Guy Norris/SEATTLE BOEING'S stretched 777-300 carries a list of superlatives almost as long as the aircraft itself. The latest member of the Boeing family is the largest twin-engined aircraft ever built, the world's fastest widebody twin, the longest airliner ever made and the first transport big enough to replace the company's original leviathan, the 747.
Yet, despite this impressive line-up of "firsts", the driving principle behind the -300 was to keep all changes to a minimum. Boeing says that it did not deliberately set out to enter the record books: essentially, it took the 777-200 and enlarged it for one main purpose - to replace its own early "Classic" 747s. The company wanted to create a "people mover" with compelling economics, and the new twin, with its big wing and powerful engines, represented its best platform for growth into this role.
The concept did not emerge overnight, although the 777 (like all Boeing jet airliners, including the 747) was designed from the outset to be stretched. The basic size of the extension was determined by Boeing's tried-and-tested stretching formula. As with its other extended-family members, ranging from the 727 to the 767, the -300 would be viable only if it could carry up to 20% more passengers. This meant growing the fuselage to take an extra 75 seats in two classes, or 60 more in a tri-class layout. That took typical dual-class seating to 451 and tri-class to almost 370. In all-economy, this same fuselage would accommodate up to 550 passengers, roughly equal to the capacity of the 747SR (short-range) variant.
Another key parameter was range. To offer airlines a true 747-100/200 replacement, the 777-300 would need a range of up to 10,500km (5,700nm), giving it the legs to operate trunk routes from San Francisco to Tokyo, or Los Angeles to London. The 777's large wing, and the 171,200-litre fuel capacity developed for the -200IGW (increased gross weight) model, provided a quick and simple solution for the -300 range requirement. By combining this range and payload with the operating costs of a twin, Boeing was able to offer airlines an aircraft that consumes one-third less fuel and will have maintenance costs 40% lower than those of the 747 Classics - particularly the -100s - that it will replace.
One of the last parts of the jigsaw to fall into place was the availability of higher-thrust engines. The virtually instant growth of the -200 series from the initial "A" market to the longer-range "B" market (later known as the -200IGW) pushed the equally rapid development of higher-thrust engines from all three manufacturers. With a baseline maximum take-off weight (MTOW) of 263,300kg, Boeing sized the -300 to operate with the first generation of -200IGW powerplants rated at 374kN (84,000lb) thrust. With further growth inevitable, the structure was designed for an MTOW of 299,600kg and 436kN engines.
General Electric's GE90-92B, rated like the other -200IGW engines at thrust levels of up to 409kN, therefore became a candidate for the -300, as did Pratt & Whitney's PW4090 and Rolls-Royce's Trent 892. In response to urgings from Korean Airlines, which particularly wanted the higher MTOW variant, P&W also agreed to commit to a 436kN engine for the -300, the PW4098.
By late 1997, GE's reluctance to commit to a higher-thrust version of the GE90, and strong competition from P&W and R-R, led to the US engine maker dropping its powerplant as an option on the -300. The remaining two, meanwhile, continued to study higher-thrust options for the proposed -200X ultra-long-range and -300X extended-range 777 variants.
Cathay Pacific Airways, one of the original partners in the "working-together" group which was instrumental in the 777 design, was a staunch champion of the stretch concept and was among the initial launch customers when the -300 was given the go-ahead at the Paris air show in June 1995. Other customers which, together with Cathay, placed $3.1 billion-worth of launch orders for 31 aircraft, included All Nippon Airways, Korean Airlines and Thai Airways International. By December 1997, two months after the first flight of the -300, the order tally had grown to 51, with Asiana Airlines, Japan Airlines and Malaysia Airlines joining the exclusively Asia-Pacific-based customer group.
With many early 747s coming up for retirement, or conversion into freighters, Boeing believes that more than 170 777-300s could be delivered by 2006, and has outlined plans to make as many as 28 a year by 2002.
To create the 777-300 from the -200, Boeing has inserted two plugs into the fuselage and redesigned the overwing area known as Section 44. The forward stretch involves adding 5.3m, or ten frames, to Section 43, while the aft-fuselage Section 46 has been extended by nine frames, or 4.8m. In theory, with the relatively tall main gear of the basic -200, the airframe was capable of even further stretching, but Boeing stuck to the 10.1m extension to maintain adequate lift margin and minimise handling problems at airports around the world.
Nine-frame stretch
"The decision on a nine-frame stretch for the aft section was based on lift margin, because that was the point where take-off speeds and rotation margin came together. That allowed us to use the same-length landing gear," says 777-200X/300X and former -300 programme manager Jeff Peace. As the size of the aft extension was finalised, so the proportionate length of the forward plug was settled for centre-of-gravity and balance considerations.
The longer aircraft cannot be rotated as steeply as the shorter -200 can, so the -300 needs more acceleration to create the same amount of lift for take-off at equivalent weights. In its efforts to launch the proposed -300X (which has a higher MTOW of around 324,600kg), the company is studying a "semi-lever" main gear to help reduce the take-off run for the same reason. The semi-lever, or articulated gear, offsets this limitation because actuated linkages attached to the oleo and forward end of the gear truck hold the aft end of the bogie on to the runway. "This effectively increases the length of the gear," says 777 preliminary-design chief project engineer Mike Burtle. "This increases the effective rotation angle of the aircraft, which lowers the take-off speed."
The driving principle behind the structural changes to the fuselage was "to keep a neutral position from the loading point of view", says body-structures senior principal engineer Scott Tomkins. "The plug in front is designed to keep the frame stations the same forward of the front spar of the wing. It is not a true plug-it is really a new length of constant section because of the interface at the front spar." Tomkins describes the nine-frame aft stretch as "a real plug attached to Section 46".
While much attention was given to stretching the fuselage, most of the design effort went into re-working Section 44 and the keel area beneath, called Section 45. "All the major structural changes were made in 44, which has now got a pretty thick skin," says 777 body-structures unit chief engineer Jack Donnan. Approximate maximum skin thickness in the -300's Section 44 is 11.4mm compared with 6.3mm for the same area of the -200.
The design team faced several challenges, ranging from how to strengthen the area to take an expected 40% load increase without altering the outside contours, to where to locate the new over-wing door. Two new Type A doors were required to meet cabin-evacuation criteria with the extended fuselage.
"We didn't think we could get a door in there at first because of the location of major bulkheads coming off the wing rear spar, and the transfer of loads in that location, which are significant," says Donnan. "Somehow, we knew we had to find a way - it was critical to the entire stretch concept." In the end, the correct location was defined and thicker skins were designed to surround the doors. "They are double thickness around the doors - just about 22.8mm, with doublers, skin and triplers. A lot of complex local strengthening was required to keep contour changes to a minimum." Maximum thickness at the door cutouts for the -200, by comparison, is 12.7mm. An added pressure was the need to keep changes in tooling to a minimum because of the common production line, says Tomkins.
Conformal doors
The overwing doors are of the same overall configuration and design as the other main doors in the existing -200, but are straight rather than curved from around the midway point down. This makes them conformal with the straight-sided contours where the wing meets the fuselage.
The overwing area required strengthening not only for the higher gross weight, but also to cope with "a really significant increase in axial or shear loads", says Donnan. Most of the interior strengthening is achieved with a "huge" increase in the keel beam between the aft pressure bulkhead and the wing where all the body bending is accommodated, says Tomkins. Two keel beams are extended aft in gradually tapered form under Section 45 to Section 46, where they help take the extra tail and landing loads. The keels were ultimately extended aft by seven frames - roughly 3.6m. "They also help reduce the 'kick' loads into the body [from landing] to a minimum," says Donnan.
With all the extra strengthening, particularly in the keel beam, "we needed to keep a close eye on the empty weight", adds Donnan. "As you go through the design steps, you normally keep track of the changes to see how that will affect weight. With this aircraft, we didn't have to go back and do any weight reductions as such. We really did not have to revisit a lot of the design, which is a testament to the original design of the aircraft and, of course, to the big engines." The operating empty weight of the -300 in typical tri-class layout with Rolls-Royce engines is slightly under 155,720kg, compared with 139,380kg for a similarly configured -200.
Some weight was also added by the strengthening the main-undercarriage legs and inboard wing structure. Most of this was accomplished with gauge increases, particularly of the upper and lower wing skins, as well as the forgings for the main gear. Boeing plans to boost the MTOW of the -200IGW variant by a further 7,040kg by using the same beefed-up gear. This will enable the shorter aircraft to fly about 14,060km with 375 passengers. The option is "for sale" and available from 1998 onwards when the -300 gear is in full production.
New Features
Although Boeing stuck religiously to its "minimum-change" philosophy throughout the development effort, some changes were unavoidable because of the sheer size of the -300.
The tail-skid device, for example, is the most externally visible new feature. The retractable, electrically actuated, tail skid is based on similar designs used on the 767-300 (and -400), as well as on the stretched 757-300. The tail skid is augmented by a body-contact sensor which will alert the crew to a more serious contact on rotation or touchdown. The tail-strike indicator consists of a small frangible foil extending down from the fuselage. If the foil makes contact with the ground, a message flashes on the engine-indicating and crew-alerting system (EICAS).
Another new feature, barely visible to the untrained eye, is the ground-manoeuvre camera system (GMCS). The new system, although not required for certification purposes or considered part of the minimum-equipment list, has been devised to help the crew during taxiing. It consists of three cameras, one mounted in the leading edge of each horizontal stabiliser and the third beneath the forward cargo bay aft of the nose leg. The 777-300 is the first aircraft to be fitted with such a system.
In another throw-back to the 747 days, Boeing built a large ground rig called the LAGOS (large-aircraft ground-operating system) to simulate ground handling. Pilots from its airline working group were invited to trundle around the Everett ramp in it and make comments. Unlike those with "Waddell's Wagon" (named after the 747 project pilot Jack Waddell), the trials involving the LAGOS proved that a ground-camera system was a good idea. It was considered necessary mainly because of the extended wheelbase of the new stretch, now 31.2m compared with 25.9m for the -200, making it the largest wheelbase of any aircraft in the world.
The front camera has a horizontal field of view (FoV) of 89.4¹ in azimuth and 30.4¹ vertically, although the vertical FoV seen on the display unit on the flightdeck covers 18.6¹. Similarly, the tail-mounted cameras can cover an FoV of 26.4¹ with electronic panning, and show a standard 18.6¹ vertical FoV on the display unit. Changes in stabiliser trim-setting (0¹ to -2¹) can change the camera's sweep angle by up to 3.5¹ in horizontal coverage and 7¹ in the vertical plane.
The nose-gear viewing camera is mounted in a sealed unit between fuselage stations 592 and 613. The unit's main connection to the aircraft structure is surrounded by a stand-pipe designed to stop moisture (from condensation within the belly) from dripping down into the camera housing. The main-gear viewing cameras in the tail are removable from the leading edge as single line-replaceable units. The units are sealed for moisture control and to prevent anything in the electronics acting as a potential ignition source. The front of each camera is protected by a sapphire window with a conductive coating to keep it from fogging up.
The camera images are presented on a T-shaped split screen, with the nose gear shown across the top of the T and the main gears shown on the bottom left and right. The picture can be shown on any three of the 777's six Honeywell flat-panel displays. Those allocated to the GMCS include the two inboard navigation displays and the main multi-function display below the EICAS. Camera lights, needed to help illuminate the ground for night operations, are controlled by a new panel switch on the glareshield.
Another new feature, and one of the most novel engineering aspects of the -300, is the unusual over-wing escape slide. This is packaged into a container in the aft fairing beside the new Section 44 door. Described by Tomkins as a "pretty unique and involved design", the slide is deployed aft and above the wing. As it inflates, the slide forms up to the door while another part deploys and slides down to the ground. The combined slide therefore forms an angle as it is deployed, and is fitted with a rail on the outside of the curve. Integral lights are built into the slide, and additional lights are mounted in the aircraft to illuminate the entire area.
Manufacturing challenge
When it launched the 777-300 on 26 June, 1995, Boeing set itself the ambitious target of getting the aircraft into service with launch customer Cathay Pacific some 32 months after the firm configuration was set in October 1995. The 25% product-definition stage was reached successfully in June 1996, allowing the release of major parts, plans and tools for production. Detailed fabrication, meanwhile, began in February 1996, with the two efforts, which Boeing terms "define" and "build", culminating in the start of major assembly in March 1997.
The true length of the -300 became apparent to onlookers for the first time at 0130 on the morning of 21 July this year, when the final body-join of the first aircraft, WB501, was completed. At 73.8m in overall length, the -300 is 3.4m longer than the 747. "The join came together just like the first -200," says Liz Otis, director of the 777 manufacturing business unit. The production challenge was compounded by the fact that, at the same time, Boeing was stepping up 777 production.
The first -300, line number 94, was rolled out at Everett on 8 September and prepared for flight tests, which began with a "flawless" 4h 6min sortie on 16 October. The main focus of the test effort was to evaluate the effects of the stretch on overall handling, including ground operations and use of the cameras.
The test team predicted that the -300 would be more controllable in yaw because of the larger tail-moment arm, but that pitch control could be adversely affected by the greater inertia. "Drag will be higher and there will be less flow down on to the tail," says Boeing flight-test chief pilot John Cashman, who was in the right-hand seat for the maiden flight. The team was also on the alert for signs of increased susceptibility to "airplane-pilot coupling" in pitch, formerly known as pilot-induced oscillation. "It could be worse, depending on the control response," says Cashman, who expects the -300's handling qualities to be on a par with those of the -200, thanks largely to the adjustable gains of the digital flight-control system.
Although the test fleet of five 777-300s will be used for plenty of runway work, some of the more daring exploits of the -200 test programme will not be repeated. Boeing 777 programme chief pilot Frank Santoni says: "We won't have to do maximum brake-energy tests and Vmu [minimum unstick speed]. You might have thought we would have needed to perform these, but we can do it by analysis. We extrapolate the results from the -200 Vmu and rejected- take-off tests which were done at 287,200kg and 183kt [338km/h] with 100% worn brakes. We will be doing abused take-offs at Edwards AFB, California, however."
The first flight passed off with no technical problems - a rare event. "We spent 4h shaking the aircraft down pretty thoroughly," says Santoni. He adds that initial handling qualities were "remarkably similar" to those of the -200. The GMCS was also used for taxiing and in flight. Cashman reveals that Boeing is considering recording the images for flight-data purposes.
The first flight was also made with Trent 892 turbofans, which represents a significant milestone for the UK engine-maker. This was the first time that non-US engines had powered a Boeing-made widebody on its first flight. The first flight included engine shut-downs and re-lights with windmill air-starts at 249kt.
Early flight testing was aimed at clearing the high-speed envelope beyond the initial 250kt restriction. This was an important hurdle to clear, because the -300 has a maximum operating cruise speed of Mach 0.89, higher than the -200's M0.87. To validate this, the tests included dives at speeds of up to M0.96, or about M0.02 faster than for the -200.
By early December, the test fleet was joined by WB531, the first PW4090-powered 777-300. By this stage, the first two R-R-powered aircraft (WB501 and 502) had amassed more than 150h of flight testing, including a visit by WB502 to the Dubai air show and the UK. Milestones achieved included the successful passing of flutter- and cruise-performance tests, as well as most of the air work, including stalls.
The total 777-300 test-flight programme is due to encompass about 1,500h, of which some 60% will be in-house Boeing testing and the remainder certification work. Of this, WB501 was expected to fly around 500h of tests aimed primarily at aerodynamic testing, flutter clearance, control-law stability evaluation and take-off performance work. WB502 is to be used for performance testing and validation of updated navigation computers and other avionics.
The first P&W-powered aircraft is expected to be used for around 200h, mainly on fuel- consumption and systems tests, while the second, WB551, has been allocated to more-stringent engine-related tests to focus on the performance of its PW4098 powerplants. This aircraft, line number 120, is due to join the test fleet in early February 1998, and will be subjected to up to 400h of tests through to the planned certification in late August 1998.
Certification of the PW4090-powered version is due to pre-date this by almost three months, while the R-R-powered versions are on schedule for certification in May 1998. The fifth test aircraft, WB503, is due to be rolled out in mid-December and will join the test programme for only two months for evaluation of cabin and in-flight-entertainment systems for a total of about 80h.
With fast progress to date, it seems that the 777-300 is living up to early expectations. Boeing's hope now is that the airlines are equally encouraged, and that the orderbook may some day grow to "Classic" proportions.
Source: Flight International
