Guy Norris/LOS ANGELES
This month, Condor took delivery of the first 757-300, the longest single-aisle aircraft ever built by Boeing in Renton
It has been a long time coming, but the stretched 757 is here. An astonishing gap of 18 years separated the launch of the baseline aircraft and that of its -300 derivative in 1996. That gap is the biggest in the history of Boeing jet airliner derivative development. It is likely to be beaten only if the company stretches its 747.
The idea behind the stretched 757-300 is simple - increase seating by 20% and reduce direct operating costs (DOCs) by 9% over those of the -200. Boeing made the programme as quick and simple as it could. It aimed to complete development and certification in just over 27 months from go-ahead to type approval, achieving this unprecedented target in January.
Boeing began studies of a stretched 757-300X around 1993, but it was not until two years later that the concept of this low-cost people mover grabbed the attention of German charter airline Condor Flugdienst. The carrier was looking for a modern alternative to its fleet of older McDonnell Douglas DC-10s on its busy "Canary conveyer" run from Germany to the Atlantic holiday islands. The -300 seemed the ideal choice.
Boeing's studies suggested that, with about 40 more passengers, the -300 would still be capable of ranges of up to 6,475km (3,500nm). It also said the stretch would have a 13% DOC advantage over the Airbus A321-200, which had begun to make a significant penetration of the 757 market. Based on a dollars-per-kilometre percentage scale, Boeing also estimated that the -300 would undercut the DOCs of a 232-seat 767-200 by up to 10%, and those of an identically configured Airbus A310-300 by 19%.
Boeing froze the stretch design with room for up to 289 passengers. This was seen as the best solution in terms of economics, structure, system and cost.
The compelling economics of the -300 persuaded Condor to launch the programme in September 1996 with an order for 12. Further orders have come from Icelandair, Arkia and an additional order from Condor for one more. Boeing hopes the DOCs of the stretch will also prove attractive to key US majors, including carriers like American Airlines, which is looking for a DC-10 replacement on some low-yield routes such as that of Chicago-Hawaii.
Structural changes
The -300 is stretched by 7.1m (23.3ft) to an overall length of 54.5m, compared with 47.3m for the standard -200. This is achieved by adding 4.05m to the front fuselage (Section 43) just ahead of the wing-to-body fairing and 3.05m to the aft fuselage (Section 46). The additional length is provided by building larger skin panels, rather than adding plugs in the conventional way. Boeing decided this would save weight, cost and complexity. "We are also able to use common tools," says Ray Halbert, co-leader of the body integrated product team (IPT).
"It is much easier to manufacture," says 757-300 chief project engineer Dan Mooney. "If you build a doughnut [a plug], then it's another join. It adds weight, because you need extra splices and, by doing it this way, it was an easier fit with our improved manufacturing process." The only real drawback of this approach was that some skin pieces are very long - up to 10.1m. This meant that some tools, and even parts of the assembly building at Renton, had to be extended to accommodate the new aircraft's bigger parts.
The gauge of stringers, stiffeners and skins was also increased in the overwing body section and lower fuselage area (Section 44) where the keel beam was strengthened. Bulkheads and frames in the tail area around Section 48 were also strengthened to accommodate a tailskid. The horizontal stabiliser is built with thicker skin panels to cope with the increased loads on rotation resulting from the greater moment arm of the longer aircraft. The front Section 41 nose bulkhead, behind the weather radar, was also strengthened in line with the latest birdstrike certification standards. This revised design will also be incorporated on the 757-200 from line number 805 onwards.
"The key thing is that we did not change the load paths on the structure," says Mooney. "We just increased the gauge. There was no full-scale fatigue or static test needed because our data was so good." The structure was analysed using an advanced finite element model that was integrated with CATIA - the IBM/Dassault-developed computer-based design tool adopted throughout Boeing.
The 757-300 design is digitised to a new level, thanks to production improvement initiatives known as FAIT and ASAT. FAIT, or fuselage assembly improvement team, saw hundreds of original, two-dimensional, Mylar drawings scanned into digital converters and reproduced as CATIA datasets. The ASAT, or automated spar assembly tool, uses the digital results of a similar process used to convert the wing design drawings.
The ASAT automatically drills and installs more than 2,600 fasteners in the wing, reducing manufacturing time and cutting costs. As with the FAIT programme, which achieves better first-time fit for the body panels, ASAT improves quality. The FAIT achieves the same end by taking CATIA data to develop operating programs for computerised numerical control (CNC) machines. The CNCs drill body panels, stringers, frames and shear ties which then almost snap together.
To enable the basic fuselage design to meet emergency exit requirements with plenty of margin, Boeing combined two basic production versions of the 757-200 into one for the -300. The modified forward and aft fuselage sections of the four-door variant, operated by airlines such as Delta, was mated with the overwing exit variant. The largest user of this version is American Airlines. The hybrid configuration therefore has eight main passenger doors and four overwing exits.
The heavier aircraft has a maximum take-off weight (MTOW) of 122,500kg (270,000lb) - 6% more than that of the -200. Its maximum landing weight of 101,700kg is 6,350kg more than the standard aircraft. Maximum zero fuel weight, by comparison, is 95,340kg for the -300, against 85,350kg for the -200.
Heavier weight means higher landing and take-off speeds. This has resulted in several changes to the landing gear. The entire assembly has been strengthened and two-ply tyres are used for the first time to cope with higher runway speeds. In flight tests, the rotation, climb and take-off speeds averaged 4-5kt (7.4-9.2km/h) faster than those of the -200.
The only discernible exterior feature to differentiate the -300, other than its overall similarity to the elongated shape of the Douglas DC-8 "Super Sixty" series, is the tail skid unit. The skid can be actuated at airspeeds up to 270kt and is plumbed directly to the landing gear retract and extend circuits just downstream of the landing gear selector valve. When the landing gear control lever is placed in the "up" position to retract the gear, pressure is applied to the head end of the tailskid actuator to extend it and retract the skid.
Unlike earlier tailskid designs such as that of the 767-300, the 757-300 unit contains a crushable cartridge rather than a compressible oleo strut. This was derived from the design developed for the 777-300 and is built to absorb the energy of an impact on take-off or landing. If the body itself makes ground contact, the crew will receive a visible warning on the engine indicating and crew alerting system (EICAS).
The system is triggered by damage to a small rangible foil that extends from the fuselage. The EICAS indication means that the crew must turn back for an inspection because any ground contact could threaten the integrity of the aft-pressure bulkhead and rear fuselage structure.
Modified systems
Although Boeing stuck religiously to its "minimum change" philosophy wherever possible, some changes were inevitable. Some were due to the nature of the stretch, while others took into account the time lapse since the launch of the -200.
Key areas of modification were the yaw damper and stabiliser trim systems. The yaw damper system supplies rudder commands to dampen undesired yaw based on inputs from the air data inertial reference units. Modal suppression accelerometers add extra inputs to improve ride quality and passenger comfort. Boeing expected the longer -300 to exhibit greater movement in yaw, and uprated the system accordingly.
Commands from the yaw damper systems are fed through the yaw/stabiliser modules (YSM) to the yaw damper servos and on to the rudder. Maximum rudder authority is 3í left or right for each yaw damper and the outputs of both left and right systems can be summed mechanically for a total deflection of 6í with both systems active. The YSM also controls longitudinal trim through the movement of the horizontal stabiliser. This is achieved through several modes, including automatic stabiliser trim, Mach trim, speed trim and manual trim.
The YSM was fitted to the first -300 from the beginning of the flight test programme, although the fine tuning of ride quality was achieved by adding a modal suppression system. This integrated special ride quality algorithms into the YSM and dampened out unwanted motion, felt particularly in the flightdeck.
Also integrated into the YSM was a device that delays deployment of the three outboard wing spoilers on each side in the event of an abused landing. The system links the pitch attitude sensors to the control modules which supervise actuation of the spoilers. "If the pilot is abusing the landing and coming in nose-high [above 8í] and too slow, the sensors will delay spoiler deployment," says Mooney. Spoiler deployment, particularly of those on the outboard wing section, induce a significant pitch moment on the 757 with flaps down.
By changing the deployment sequence, Boeing was able to devise a method of forcing the nose back down if the aircraft was coming in too nose-high. The 757-200 suffered its share of tailscrape incidents before crew training was revised and the problem largely eliminated, but Boeing was concerned that the problem could resurface with the -300.
Leon Robert, 757-300 chief project test pilot, says: "The action will increase the nose-down force and assist the pilot in doing what he should be doing by then. At that angle he wouldn't even be able to see the runway over the nose." The system delays spoilers 1 to 3 and 10 to 12, and cuts out when the nose drops below 5í.
Boeing also studied major updates to the flightdeck, but rejected these on cost grounds. Condor pushed for a more extensive update based on flat panel liquid crystal display technology, but backed off because of the costs.
"We would have preferred a 737 Next Generation cockpit because it would have allowed us to have thoughts of the 777 for the future," says Condor's former managing director, Dr Dietmar Kirchner. "We expected more in the cockpit, but minimum change comes with minimum costs, so you can't complain too much." Kirchner is now senior vice-president of corporate purchasing at Condor's parent company, Lufthansa.
Some significant flightdeck improvements were made, however, including the use of Honeywell's recently developed Pegasus flight management computer (FMC). This replaces three separate units with a more capable single system and brings global positioning system (GPS) navigation into the 757 flightdeck. It gives the flight management system the capability to operate with new communications, navigation, surveillance/air traffic management systems.
The 757-300 is also the first Boeing derivative to incorporate AlliedSignal's enhanced ground proximity warning system (EGPWS), says systems IPT leader Terry Waldron. The EGPWS gives a warning of high terrain on the horizontal situational indicator of the electronic flight instrument system (EFIS). It is fully integrated into the avionics rather than being "tacked on", says Waldron. The -300 also has a Rockwell Collins predictive windshear radar into the displays, and incorporates the recently developed air data/inertial reference system
Other systems to change with the -300 include the environmental control system (ECS). This was extended in the longer aircraft to provide a four-area automatic temperature control zone for the -300's three main cabin zones and the flightdeck. The pre-coolers in the engines were increased in volume to accommodate the higher requirement, and the capacity of the air conditioning pack flow control valve was raised.
One exterior clue to the bigger ECS requirements is the ram air inlet in the wing-to-body fairing. This is positioned slightly further forward than the same inlet on the -200 and is enlarged to scoop more air. "The recirculation fans are also larger, but we do provide a little more fresh air," says Waldron.
The cabin interior is modified extensively with the new-look architecture developed originally for the 777 and perfected for narrow-body aircraft on the Next Generation 737. The side and ceiling panels are sculpted to give an impression of spaciousness. The cabin is illuminated with indirect lighting. The storage bins are also enlarged and have been extended from a 1.4m module with a single door to a 2.03m module with two doors.
The entry areas in the doorways have also been standardised with the same lights and systems to enable Door 2 to be used as an entry door. Internal cabin communications systems have also been improved, with new control panels for flight attendants. Centreline overhead stowage bins with a 63kg capacity have been fitted for carrying life rafts and other emergency equipment. Up to seven positions can be occupied but only five liferafts are installed on the Condor aircraft.
An important upgrade has been made to the aircraft's vacuum-system lavatories. The extra passenger capacity requires a dual system for the -300 which has two tanks, two vacuum pumps and 50mm-diameter steel tubing throughout. Interiors IPT leader Bill Smith says: "We used CATIA to revise the design and to reduce tight turns in the pipes which produced problems with similar systems in the past." Changes were also made below decks in the cargo compartment. The increase in length boosted cargo capacity by a staggering 48%, but required substantial changes in terms of smoke and fire detection and suppression systems. "We really stepped up to the latest rules," says Waldron, adding that the 757-200's 15kg halon gas canisters were replaced with 25kg bottles.
Unlike the older system, in which the two bottles dumped their entire contents in one go, the new system lets one bottle dump and meters the discharge from the other. The sensitivity of the smoke detectors is also slightly increased, as is the number of sniffer ports throughout the compartment.
Busy flight test
In January, the Boeing 757-300 test team spent a busy few days receiving its US Federal Aviation Administration type certificate, production certificate, 180min extended range twin-engine operations approval and European Joint Aviation Authorities validation.
The flurry of clearances, all of which were granted in less than 72h, followed a five-and-a-half-month flight test programme - the shortest period for any Boeing derivative since that of the 737-400 in 1988. The three test aircraft together amassed 1,286h of ground tests and 912 flight test hours over 356 sorties, which began with the first flight on 2 August, 1998. Most of the hours were built up on aircraft NU701 (the 839th 757 built), while 721 accumulated 308h and 722 totalled 168h.
NU701 was used mainly for basic handling, including assessing the potential for pilot-induced oscillation. Flight testing chief engineer 757 Art Fanning says: "We did that particularly in roll. We tried some things that were not necessary as it turned out." These included a damper in the control wheel and a notch to keep the wheel centred, once set.
Because engineers suspected the possible effects of aeroelastic interaction between the longer fuselage and the wings, the front spar and rib at the tip were fitted with weights to counter any flutter. NU701 was instrumental in proving that these weights, accounting for 340kg, were not needed.
NU721 proved a reliable workhorse and was used for ground effects, automatic landing and fuel consumption tests. It was also used to test the nose gear water spray deflector, which was found to be unnecessary. NU722 cleared the aircraft through the new smoke penetration and testing trials, as well as becoming the first 757 model to be put through HIRF (high-intensity radiated field) tests. It also undertook the crucial service-ready demonstration with Condor on the airline's route network over four days in December 1998.
The 757-300 was also the first Boeing model to be cleared by the FAA using a new certification method called the Project-Specific Certification Plan. This was a formal agreement signed by the FAA and Boeing, under which both parties worked to a common schedule and preset milestones.
"It captured a working agreement we could use to hold to our schedule and meet the FAA's requirements," says 757-300 certification manager Randal Corman. "It was a new thing. The FAA casually mentioned it and we picked up on it. We became the first programme to try it out."
Although the JAA did not sign the agreement, it shared a similarly close watching brief on the certification process through a co-signed procedure information document. JAA 757-300 project certification manager David Clement says: "The programme went very well because of the way all the teams worked together." The new FAA process hinged on setting up 24 milestones and associated status meetings. "It kept all the issues bubbling, and kept us all in tune," says Clement.
Condor is scheduled to take seven -300s in 1999 and a final batch of six in 2000. All three test aircraft are now in refurbishment, with NU701 the last to join after completing a separate test effort for Rolls-Royce. The aircraft was used to test the engine maker's Phase V combustor for forthcoming application in the 757-200/300's RB211-535E4B powerplant. All aircraft are flying with engines fitted with the standard Phase II combustor, although R-R plans to change to the new standard based on combustor technology developed for the Trent 800.
In the meantime, the real test for the aircraft is beginning. Many airlines are interested in the 757-300 for obvious cost reasons, but are waiting to see how Condor copes with issues such as passenger loading and unloading. Boeing hopes that the airlines like what they see, and will soon begin to bring the 757-300 orderbook up to the levels of its shorter sister.
The stretched Boeing 757-300 has arrived on the production line at Renton (below), achieving certification in January after 27 months of development, including testing at Yuma, Arizona (main picture and below left)
Changes to the cockpit were kept to a minimum on cost grounds, but improvements include the addition of AlliedSignal's enhanced ground proximity warning system (below)
"We expected more in the cockpit, but minimum change comes with minimum costs, so you can't complain too much." Former Condor managing director Dietmar Kirchner.
All 757-300 customers to date have selected the Rolls-Royce RB211-535E4B engine, and an advanced "Phase V" combustor has been tested which will become standard. Pictured below is the 757-300's new tail skid
Source: Flight International
