Popular Science has a nice write-up on the “effects of volcanic ash on jet engines”:http://www.popsci.com/science/article/2010-04/why-cant-planes-fly-through-volcanic-ash-because-nasa-tried-once. The scariest thing I read in the article is the fact that pilots often can’t tell that they’re about to fly through an ash cloud. Couple that with the fact that the ash can actually melt inside the engine and collect on the components and you’ve got a regular old Twilight Zone episode on your hands.