I think in-flight structural failure is pretty rare, isn't it? I can understand how stuff like that happens in old aircraft (i.e. tankers-- metal fatigue) and amphibians/seaplanes (corrosion), but you're probably more likely to have a cable fray apart or an engine quit. Or an elevator jam due to a coin/flashlight etc. getting stuck in the works. Cracks leading to thrown props aren't much fun either, at least from what I've been told.