Boeing 787 wing structural testing was a success

ALTHOUGH THE BOEING CO.'s 787 Dreamliner has been beset by a series of embarrassing delays that have now stretched more than a year and a half, engineers accomplished a significant structural milestone when they broke a 50-foot-long section of one of the composite wings.

The wing did just what it was supposed to -- withstand, with a substantial safety margin, the highest aerodynamic forces the Dreamliner is ever expected to encounter in flight.

Now, engineers and 787 program officials must decide if they want to break the wing on one of the 787 test planes. Other than first flight, it is arguably the most dramatic moment during the testing of an all-new commercial jetliner before it can carry passengers.

For a couple of years, Boeing engineers have had a lively internal debate over whether to break the wing as part of the static ground tests. The 787 can be certified without actually breaking the wing, which could damage expensive test equipment and shower part of the Everett plant with tiny shards of composite material.

Boeing released a video of the wing box being stressed until it exploded, a jagged tear ripped across the composite structure.

Before a new commercial jetliner can be certified to carry passengers, the Federal Aviation Administration requires that the wings be able to withstand loads up to 1.5 times, or 150 percent, of the highest aerodynamic load that the jet could ever be expected to encounter during flight. After holding at that load for three seconds, the test is considered successful. There is no need to keep going until the wings break. But Boeing has done so in the past, to the breaking point, to demonstrate that the wings of its jetliners have a safety margin greater than required and to validate the analytical tools that engineers use in designing new jets.Engineers previously had loaded the wing that was destroyed Saturday up to 150 percent of the limit load, but then backed off because they did not want to actually test the wing until it broke.This time they did.

Boeing would not say how much above the 150 percent threshold the wing broke, just that it was "well in excess" of that figure."The technical team doesn't feel comfortable doing that," the Boeing spokeswoman said when asked why the break-point figure was not made public. "They said that on its own, the number is meaningless and people would try to make inferences that would not be founded with the proper context."

On the Boeing video of the test, Mark Jenks, vice president of 787 development, said the break occurred in the wing box exactly where engineers predicted.The 50-foot-long wing box weighs about 55,000 pounds. Boeing and its 787 partners, Fuji Heavy Industries and Mitsubishi Heavy Industries, which manufactures the 787 wing in Nagoya, Japan, have been doing structural testing on the 787 wing box for a couple of years at Boeing's research center on East Marginal Way South across from Boeing Field.Although it is full-scale, the 50-foot piece represents only a portion of the wing section, beginning at about the center of the 787 and stopping at about two-thirds of the span of the wing.

Boeing announced the wing-test project in July 2007 at the Farnborough air show near London and said the section of wing would be stressed until it broke. At the time, however, then-787 program boss Mike Bair said there was a "raging debate" within the 787 program about whether to actually break the wings of the static test plane as part of the certification test.

Static testing involves a one-time loading of areas of the plane to determine their ability to carry load, typically the maximum load expected in the lifetime of the airframe. There is also a fatigue-test 787. Fatigue testing helps engineers determine the plane's durability and involves cyclic loading of the structure to simulate repetitive flights.

The 787 wings are composite rather than aluminum. And that has allowed them to be very long -- 197 feet from tip to tip for the 787-8. The longer wingspan reduces drag, and that makes for a more efficient plane. But it never would have been possible without composites. The lighter and stronger composite material means more of the load can be carried on the outward part of the wing.

The 787 wings are nearly as long as the wings of the much bigger 777, which Boeing engineers broke during its static-certification testing. In January 1995, the wings were bent 24 feet above their normal position before they splintered -- at 154 percent of the design load.Airbus also broke the wings of its latest jet, the A380. But they failed at only 1.45 times the limit load.

At the time the A380 wing broke, the wingtips were deflected just over 24 feet -- much farther than they could be expected to ever bend in flight. The A380 was subsequently certified by the FAA and European regulators to carry passengers, but Airbus had to make structural modifications to the wing.The 787 wings are so long that some Boeing engineers think the tips might actually touch above the plane before they snap.