My aircraft has been parked outside -in sunny Lancashire- unmoved since Thursday. There is now a significant coating of dust - enough that I could not fly without washing the screen. As a PPL I dont have to fly, and my take on this is, I wont till its gone. -I pay the maintenance bills. I doubt safety is a problem for piston aircraft but the maintenance issues are unknown. I suspect blade paint will be the first thing to suffer followed by high cylinder leakage at the next annual. If the ash gets past the air filter and embedded in the pistons rapid cylinder wear will occur. Oil analysis should reveal if this has happened but by then its too late. I do not know if the ash will pass the air filter. Not worth the risk for me.