n May 2009, inside a test hall at Boeing’s facility in Everett, Washington, USA, engineers began bending the composite wings of static test airframe ZY997 upward. It was supposed to be routine – a limit load test applying forces equivalent to the most extreme loads the aircraft would ever experience in service.
It was not routine. Just above the limit load, with the wings deflected around 18ft (5.5m), the carbon fiber reinforced plastic at the wing-to-body join began to delaminate.
The damage centered on the 17 stiffening rods or stringers, running along the upper skin of each wing box. The co-cured composite stringers are bonded to the wing skins in an autoclave during production, forming a single structure.
At their inboard ends, where the Mitsubishi Heavy Industries-made wing box meets the center wing box made by Fuji Heavy Industries (now Subaru Corporation), each stringer terminates in a widened cap that makes the structural connection at the side-of-body join. Under load, the fiber layers at these caps ruptured and separated from the skin. The stringers effectively peeled away from the wing.
Corresponding damage appeared on the fuselage side of the join, inside the center wing box. The problem was bilateral, and significantly more complex to fix than anyone had anticipated.
The real sting was that Boeing’s computational models had not predicted the issue. The analysis showed a clear disconnect between what the finite element models expected and what the physical structure actually did.
The load path through the titanium fasteners at the side-of-body join was not transferring forces as designed. The resulting stress concentrations drove delamination at loads well below the 150% ultimate load target required for certification by the Federal Aviation Administration.
Boeing attributed the failure to manufacturing problems, while insiders pointed to a design flaw. “The area in question is a few square inches in the side of body,” Scott Fancher, vice president and general manager of the 787 program at Boeing, reassured media at the time.
The fix, however, was anything but small. Engineers designed a modification that included U-shaped cutouts at each stringer end to redistribute load to the titanium fittings, with new reinforcement fittings installed at 34 stringer locations within the wing-to-body join.
On November 30, 2009, the modified ZY997 test airframe was retested and the wings deflected to the limit load with no delamination.
The 787’s first flight finally took place on December 15, 2009, around 18 months later than the schedule in place before the wing test failure.
On March 28, 2010, the aircraft passed its ultimate load test at 150% of maximum expected service loads, with the wings bending approximately 25ft (7.6m) upward.
According to the Seattle Times, Boeing’s chief structures engineer Jim Ogonowski described the moments approaching ultimate load as “the quietest 90 seconds on Earth in the control room.”
What was learned
The 787-wing test failure remains one of the most instructive case studies in composite primary structure certification. It demonstrated that delamination in large co-cured composite assemblies can behave differently from metallic failure modes, and that analytical models may not capture localized stress concentrations where different composite sections meet.
The case reinforces the important principle that the computer is a tool, not an oracle. For novel materials and structural configurations, full-scale physical testing is when you find out what the model does not know.
