On the TV last week was a report on an aircraft crash. Pitot tubes are what registers the airspeed on modern planes. It seems that they were clogged with "supercooled water" which created a world of hurt for the crew and unfortunatly the entire electrical system failed, causing the plane to crash. Basically this water is below 0.deg centigrade, but doesn't turn to ice. Took a bit of working out for the investigator...