The structural flaw that delayed the first flight of the 787 Dreamliner is more complex than originally described by the company, and the plane's inaugural takeoff is likely at least four to six months away, say two engineers with knowledge of Boeing's problem.
The structural flaw that delayed the first flight of the 787 Dreamliner is more complex than originally described by the company, and the plane’s inaugural takeoff is likely at least four to six months away, say two engineers with knowledge of Boeing’s problem.
“It’s got to take at least three to four months just to get something installed on an airplane,” said a structures engineer who has been briefed on the issue. “It’s definitely a costly fix to go and do this work.”
A second engineer, who is familiar with the details of Boeing’s construction method, said the fix must first be made on the nonflying test airplane in the Everett factory. Assuming that’s successful, it will take another month or two to install the fix on the first airplane to fly.
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Both engineers said the issue requires a thorough redesign of the plane’s wing-to-body join, and the necessary parts will be very difficult to install on the test airplanes that have already been built.
The engineers’ accounts differ from Boeing’s description June 23 when it acknowledged a problem and again postponed the first flight of the much-delayed plane.
Dreamliner program chief Scott Fancher said then that the fix would be “a simple modification” requiring only “a handful of parts.” But almost a month later, heading into today’s quarterly earnings report, Boeing has neither set a new schedule nor outlined its planned fix of the problem.
The second engineer said the problem is caused by high loads at the ends of the stringers on the upper wing skins. Stringers are the long composite rods, shaped like I-beams, that stiffen the inside of the wing skin.
There are 17 stringers on each upper wing, all of them subject to compression forces when the wings flex upward in flight. At the point where each stringer ends, close to where the wing and body of the plane are joined, those forces pull the stringer away from the skin.
During a wing-bending test in May on the ground-test airplane inside the Everett factory, the fibrous layers of the composite plastic material delaminated at these stress points.
Such a separation of the material isn’t likely to lead to catastrophic failure of the airplane, but it would require constant monitoring and potentially costly repairs by the airlines. Any tear in the material would have to be promptly fixed to avoid spreading of the delamination.
If Boeing’s initial fix fails to divert enough of the load away from the stress points, the delay in first flight could extend beyond six months, pushing the date out into 2010.
“There’s no guarantee that what (Boeing) is doing will work,” the second engineer said. “If the testing or analysis shows it doesn’t get rid of the load, then the engineers are back to square one.”
Beyond first flight, solving the structural flaw could also further slow the plan for ramping up production.
Boeing’s current focus is on an interim solution to the stringer problem for the test planes that it has already built. The first engineer said Boeing hasn’t had time yet to figure out at what point in production to introduce a permanent redesign on all subsequent jets.
“None of that is nailed down yet,” he said. “There’s no schedule.”
Boeing has a large team of engineers working on the analysis, the redesign and how the fix could be implemented.
Those engineers are focusing on a solution that will send mechanics inside the wings of the assembled planes to trim the ends of each upper wing-skin stringer. They will create a U-shaped cutout in the end of the stringer, leaving the flanges at top and bottom untouched.
The U-shaped cut in the stringer ensures that the load on the flange away from the skin, the inner flange, will transfer entirely into the strong titanium fitting at the wing-body join and not into the wing skin, the engineers said.
The hope is that will reduce the stress point load enough to prevent future delamination.
The reshaped stringer ends must be refastened with newly designed parts to the titanium fitting, which connects the wing stringers to similar stringers on the fuselage side of the join.
And the design must accomplish this without creating another stress point somewhere else.
To reinforce the stringer ends, mechanics will also add some fasteners that go through stringer and skin. The 17 stringers on each side don’t all require the same reinforcement, but Boeing wants one design fix for all, so whatever is the beefiest reinforcement needed will likely be done for all the stringers, said the first engineer.
This retrofit will be tremendously difficult to implement on the airplanes already built because the mechanics will have to do the tedious and meticulous work inside the confined space of the wing.
“Drilling holes in titanium is difficult. Drilling holes in composite is difficult. And the access will be very difficult,” said the second engineer.
And when Boeing finally comes to do the job on Dreamliner No. 1, it will first have to empty the fuel from the wing tanks so that repair crews can work inside.
Excessive loads at stringer ends (known to engineers as “runouts”) is not something that should have struck Boeing out of the blue.
“The problem with stringer runouts has been identified in the past and recognized as a problem,” the second engineer said. He said the issue has arisen on other composite airplanes.
Indeed, the first engineer said the stress point at the end of the 787 stringers showed up as a “hot spot” in Boeing’s computer models before the delamination in the wing bend test — but for some reason it was never addressed.
The delamination happened after the wing bend test reached ultimate load, which is 50 percent higher than the maximum load expected in service.
The second engineer said reaching that load proves that Boeing’s heavy titanium structure is as strong as it needs to be. However, the delamination of the wing skin could have begun well before that load was reached, he said.
In the tests of the proposed fix that lie ahead, he said, engineers will have to inspect the stress points for delamination closely at every increment up to the highest loads.
Boeing spokeswoman Yvonne Leach said the company “will provide details on the technical solution in due course as we finalize our plans for implementation.”
The company reports its quarterly earnings before the stock market opens today.
In an early-morning teleconference after the earnings news is released, Boeing management will be quizzed closely by Wall Street analysts for more detail on the expected program delay. It’s likely the executives won’t yet have precise answers.
Dominic Gates: 206-464-2963 or firstname.lastname@example.org