The volcanic ash is not only dangerous for jets, they raise hell with any kind of engines exposed to the ash. Mt St Helens created a lot of misery for people downwind. I had flown around the mountain for a photo shoot just weeks before it blew. A good friend flying ag over near the Palouse in eastern WA told me when the cloud reached his area, he was flying trying to get in the last few loads. On the last load he said it was weird when his whole cockpit seemed to lose its reflectivness and it caused him to go IFR inside!
The locals of the Columbia Basin told me of nighttime conditions in mid day, ash piling up all over the place, cars losing power as their filters clogged, and eventually trashing their engines. A/C units were trashed for the same reasons, and countless aircraft engines lost hundreds of hours TBO if they weren't completely beyond repair.
