Aircraft avoid airspace that has volcanic ash as it can wreck the flight ability of propeller and jet aircraft. The ash is so fine that it will invade the spaces between rotating machinery and jam them – silica in ash melts at about 1,100 degrees and fuses to turbine blades and nozzle guide vanes (another part of the turbine assembly), which in modern aircraft operate at 1,400 degrees.
That can be very dangerous – as the crew of two aircraft, including a British Airways Boeing 747, discovered in 1982 when they flew through an ash cloud from the Galunggung volcano in Indonesia.
All four engines on both planes stopped; they dived from 36,000ft to 12,000ft before they could restart the engines and make emergency landings.
That is not the only problem. Ash can pit the windscreens of the pilots cabin, damage the fuselage and light covers, and coat a plane so much that it becomes tail-heavy. At runways ash creates an extra problem as takeoffs and landings will throw it into the air again – where the engines can suck it in and it will cause major damage to moving parts. The Icelandic ash plume has been thrown into the atmosphere to between 6km and 11km – exactly the height that aircraft would be flying at.