Stretching and testing

Source:
This story is sourced from Flight International
Subscribe today »
US and European certification is imminent for Boeing's biggest 767, the -400ER

Guy Norris/SEATTLE

Boeing's third and final stretch of the 767, the mighty -400ER, is set to receive joint US and European certification as the Farnborough show takes place. To boost the sales prospects of its new entrant and underline its renewed attack on the mid-size market, Boeing has chosen Farnborough as the chief venue for a five-week 767-400ER world tour.

Launched in early 1997, the first 767-400ER, VQ001, was rolled out last August and began flight-testing on schedule on 9 October (see Strategic stretch, Flight International 25-31 August 1999). Getting the aircraft to Farnborough has been no mean feat for the test team, which had amassed almost 1,150 flight-test hours since the prototype made its first flight eight months ago.

Despite the overall success of the test programme, Boeing admits that the -400ER is late and took longer to certificate than was planned. Delivery to launch customer Delta Air Lines has slipped from May to August. The first aircraft for the new model's other customer, Continental Airlines, is close to being completed at Boeing's Everett, Washington, plant, and is due for delivery during the third quarter.

One of the main culprits for the delay was the 40-day engineers' strike that hit Boeing in early 2000, just at a critical build-up time for the three-aircraft test fleet. A fourth aircraft, VQ004, designated as a test vehicle for some last-minute in-flight entertainment and other cabin-related evaluation, was largely unaffected by the strike but did not join the certification effort until the end of June.

"Things were going almost too well and I thought something had to go wrong," quips former 767-400ER chief project engineer Ken Kirwan, who says the 30% growth in the test programme over the 805 hours originally predicted was in line with previous certification and development programmes. "Ideally it would have been a tighter estimate on ferry times because I believe that numerically, compared to other programmes, it really did not exceed our planned times," says Kirwan. The largest overseas ferry trips, which totalled about 100 hours, included flights to Australia and double trips to Kona, Hawaii.

At least 100 hours were also added to the test effort to reschedule the leading edge flaps and test the position of vortilons that were added to correct a "roll-off" condition discovered during stall testing. The extra test hours also included "a lot of work on smoke detection and fire detection, and the proper positioning of the halon extinguisher bottles", says Kirwan, who describes the painstaking work as a "black art". The extra lower deck cargo space of the stretch, plus stringent updated requirements for coverage during extended range twin operations (ETOPS), meant a heavy workload for the test team in this area.

"The roll-off probably made me a little nervous," says Kirwan. "We were on a tight schedule. We had to get to Edwards AFB, California, before Thanksgiving (November 1999). We had the scaled unit in the 9 x 9 (feet), low-speed windtunnel by Friday, and by Saturday we had the model in the tunnel. By Sunday we were running various vortilon patterns and simultaneously building test units using stereolithography, and by Monday we flew four times and had an answer that night."

The answer turned out to be a combination of sealing gaps in the inboard leading edge slats and adding tiny vortilons, which are located on the leading edge about mid-span between the engine strut and the raked wing tip.

Part of the problem was knowing whether to even bother correcting the roll-off that occurred during turning stall manoeuvres with landing flap set at 15°. "It was not really a big issue and we had big debates about whether to do anything or not because the roll went right to 20° after recovery, which is the limit," says Kirwan. "So the question was, what would a line pilot experience? The rolling moment had a fairly sharp break, so aerodynamically we tried to smooth out that rolling motion. The target was that, even if you get flow separation, it propagates inboard to the vortilon, but then stops." The vortilon forms an aerodynamic wall, or dam, which prevents the stalled section of wing from spreading inboard towards the root.

Sealing the gap in the inboard slats mainly helped to maintain positive stick forces right up to stall. "During testing, we noticed a lightning of forces at the aft CG (centre of gravity) part of the envelope," says Kirwan. "We were trying to meet the regulations which call for positive forces all the way to aft stick, and although we didn't get a reversal, the forces were real light." Luckily, Boeing had a quick answer in the slat seal move, which had been learned during test flights of the E-767 AWACS (airborne warning and control system) version.

Virtually all the aerodynamic tests, fixes and trials were performed using VQ001, which was also in the forefront of evaluating the reduced drag benefits of the patented raked tip, used for the first time on the 767-400ER. "The results look quite a lot better than expected," says Kirwan, adding that a further batch of test flights were scheduled to take place before Farnborough to quantify the exact benefit. "It is certainly 5%, but it could be as much as 6% - so the aircraft is doing 1% or better." Some of this is undoubtedly down to the increased fineness ratio of the -400ER's longer fuselage, he adds, but the overall result - no matter what the cause - is 1% better than guarantees.

Tests will be conducted using a sophisticated data retrieval and recording system which, when fully operational at the start of the programme, collected 1,600 unique measurements while monitoring 22,000 digital parameters. By the end of the entire test effort, Boeing expected to have amassed about 60 billion data points. A suite of 15 mini-cameras was also included in the system, two of which were mounted on the wingtips during the roll-off investigation.

Some of VQ001's workload, which was expected to add up to 500 flight test hours by the end of the programme, was also dedicated to gathering data for the first -400ER simulator. "To get a Type 6 rating, we needed to fly all conditions and load them into a simulator," says Kirwan. "So it was a fairly extensive package." He says the -400ER is being certificated as a derivative based on the original 767 type certificate.

Current 767 crews will see, rather than feel, the biggest changes in the flightdeck simply by looking at the 777-style flightdeck. Adopted late in the programme in December 1997, the Rockwell Collins flat panel display-dominated flightdeck provides commonality with Boeing's latest products, but still allows current 757 and 767 crews to obtain ratings on the stretch. "It was a challenge because it was a short programme and we didn't freeze the configuration until June 1998," says Kirwan, who adds that the overall system was virtually fault-free during tests that were mainly conducted using VQ002. This aircraft was assigned to systems and performance tests including high-speed nautical air miles (NAMS) fuel consumption tests, auto-land and cross-wind tests, some of which were made in Iceland. It also evaluated the performance of systems such as traffic alert and collision avoidance (TCAS) and weather radar.

Aircraft VQ003, which amassed just over half of VQ002's 400 hours, was dedicated mainly to interior system tests as well as smoke detection and halon fire-extinguishing system work. The interior tests were particularly important given the -400ER's leap to the 777-style interior for the 767. This feature has since become one of the first elements of the -400ER programme to be spun off into the rest of the 767 family. Continental Airlines is due to receive the first 767-200ER with the new interior in the third quarter of 2000, followed shortly by Lauda Air with the first remodelled -300ER. Boeing eventually added over 50 hours more than expected to VQ003's workload, says new -400ER chief engineer John Bruemmer, because of "a lot of work in the smoke and fire-extinguishing areas". Changes to fix "leakage" were also required, again adding to the test time.

The final test aircraft, VQ004, had a relatively minor role to play in the programme and did not even make its maiden flight until the end of June this year. Fully equipped with production standard avionics, the Collins-developed Ethernet databus and a complete in-flight entertainment (IFE) system, the -400ER has been used in recent weeks to prove the aircraft's resistance to electromagnetic interference. It was also scheduled to be used by Delta for initial route proving and crew training.

VQ001 and 002, their testing days over, were brought in for refurbishment at the end of May and will be delivered to Delta in November. Aircraft VQ003 will have its demonstrator livery stripped and will be refurbished and repainted in Delta livery before being handed over in the first quarter of 2001.

Inside the -400ER

Delta will accept delivery of the first -400ER about four and a half years after the project was first revealed at the Dubai air show in 1995. The -400X, as it was called, was designed to fulfill two major roles: replace Delta's Lockheed TriStar fleet, and compete against the newly developed Airbus A330-200. Through it, Boeing also injected fresh life into the 767 family and provided new upgrade opportunities that would otherwise have been unjustifiable.

The -400ER, as it became when launched with an order from Delta in March 1997, was defined with an overall length of 61.4m. This was achieved by extending the -300 fuselage with a 3.4m forward plug and a 3.0m plug aft. The extension created space for up to 20% more seats, 25% greater lower deck volume and 10% lower seat-kilometre costs than the 767-300. The extensive stretch required fuselage strengthening and the redesign of panels, which was done using the Dassault/IBM CATIA digital design system. This had been used first on the design of the original engine pylon for the Rolls-Royce-powered 767-300, and later for the entire 777 programme. Boeing says about 80% of the -400ER was digitally defined.

Structural innovations and changes included the use of revised, composite floor grids with built-in "raceways" to house the vast network of IFE cabling as well as support the 16g stressed seats. A stringerless window belt with elliptical cutouts for 777-style windows was also incorporated to save up to 270kg (595lb) in weight per aircraft. Changes were also made to the interior of the Section 48 tail cone to house the more powerful Honeywell 331-400 auxiliary power unit, which pumps out up to 120kVA, rather than the 90kVA of the original 767 APU.

Higher airflow needs

The wing root area was also redesigned to house higher-capacity Hamilton Sundstrand air-cycle air conditioning systems which were required to cope with the higher airflow needs of the larger cabin and its IFE system. The units, identical to those developed for the 777, provide a fresh air flow rate one-third higher than that of the 767-300ER.

Poking out from beneath the tail is another new feature of the -400ER - a retractable, hydraulically actuated tail skid. This was based on a similar unit developed for the 757-300 stretch and contains a crushable cartridge that can absorb the impact of a tail scrape with the runway. Use of the cartridge allowed a gain of 0.2° in rotation angle margin and permitted major design changes to the main undercarriage for the same reason.

"That was a major success story," says Kirwan. "We did lots of engineering to get it to work, plus we had to do lots of tests of flailing and flopping tread, exploding tyres, hot tyres and thrown treads, either down or up and spinning." The changes to the gear mean it is 1,360kg heavier and 46cm taller than the -200/300 unit, yet still fits into the same wheel well with only a few changes. The wheels, tyres and brakes are identical to those of the 777, involving the use of 127cm radials rather than the 117cm-diameter bias-ply tyres of the standard 767.

"Unfortunately, you couldn't pick a worse spot to have to make those changes," says Kirwan. "The flight control cables, hydraulic, fuel, electrical and power generation lines all come off the wing around this area, so the tests were very thorough." Hydraulic lines and the wing-mounted trunnion were shifted to ensure the gear would fit, and a vertical beam on the rear spar was also moved to the aft side of the panel. To be on the safe side, Boeing also redistributed several systems around the bay to ensure they would be protected in the event of a tyre exploding. The only external clue to any change in the main gear is a small aerodynamic blister fairing at the forward end of the adjacent flap track housing. This covers a new trunnion housing which is moved 25cm further outboard and 10cm downward.

Another external change is the raked wingtip, chosen as the best solution to increasing span without incurring the weight penalty of strengthened outboard sections. Wind tunnel tests revealed that this would have been needed to counter torsional loads imposed with the winglets that Boeing had originally planned to fit to the aircraft. Another benefit was the overall span being increased to 51.9m against the 55.2m originally sketched out with the winglet addition. The 2.34m-long tips are made largely of composites with a metal spar and aluminium leading edge for protection against erosion.

The changes to the tip required repositioning of the outboard leading edge slat and the redesign of the wingtip navigation light fixture. Flight control surface changes hinge mainly on the adoption of a single yaw-damper/rudder ratio change, stabiliser trim module (YSM). This replaces the separate yaw damper, rudder ratio change and stabiliser modules, freeing up space, reducing weight and improving efficiency. Higher gross weights also drove changes to the flap/slat electronic unit, a schedule change to the flap load relief system, strengthening of the inboard flaps and the addition of inboard aileron droop for improved lift at low speeds.

Powering the stretch are General Electric CF6-80C2B7F1s rated at 62,000lb (276kN) thrust, or 8Fs rated at 63,300lb thrust, with Pratt & Whitney PW4000s offered as an option. The B8F has GE's newly developed "boltless" high-pressure turbine at its core, and is attached to the wing with larger shear pins and a rear mount forging made from Inconel.

Looking for customers

Despite the huge technical effort by Boeing, it has so far secured just one additional customer for the new model, Kenya Airways, since the 1997 launch deals from Delta and Continental for 21 and 26 aircraft, respectively. The aircraft is pitched against a worthy rival from Airbus, the A330-200, which has taken the upper hand in most recent sales campaigns, while several leasing companies have dropped 767-400ER plans due a lack of significant customer interest.

Boeing is now pondering its next move with the twinjet which could see a longer range " 400ERX" developed, along with sweeping changes to the earlier -200/300 models. Whatever it decides, the effort made to create the -400ER has provided Boeing with a very capable platform from which to develop.