Black box data is the ne plus ultra of aircraft accident investigation. But it is not the only kind of physical evidence. Pieces of debris—in particular, their dents and fractures — can tell a vivid story in themselves.

When a plane crashes, it’s common for all different parts to exhibit different kinds of failure. Imagine a plane whose wingtip hits a tree. The impact would crush the leading edge of the wingtip—compression failure—and then wrench the wing backwards from the body of the plane, causing a tension failure at the forward wing root and compression failure at the aft end.

By collecting many pieces of debris after a crash, investigators can place the mechanical failures in a chronological order to tell a story that makes sense, much as you might arrange magnetic words on a refrigerator.

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