However, reduced runway visibility is not the biggest risk d
uring a volcanic eruption. Rather, the biggest risk is that the volcanic ash may affect the aircraft engines. The abrasive particles can even scour the windscreen, making it impossible for the pilot to look out the window.
Found this old, but cool video about the subject.
http://www.youtube.com/watch?v=pjftxS7CIFY
Thanks AMS Aviator Stan Tham, for the idea!