As for the visible versus invisible ash question - see ZQA'a link.
CaptainPaddy.
I did listen to it and the incident report was very interesting. visible versus invisible? Listen to the report again and you will note that his flight and encounter was at NIGHT!
Alaskan airlines limited their flights to daytime only which I would also support.
At night they would not know what they were flying through.
I also noted his referal to NO indications whatsover not even smells which if true would be very worrying.
I would also note that while possible that the damage they discovered from this accidental encounter was from the ash, there is nothing that proves it from his report.
What I would like to ask you is how would you have treated this possibly ongoing problem without causing major damage to an already strangled industry?
all the best
Pace