I still haven't seen a reply to what was different about this eruption compared to the thousands of others that have happened since the advent of turbine engines, which have had no ill effects reported beyond a few where the aircraft was actually close enough to be visual with the eruption, and which were avoided by all other aircraft by common sense rather than a continent-wide ban.
When Mount St. Helens blew in 1980, for example, the ash generated was so thick that snowploughs had to be deployed on roads well downwind, yet there were only local aviation closures and diversions, and no adverse effects were reported from the continuing use of other airports and airways beyond the immediate and obvious zone. Why was a different approach taken this time, and what was the basis for it ?