Picture the scene. A Boeing 757-300 emerges from the blackness of a North Atlantic storm, fighting a vicious crosswind with gusts of 40kt (75km/h) or more. Ahead, through the gloom and screaming wind, lies the rain-slicked runway of Iceland's Keflavik Airport. Testing the automatic landing system to its limits, the 757 seems almost to be crabbed side-on to the runway as it crosses the threshold. Then the spindly main undercarriage legs make contact - first one, then the other.
The landing is a success and, within hours, the crew prepares for the next task - another autoland approach, this time into Cork, Ireland. For this landing, the 757 will make the approach into a buffeting 25kt headwind - the highest wind on the nose the crew can find over any runway around the entire north Atlantic that day.
Most crews would consider themselves unfortunate to face the conditions at either landing site on a regular basis, let alone at both sites on consecutive days. Yet to the Boeing test team, the furious winds spilling out of the aftermath of Hurricane Mitch present a golden opportunity to wrap up critical phases of the test programme in record time. They feel lucky.
"I don't know who is doing the weather for us, but they're doing great and they deserve a lot of credit," says 757-300 chief engineer, test and aircraft validation team leader Art Fanning.
The effort to find the right winds has sent the 757-300 test team scurrying across the length and breadth of the USA, as well as to the northern and eastern edges of the Atlantic. The task is urgent because the test programme, involving three aircraft, is planned to be completed by January 1999. Assuming certification is granted, this will allow first deliveries to begin early the next month to launch airline Condor.
"We've spent a lot of time on wind watch, waiting for the right conditions to emerge," says Fanning, who describes the recently completed cross, tail and headwind assessments within three days as the "triple crown of flight testing". Although traditional meteorological sources continue to provide most of the data, the wind watch team has also become adept at surfing the Internet for up-to-the minute changes.
"It is one area where the worldwide web really helps," says 757-300 chief project engineer Dan Mooney. "One of the toughest requirements was to find still conditions at Edwards AFB [California], where we needed wind speeds below 10kt. We found a web site covering the anemometers across Edwards which allowed us to find calm winds in some areas for take-off performance tests."
The pressure on the 757-300 flight test team is intense, particularly because the planned 27-month effort is the shortest design-to-production-to-delivery cycle of any Boeing derivative. With a total planned test programme of fewer than 900 flight hours, the flight test effort is currently at around the half-way point. By mid-November, aircraft number one, NU701, had amassed 270h; aircraft number two, NU271, had 210h; and number three, NU722, had about 5h.
The first -300 began the flight test programme with its maiden flight on 2 August and will fly in this role until mid-January 1999 "and possibly slightly beyond", says Fanning. The aircraft was used for initial airworthiness and basic controllability tests. "We did flutter demonstrations with 701 to make sure that a 15% overspeed did not cut into the flutter envelope," he adds. The aircraft was also used for stall tests, stability and control, as well as envelope expansion.
As a result of this early work, minor changes were made to the flying surfaces and control system. These included adding vortex generators to the leading edge of the outboard flap sections to improve roll characteristics "-and give an even roll response", says Fanning. The modification, although desirable, was not considered urgent, says Mooney, who adds that the change could also be retrofitted to -200s.
Changes were also made to the scheduling of the control column deflection in pitch to make the -300 "-feel more like the -200 to pilots", says Mooney. This involved changes to the elevator feel computer and the addition of a mass balance to the column. The adjustment of the column forces "-smooths out the amount of variation that was being felt by the pilots with different loads due to the increased length, speeds and weights of the -300," he adds.
Other minor changes included altering the diameter of a slat seal to cure an acoustic resonance which developed as the slat was retracted. The resonating stopped with the change in the seal and the modification has already been applied to a newly built -200 which exhibited the same phenomenon in flight test.
NU701 was also used for rejected take-off tests, which were conducted at Edwards AFB in October. The tests were made at weights up to 124,400kg (274,000lb) - 1,360kg above current maximum taxi weight - and speeds of up to 190kt before maximum braking was applied with 100% worn brakes.
The tests, which require the aircraft to come to a full stop before taxiing off the runway and standing for 5min before firefighters can douse down the red-hot wheels and brakes, were successfully passed by both brake vendors, BFGoodrich and Dunlop.
The aircraft was scheduled to rejoin the test programme in late November from a brief lay-up, during which a Honeywell air data inertial reference unit (ADIRU) was installed. The system, which incorporates Sextant Avionique's air data modules, is representative of the production configuration ADIRU and will be used for final autoland trials. For the near future, NU701 will be used for stability and control and stall demonstration certification and thrust reverser demonstrations.
"Following ADIRU certification demonstration, we have the first version of the YSM to fit," says Fanning. The YSM (yaw damper rudder ratio change stabiliser module) will provide modal suppression and improve ride quality in the long, thin -300. Like the -200, which also has a modal suppression system driving the rudder to damp out lateral vibrations, the -300 is expected to benefit from the system.
The YSM is not being fitted until this relatively late stage in the test programme to ensure that "-we have all the right data to feed into it", says Mooney. "Now it's about ready to go and we have to prove it before the service ready demonstration starts. It's going to be kind of a tight squeeze, but we should do it."
A spoiler rescheduling system, introduced on the -300 to help reduce pitch-up in the event of a slow landing, is also programmed into the YSM. The system modified the spoiler deployment sequence if pitch attitude exceeds 8¼ on landing. With all spoilers deployed, the pitch attitude would normally increase, leading to a higher probability of tail strike. The system automatically delays all the spoilers deploying, cutting the chances of the tail strike.
Because the longer -300 was expected to be more prone to this problem, it is also fitted with a new tail skid actuator. "It has not been a significant event in flight tests and where we did have tail strikes, it's been where you'd expect them," says Mooney. Fanning adds: "We've compressed a few cartridges and replaced them, but it hasn't been a recurring theme. We'd laid in a supply of spares at the start of the programme, which has also involved several Condor pilots, not just our own test pilots, and we have not crushed any of them."
Intriguingly, NU701 is also expected to be used for tests in December which could reduce the aircraft's empty weight by a precious 340kg. This unusual, and significant, last-minute weight reduction is anticipated from final flutter tests which may enable Boeing to remove up to 170kg in weight from each wingtip of the aircraft.
The tungsten weights were added to the already beefed-up -300 wing structure to ensure flutter was dampened out. "We believe we can take out that weight, and we'd like to get rid of them," says Mooney.
The second test aircraft, NU721, has been building up test hours at an almost unprecedented rate for any Boeing airliner in this early stage of its development. "It flew through 120h in its first month and averaged around 110h in its second month, and 90h in its third," says Fanning. "From my standpoint, that's pretty amazing stuff. We normally consider 80h per month pretty aggressive."
The long hours are partly related to the tasks allocated to 721, and partly to its mechanical reliability. "The aircraft has behaved well and, what with all the recent experience on other test programmes [777 and Next Generation 737], we have a particularly professional flight test crew available right now," he adds.
Tests conducted by the aircraft include ground effects, autoland (including the varying wind conditions assessments already mentioned), and nautical air miles/pound of fuel - or NAMS fuel consumption tests. The aircraft was also used to test the nosegear water spray deflector, which was discovered to be unnecessary. The result was a relief to Condor, says Mooney. "It was particularly concerned about it because of the towbarless system at Frankfurt."
The aircraft, one of two painted in Condor livery, will be used for a final series of certification tests, including some for avionics before going into refurbishment around mid-December. The date is still uncertain, says Fanning. "We might want to keep 721 available as a sort of contingency vehicle for any further tests that might be needed." The aircraft is not scheduled for delivery until June 1999 and so offers the biggest delivery margin, he adds.
The third test aircraft, NU722, has been used for HERF (high energy radiated field) electrical tests. These are now required for US Federal Aviation Administration and European Joint Aviation Authorities certification, but were not needed when the 757-200 was developed. The HERF attenuation tests were conducted at Glasgow, Montana, and involved checking the effect of the fuselage itself on HERF frequency and oscillation.
Smoke penetration, detection and suppression tests were also made using NU722, which has a redesigned fire suppression system to suit the extended cargo compartments of the stretch. The new system meters some of the halon gas rather than discharging it into the hold in one blast when smoke is detected. The system is also equipped with extra monitors to meet JAA requirements.
Possibly the biggest task awaiting NU722, and one which Boeing marketeers will be anxious to capitalise upon, is the upcoming service ready demonstration with Condor. Beginning shortly after 7 December, the aircraft will fly a four-day period of simulated airline operations from Frankfurt.
Fully equipped with seats and the standard interior fittings, it will be operated without passengers to several of Condor's principal holiday destinations. "It will prove the systems and make sure there will be no big surprises when the aircraft enters service," says Mooney.
The demonstration will also include some turn time evaluations, with airline staff taking the place of passengers. Turn time has become a major issue with potential -300 operators, which are eager to see what Condor's experience will be. Some remain sceptical about Boeing's claims that the -300 will require only four more minutes to load and 2.5 minutes to unload compared with the -200.
With final certification expected in January, the results of the turn time trials will be vital to Boeing, which is anxious to add to the 757-300 orderbook.
Source: Flight International