The Periodic Table of Videos has an interview with an aeronautical engineer on how a jet engine works, and what happens to it when volcanic ash passes through it. The result is not good. It's very bad as a matter of fact. Volcanic ash contains glass like particles and the combustion temperature in the jet engine can reach over a 1000 °C. The ash melts an coats the turbines with liquid glass. The engines tend to stop after a while.