The wing damage that grounded Boeing's new composite plastic 787 Dreamliner occurred under less stress and is more extensive than previously reported.

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The wing damage that grounded Boeing’s new composite 787 Dreamliner occurred under less stress than previously reported — and is more extensive.

An engineer familiar with the details said the damage happened when the stress on the wings was well below the load the wings must bear to be federally certified to carry passengers.

In addition, information obtained independently and confirmed by a second engineer familiar with the problem shows the damage occurred on both sides of the wing-body join — that is, on the outer wing as well as inside the fuselage.

The structural flaw in the Boeing design was found in May during a ground test that bent the wings upward. Stresses at the ends of the long rods that stiffen the upper wing skin panels caused the fibrous layers of the composite plastic material to delaminate.

The damage at the end of each of the 17 long stiffening rods, called stringers, on each wing’s upper skin happened just beyond the aircraft’s “limit load,” which is the maximum load the wing is expected to bear in service.

Last week, The Seattle Times mistakenly reported that the damage occurred later in the test, just beyond “ultimate load.” That is defined as 50 percent higher than the in-service limit load and is the Federal Aviation Administration’s test target. The tearing at the end points of the stringers well before the wing reached ultimate load means the problem is worse than suggested in last week’s story.

Because the wing test fell short of the ultimate load target, the plane could have flown only under restrictions that would have severely limited the usefulness of a test flight.

It also helps explain why Boeing canceled the first flight planned for the end of June.

The fact that there is corresponding damage on the fuselage side of the wing join adds to the complexity of any fix and the time and cost involved in implementing it.

The wings of the 787 are made by Mitsubishi Heavy Industries in Japan.

Inside the fuselage, on the other side of where each wing joins the jet’s body, there is a structure called the “center wing box,” made by Fuji Heavy Industries, also in Japan.

This center box is constructed much like the outer wing, with composite-plastic skin panels stiffened by composite-plastic stringers.

The stringers on the fuselage side mate at the wing join, fitting with those on the wing side.

Because the wings are designed to transfer the loads into the fuselage box, the damage that occurred in the test was mirrored on either side of the join.

Though a single fix, once designed and tested, will work on both sides of the join, mechanics performing the necessary modifications inside the airplanes already built will have to duplicate the work inside the wing and inside the fuselage.

According to the engineers, Boeing is focusing on a solution that will require mechanics to create a U-shaped cutout in the end of each upper wing-skin stringer.

This would have the effect of transferring part of the excess load into the titanium fitting at the wing-body join instead of into the wing skin.

The mechanics must then fasten the reshaped stringer ends with newly designed parts to the titanium fitting.

The goal is to reduce the stress-point loads enough to prevent future delamination.

The delamination of the composite-plastic 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 would have to be promptly fixed to prevent it from spreading.

The way the stringers terminate and mate at the join, the focus of the problem, is Boeing’s responsibility and not that of its Japanese partners. Boeing will have to pay for the cost overruns.

Engineers will have to validate Boeing’s chosen solution in tests before they modify the wings and center wing boxes already built.

Company spokeswoman Yvonne Leach said 10 Dreamliners have been completed, including two ground-test airplanes. About 30 more are in various stages of production.

The Dreamliner is already two years late.

CEO Jim McNerney said last week that a new schedule for first flight and delivery will be ready within the next two months.

Estimates by the two engineers of the minimum time needed to fix the problem suggest the plane is now unlikely to fly until next year.

Until the new production timetable is announced, Wall Street analysts are unable to calculate the precise additional cost of this latest delay.

Analyst Joe Campbell, of Barclays Capital, this week downgraded Boeing’s stock. He cited an increased risk that the company will book a large accounting loss this year to cover the extra expense of the repeated delays.

In a note to clients, Campbell estimated the total cost overrun of the Dreamliner program so far — extra startup and engineering costs, penalties owed to customers for delivery delays and contractual obligations to suppliers for engineering changes — as “in the vicinity of $11 billion.”

Because 850 Dreamliners have already been ordered, Campbell still believes the jet can be “highly profitable” over two decades of full production.

But with that level of cost overrun, Campbell said, “Boeing is highly likely to lose large sums of money on the first 400 to 600 aircraft.”

Dominic Gates: 206-464-2963 or