Fraser
Obviously if you fly into thick dense volcanic ash there will be a problem
we have all seen the towering cumulous like black dense clouds of ash billowing out of a volcano.
As with any particles in clear air whether moisture or volcanic ash they are visible in day light in clear air in the form of clouds or mist.
The argument as before is how dense does that ash need to be to risk the safety of an aircraft and its occupants?
As stated in the history of aviation going back to times when there was not the technology available today there has never been a fatality cause by an intrusion into ash.
the same cannot be said for bird strikes which have bought aircraft down.
One is perceived risk the other demonstrated risk.