A smokey environment is one thing, volcanic ash is a whole different issue - volcanic 'ash' is a bit of a misnomer - it's really rock, not the sort of ash a fire produces.
If we're talking a piston engine, conventional air filters can do a reasonable job (although replacement would need to be very frequent, perhaps every flight). Turbine engines require very elaborate systems to filter out something like volcanic ash - some military helos use such a system - but I've never seen them on a fixed wing aircraft. Ingest a large amount of volcanic ash into a turbine engine, and you're not talking hours to potential failure, you're talking seconds...