore have a new sculptured ceiling and indirect overhead lighting. It will also have 0.5m-longer overhead bins up to 2.03m in length. Boeing hopes that airlines will be attracted by the family commonality of the interior with that of the next-generation 737 in spares holdings and maintenance.
Airlines will also have the option of choosing a new continuous handrail which extends along the bottom of the bins and which resembles a similar design used in the MDC MD90. Another option will be convertible seating, which enables a rapid conversion of the seating layout from six-abreast economy class to five-abreast business, or vice versa.
Despite the 20% increase in passengers, Boeing believes that the typical 279-passenger -300 used on holiday charter flights will take only 4min extra to load and an extra 2min to unload. After measurements were taken of turn-arounds at Seatac Airport in Seattle, the company estimates that the larger 757 will take around 8min more to turn around than the 231-passenger charter-configured 757-200. Total turnaround time is estimated at around 66min. A mixed-class -300, seating around 240 people, is expected to take just over 6min more to be turned around than the equivalent -200. Total time for a mixed-class aircraft with bags, but no cargo, is expected to be around 59min compared with 53min for the -200. "We're working on ways to improve that," says Nicoletti, who adds that a "sliding-carpet" baggage and cargo system is offered as an option. The system has a conveyor system and a movable bulkhead, which is pushed along the belly hold by a drive system.
New manufacturing processes are being introduced into the 757 line as part of Boeing's drive to shorten cycle times from ten months to around six months. The changes have affected the current 757-200 production process and will be in place to streamline the introduction of the -300 in 1998.
FAIT takes a hand
The fuselage-assembly-improvement team (FAIT) is upgrading the fuselage manufacturing process to take advantage of computerised numerical control (CNC) machines. These are gradually replacing the conventional build-up tools which have been in constant use since the start of the programme. The old tools have gradually become worn and less reliable and, as a result, the locations of positioning holes have started to move. Boeing was therefore beginning to spend much more time and money on rework to correct quality-control problems.
Hundreds of original two-dimensional Mylar drawings have been scanned into converters to produce CATIA-generated three-dimensional datasets. These are then used by numerical-control programmers to develop operating programmes for the six CNC machines. Two of these are used to drill body panels, two to drill stringers (produced in South Korea by Samsung Aerospace), one drills frames and one shear ties. With the holes already drilled precisely by the CNC machines, the massive tools are no longer needed to hold the assembly in place. Parts are virtually snapped together and support one another during construction.
By August 1996, some 26 of the 53 skin-panel build-up tools used in the assembly of sections 43, 46 and the crown of 44 had been replaced by CNC machines. The balance were expected to be replaced by the year-end. "The goal is to have a lot fewer tools and be a lot more flexible," says Renton 757 site-operations manager Antonio Micale. "It really produces substantial savings in non-recurring costs and produces something with a lot higher quality. The big challenge has been to take the existing design, not change it, and still end up with something flexible." The process also cuts down on the amount of assembly space required. "We have eliminated positions and combined them, so we can make big chunks of the aircraft in one area," Micale adds.
With the tight timetable, the manufacturing-process improvements are a vital step in the right direction. The first wingset is due on the line in September 1997, with major assembly due to begin the following November. The first flight is set for mid-1998 and initial deliveries to Condor will begin in January 1999.
Source: Flight International
