Why is Volcanic Ash bad for aircraft?

Aircraft avoid airspace that has volcanic ash because it can damage the flight ability of both propeller and jet aircraft. Ash is so fine that it can invade spaces between rotating machinery and possibly 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.
Other possible problems is ash can pit the windscreens of the pilots cabin, damage the fuselage and light covers, and in some cases coat a plane so much that it could become tail-heavy. At runways ash creates an extra problem during takeoffs and landings will throw it into the air again – where the engines can suck it in and it may 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.
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