Boeing’s technical problems with the 787 Dreamliner go beyond the upper wing join issue publicly acknowledged by the company.
Engineers have discovered wrinkles in the fuselage skin just behind the wing that will require repair work on all the completed fuselage barrels.
Boeing minimized the flaw as “microscopic wrinkles in the skin plies” in just two locations near a fuselage door and said the problem can be fixed with “a simple patch.”
“The modification has already been designed and is being installed now in (Charleston,) South Carolina, and will be installed at completed sections in Italy and Everett,” Boeing said in a statement.
Still, the wrinkling is serious enough that Boeing’s engineers ordered the supplier of the affected section – Alenia of Italy – to stop work on new fuselage barrels until they can complete the fix of the manufacturing process.
Boeing issued a stop-work order June 23, the same day executives announced the indefinite postponement of the Dreamliner’s first flight. Boeing spokeswoman Lori Gunter said that timing was a coincidence and that the first-flight delay came because of an unrelated problem with the upper wing join.
“This [fuselage problem] was not a safety-of-flight issue and would not have caused a delay in first-flight timing,” Gunter said.
The stop-work order states that the already completed fuselage barrels on airplanes from at least No. 7 through No. 29 will have to be repaired. Alenia has already scrapped two fuselage barrels and sectioned portions of them in an attempt to understand the defect.
The problem arises in the manufacture of the longitudinal rods called stringers, used to stiffen fuselage skin.
The edges of the stringers are stepped, and Boeing’s specifications require accuracy in the manufacture of those steps to within a one-hundredth of an inch.
“Boeing engineering evaluations of the cross-sections provided by Alenia,” according to the stop-work order, showed the stringer edges on all the barrels from Dreamliner No. 7 onward “well in excess” of the required dimensions. Airplanes No. 5 and No. 6 are still being evaluated, Gunter said.
If the stringer-edge steps are the wrong depth, the skin around the fuselage wrinkles. When the fuselage bends – as when an airplane lands, for example – a wrinkled skin could cause the fiber layers to separate and tear, requiring expensive repairs.
According to Boeing’s order – signed by Jay Campbell, a senior manager of the 787 fuselage-supply chain, and James Simmons, senior engineering manager for the two defective sections – the problem was introduced into the manufacturing process in the building of airplane No. 5 when Alenia began using a new machine to build the stringers at its 787 facility in Grottaglie, Italy.
“Boeing is making every effort to work with Alenia to resolve these issues,” the order states. “Until the issues have been resolved, this stop-work order will be in effect.”
The order states that the wrinkles are big enough that Boeing’s defects analysis “does not sufficiently characterize the structural performance of wrinkles of this magnitude.”
The order also cites a Boeing technical manual cautioning that stringer-edge steps higher than 0.04 inch, compared with the specification of 0.015 inch, could “lead to significant degradation of the structure.”
Gunter said the deviations found in the Alenia structures are not that large.
The order states “Alenia has determined that they cannot comply” with Boeing’s requirements and requested the step-height tolerances be relaxed.
But Boeing decided that “this is unacceptable in that the subsequent bow wave skin wrinkles represent a risk of a major repair to every unit that is built.”
The long-term fix, Gunter said, is to let Alenia continue to build the stringers exactly as it has done, and to redesign the skin at the wrinkle points by adding extra plies of carbon fiber to make it stronger.
“Changing the stringers is difficult,” Gunter said. “The easier solution is to strengthen the skin by adding plies.”
Boeing will do that on all Dreamliners as they are built from airplane No. 30 forward, Gunter said.
As for fixing the barrels already produced, Gunter said it amounts to “putting a patch over the top” of the wrinkled skin. “The modification is relatively easy to make and requires no special access,” she said.
Responsibility for the flaw appears to lie with Alenia, although Boeing declined to assign blame. The stop-work order specifically states that the assemblies built in Grottaglie do not meet Boeing’s specification.
Alenia representatives in Italy did not respond to e-mail requests for comment before publication time.
Gunter said the fuselage skin wrinkling is an entirely separate problem from the one Boeing recently acknowledged.
On the day the stop-work order was issued, 787 program chief Scott Fancher publicly explained that first flight was being postponed because of a problem with the upper wing join, not the fuselage.
Fancher described a localized separation, or delamination, of the layers of composite fibers in a very specific area: “specifically limited to the upper portion of where the wing and side-of-body join.”