Boeing yesterday announced it was postponing first flight of the 787 citing the need to reinforce structure where the wing box meets the center wing box at the side of body of the aircraft. FlightBlogger takes a closer look at exactly what the problem is and how Boeing came to yesterday’s announcement.
Because of the need to go back into the detailed design phase for this fix, combined with the need to fabricate, install and test at component and at full scale levels, several sources with a direct familiarity to the situation estimate that the fix will take “months not weeks.”
- What is the problem?
- Historical Precedent
- The Fix
WHAT EXACTLY IS THE PROBLEM?
The issue centers around the wing-to-body join that mates the wing box(Mitsubishi/Section 12) and the center wing box (Fuji/Section 45/11).The center wing box is the combination of two pieces, the center wingtank (Section 11) and main landing gear wheel well (Section 45). Thearea of concern centers on the 18 points where Sections 11 and 12 meet.
Digging deeper, the 18 points in question on each side of the airplane(36 total) are located on the top panel of the center wing box and runport to starboard inside the structure of the center tank through tothe other wing. These 18 ‘stringers’ inside the center wing box arematched by 17 stringers on the wing box, which serve to stiffen thewing skin. The wing box has 17 stringers, but a source indicates theyare designated 2-18, hence the reference to the 18 points that need tobe reinforced.
The composite stringers, which give the wings its longitudinalstiffness, are cured during production when cooked in the autoclave andjoined as a single bonded piece with the wingskins.
On the inboard side of the wing box where the 17 stringers end andconnect to the center wing box, each has what is known as a ‘stringercap’ that widens at the end and actually makes the hard connectionbetween Section 11 and Section 12 on the side of body. The stringercaps on ZY997 sustained damage, albeit repairable, when the wings wereflexed in late May.
Boeing confirms that small areas of the wing structure separated or “disbonded” from thewing skin, though declined to specify exactly where. Sources directly familiar with the situation say theshifting tension load from the stringer to fastener head also causeddamage on the structure.
In February 2006, nearly a year after A380 had begun its flight testprogram, Airbus was conducting testing on MSN5000, the static testairframe when the wing “ruptured” during ultimate load testing. Thewing was being flexed to 150% of limit load when the wing broke betweenthe inboard and outboard engines.
At the time, the wing was deflected 24.3 feet at 147% of limit load,below the 150% requirement for certification. Airbus said it designedthe A380 wing to break just beyond 150% citing strict adherence to itsweight reduction program. Airbus said it demonstrated the structuralimprovement, not through full-scale testing, but through further”finite element models to prove the adequacy of the structure onproduction aircraft.”
Later that summer, Airbus was required to install a 66 lb wingstrengthening package on the existing A380 fleet, starting with MSN003,the first superjumbo for Singapore Airlines that entered service inOctober 2007. The European airframer decided that wings delivered fromMSN018 on would have the modifications incorporated prior to deliveryto final assembly in Toulouse.
Airbus was able to verify the viability of the fix as a part of itscertification documents supplied to the FAA and EASA for final analysisand approval.
TIMELINE – Updated
Boeing experiences the first signs of trouble on the static airframe.During that test, the wings of ZY997 were flexed and the strain measurements on thestringer caps were reading higher than predicted.
Previously, on April 21st, Boeing conducted the limit load test which saw the wings deflected over 17-feet and an equivalent of 120-130% of maximum load.
”We went in and did some inspections and saw a number of thingsindicative of what the strain gauges were saying,” said Scott Fancher,vice president and general manager of the 787 program, said on yesterday’s teleconference, implying thatthe test had left visible damage to the structure during the late May testing.
The 1G check out of the wing, which was conducted in late-March, wouldnot have stressed the join hard enough to yield the same results.Previous rumors of delamination from December 2008 still stand as falseand unrelated to the current situation which came directly from testingthis past spring.
Preliminary analysis showed that the aircraft was still cleared forfirst flight, though with a reduced flight envelope. Sources indicatethat the original plan was to fly ZA001 and ZA002 on their respectivemaiden flights to BFI as planned then park the aircraft while a fix, which was considered to be “relatively minor” at the time, wasdeveloped that would allow an expanded flight test envelope, though Boeing says this plan was never in consideration.
Scott Carson, CEO of Boeing Commercial Airplanes, said yesterday that “the airplane could enter flight test with a credibleflight test envelope as [the company] worked relatively minor modifications.”
Boeing completes final detailed analysis on the inboard wing structure and decides to postpone first flight.
“The work done by the team through the week last week narrowed theenvelope to the point where on Friday we determined that to fly wouldbe such a small envelope for us that it would be an interestingexercise in having the airplane in the air but not particularly usefulin terms of preparing the airplane for certification,” Carson said.
Boeing makes a formal announcement of the first flight postponement.The change in first flight was unknown to many of those closest to theairplane. As late as the evening of Monday, June 22, sourcesindicated first flight had shifted to July 2nd at 10 am after holdingat June 30th for more than a week before and during the Paris Air Show. Boeing says that the July 2nd date was never formally approved for first flight.
Boeing says it will be several weeks before it announces a new schedule for first flight and first delivery.
Several tasks have to be accomplished before 787 is cleared for firstflight: 1. Develop the modification & concurrently repair ZY997. 2.Test the modification on a component level. 3. Install the modificationon ZY997. 4. Conduct full-scale tests on ZY997. 5. Install themodification on ZA001.
“We have to give the team time to do time necessary to do this job,”said Fancher. “While we will proceed with urgency, we will notcompromise the process for the sake of schedule.”
The fix, once identified, will be able to be installed on the aircraftin the factory, the flight line and at supplier partners without anyanticipated schedule disruption.
Carson affirmed that the production plan will proceed as planned with a 10 aircraft/month ramp up targeted for 2012.
“At this point, that’s our judgment that we will continue with the build up that we had previously anticipated.”