Metal Fatigue Causes Bumpy Landing

calamities.jpgWhen it comes to structural integrity, plastic parts often get a bad rap. But when they fail, often the result of metal fatigue, metal parts can fail catastrophically as forensic engineer Ken Russell relates in his Calamities Column in Design News.

In the Case of the Acrobatic Airplane, he describes his investigation into a twin-turboprop airplane that was forced to land on one wheel when the other failed to extend. My worst nightmare, actually.

“The landing gear retraction mechanism involved an up-lock hook that engaged a pin. The hook had partly fractured and jammed so the wheel could not be lowered. Many airplanes of the same model used the same hook without incident. So why did this particular hook fail? Examination of the failed hook with a simple hand lens gave the answer. The fracture was next to a hole that had been reinforced by a washer spot-welded in place. The spot weld was defective in burning a hole through the bracket. Fatigue cracks started at this weld defect and ultimately caused the accident.  (The figure above is an SEM photo of a small region on the fracture surface.)”

Read the full analysis here.

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