"The world's busy air traffic corridors pass over hundreds of volcanoes capable of sudden, explosive eruptions. In the United States alone, aircraft carry many thousands of passengers and millions of dollars of cargo over volcanoes each day. Volcanic ash can be a serious hazard to aviation even thousands of miles from an eruption. Airborne ash can diminish visibility, damage flight control systems, and cause jet engines to fail. USGS [U.S. Geological Survey] and other scientists with the Alaska Volcano Observatory are playing a leading role in the international effort to reduce the risk posed to aircraft by volcanic eruptions."
U.S. Geological Survey Fact Sheet 030-97
Alaska Volcano Observatory: http://www.avo.alaska.edu/