From the BBC:
Flying through volcanic ash clouds
Imagine that the next time a volcano erupts in Iceland, the airlines are able to treat it as though it was no more than a simple weather hazard, like fog.
Scientists working on an ash cloud detection system believe that airlines will soon be able to do just that.
Volcanic ash and jet engines simply don't mix. The superheated particles turn to glass inside the engine and stop it dead.
So, those who manage the airspace above us go to great lengths to keep aircraft well away from the ash clouds that form during an eruption.
Full story:
BBC News - Flying through volcanic ash clouds