Vulcan Vesuvio Italy destroy american bombers

[SQB]

It takes a LOT of ash to kill an aircraft, that plane flew through 80km of ash before the engines stopped working, the ash had turned back into glass and built up on the edge of the inner turbine (from memory).

After that airlines have become rather paranoid about volcanic ash, honestly I don't blame them, imagine if there was a serious crash because they thought it was ok.