You may remember earlier this year (April 2010), the eruption of Iceland’s Eyjafjallajökull volcano, which caused airlines to cancel many flights into & out of European airspace. I have a friend who cancelled a trip to Europe because of the situation. A Math Department faculty member was in Russia for a conference and could not depart afterward due to the cancelled flights.
You may be interested in reading this article on Bloomberg.com Europe Airspace Shutdown Worsened by Lack of Research. The article implies a lack of research on the effects of volcanic ash on jet engines and over reliance on computer modeling.