Chairlift Failure Sends Skier Plummeting
The winter sports season is upon us, along with the usual spate of bruised buns and broken bones.. But while most skiers who run into trouble will do so on their way down the slopes, for the really unlucky there’s the occasional freak accident going the other way. The latest incident, as reported in the Concord Monitor, was a lift malfunction in New Hampshire: “A young man was hurt in a chairlift accident at the Mount Sunapee Ski Resort in Newbury yesterday…. The incident involved a mechanical failure of the North Peak triple chairlift.” No details on the actual failure mode, though I’m reasonably confident that lawyers are already busily at work finding out. But it does bring to mind another case involving a chairlift calamity in New Hampshire. In this instance, a fractured housing caused a lift chair to separate from the cable, sending it and two occupants plummeting to the slopes below. My friend, forensic engineer and metallurgist Ken Russell, investigated the case on behalf of the ski area, who was being sued by the parents of the injured teenage skiers for poor design and assembly and a suspected overtightening of the fastener holding things together. Ken’s job as forensic engineer was to figure out what caused the housing to fracture. Try as he might, he couldn’t find a good metallurgical reason for the failure — as he reported in Design News “Radiography and optical and scanning electron microscopy showed a sound microstructure remarkably free of cracks and porosity. A small specimen machined from the subject housing and loaded to failure had a strength consistent with that of the exemplar. So, the housing was plenty strong enough. The test showed only 6 percent ductility, which is to be expected of the aluminum-silicon alloy used.” The expert for the plaintiff saw Ken’s report and immediately changed his tune — concluding the failure was due to “sympathetic oscillations set up in the cable instigated solely by the powering mechanism, which caused the chair to strike a tower and fracture the housing.” Wow — good thing kids don’t try and get those chairs swinging or anything!