Boeing 787 Dreamliner - Mark Wagner [76]
One of the biggest worries of the airlines, however, was that composites could sustain potentially serious damage internally and yet look perfectly fine on the outside. Airlines, most completely unfamiliar with composite materials, had become particularly alarmed when Airbus said at a safety conference in late 2005 that the Dreamliner could be “grounded because of a scratch in the paint.” The problem, according to the European manufacturer, was that Boeing’s certification basis for the 787 called for inspections for visible damage only, without the need for nondestructive tests.
The claims infuriated Boeing. Justin Hale, then 787 chief mechanic, said, “We all know composites can hide damage, and so right up front we decided we’d certify for visible damage only.”
But what did this mean exactly? Boeing adopted design criteria for the static strength of the 787 that were related to barely visible impact damage (BVID), and for damage tolerance that were related to visible impact damage (VID). BVID was defined as small damage, such as dents of 0.01 to 0.02 inch deep, which could be caused by dropping a tool on the wing or fuselage, and which may not be found during heavy maintenance by general visual inspections using typical lighting conditions from a distance of five feet.
Anything designed to sustain BVID would have to prove, through rigorous testing, that it would maintain ultimate design strength and would not quietly grow into bigger, potentially dangerous structural damage inside the laminated skins. For VID, which included typical damage sustained by airframes from runway debris kicked up by tires, or from balls of hail, the requirements were to carry design limit loads without failure, and to carry residual strength loads until the damage was spotted and repaired. The design for VID also included the requirement that damage would not grow over time for the equivalent of an entire structural inspection interval, the first of which was not due for up to six years after service entry.
By March 2008 Alenia completed the ultimate load testing of the horizontal stabilizer at the Laboratory of Structural Tests in its Pomigliano plant in Naples. The tests, the first of their type to be conducted on a Boeing tail outside North America, proved the unit was more than capable of withstanding 150 percent of the design limit load it expected to see in its lifetime, eventually failing at 210 percent. The three-month test phase included repeated up-and-down movements as well as asymmetry at maximum load, simulating three critical design conditions for the stabilizer. Made of two monolithic co-cured side pieces and one central element, the sixty-five-foot-wide unit was cleverly manufactured in a one-shot autoclave cure cycle starting from twenty-seven uncured components. Alenia
The bottom line was that cracks did not grow, or propagate, in solid laminate composites, and neither would they weaken the overall structure. Small areas of damage would be acceptable for the entire life of the aircraft in most cases. “We have to demonstrate we’re good for these sorts of damage to the ultimate load for the life of the aircraft,” said Hale. Other damage tolerance requirements included continued safe flight following impact with an 8-pound bird hitting the tail at Mach 0.85 and 8,000 feet, safe flight after being sliced by a scything, loose fan blade, and even the ability to survive a sudden decompression caused by the opening of holes in the skin of up to 20 square feet.
Another key focus was demonstrating tolerance to lightning strikes—a relatively frequent