At last there has been sense, and the skies are reopening. But a couple of days ago I found this website, by the official US air force and it mentions the exercise planned for this time period over Europe.
My cynical mind is working overtime. 9/11 had an exercise taking place when the towers went: 7/7 in London also had an exercise going; could this exercise also have been an excuse, using the volcano eruption as a cover story?
I leave it up to you to decide.