Boeing says that its 787 Dreamliner–a midsize, fuel-efficient passenger jet that is currently in development–will be the first commercial aircraft in which major structural elements are made of composite materials rather than aluminum alloys. The changes are expected to slash component weights 20 percent, significantly boosting fuel efficiency.

Such composite materials–layers of superstrong carbon fibers and epoxy–have long been used in military jets, where money is rarely an object, and in commercial jets for parts like luggage-rack frames. But Boeing is learning how hard composites can be to analyze effectively and build economically for commercial jet structures. The company has had to delay the 787’s introduction because elements of the composite-made wing box–the major structure inside each wing–buckled in stress tests.

The wing box begins in roughly the middle of the plane and extends about two-thirds of the wingspan. This key component–more than 15 meters long and 5 meters wide–was designed and built by Boeing together with Mitsubishi Heavy Industries and Fuji Heavy Industries, in Japan. Pat Shanahan, vice president for the 787 program, said in a conference call last week that structural testing had “identified the need to stiffen elements within the center wing box.”

The fix requires adding new brackets and other parts to wing boxes already built, as well as modifying the design of boxes not yet built. The retrofits of existing boxes will intrude into wiring pathways, compounding the problems. So Boeing is pushing back the 787’s delivery date about six months, from the first quarter to the third quarter of 2009.

The issue with composites isn’t that they aren’t strong; it’s that they are so internally complex. They consist of layers oriented in different directions; those layers, in turn, are made of individual fibers that may vary somewhat in composition. This makes it difficult for engineers to accurately mimic their performance in computer models for premanufacture testing.

“Composite materials are more difficult to analyze than simple homogenous metals,” says John Hansman, director of the International Center for Air Transportation, at MIT. “You generally don’t model every fiber in the structure, so you come up with models that have simplifications.”

What’s more, he says, composites allow engineers to make custom shapes, but these custom shapes compound the already difficult modeling problem. “You have many more design options, which can be both a strength and a weakness. There are many more things I can do with composite materials–add strength in specific places, take it away–but then you have combinations of both the geometry and the particular layup of the composite materials” that are unique.

Boeing’s mechanical stress tests start with representative pieces (known as coupons), then move on to progressively larger parts of the structure, and finally to the full structure. Boeing puts the structural parts into huge hydraulic machines that bend and twist them to mimic stresses that go far beyond worst-expected conditions in real flights. It was during such tests that problems emerged with structural spars in the wing box.

Shanahan said in last week’s conference call that Boeing has traced the problem back to an error in earlier modeling analysis, but he did not explain the details. “We discovered it. We’ll go back and correct it,” he said. Shanahan added that Boeing has not lost faith in its decisions to more widely use composites; 95 percent of thousands of tests have yielded as-good or better-than-expected results. On one such test–of the composite-made fuselage “barrel”–engineers had to stop the test for fear of breaking the test equipment, he bragged.

David Roylance, a composites expert and associate professor in materials engineering at MIT, says that Boeing’s experience with the 787 shows that the industry is still on a learning curve in using composites more widely in commercial planes. “There are a whole variety of things with composites that are engineerable, but are different than metals,” he says. “So it takes time for people to feel comfortable with it.”