Aircraft avoid any airspace that has volcanic ash in it for a simple reason: The ash can wreck the function of propeller or jet aircraft.
The gas particles are actually silica so fine they will invade the spaces between rotating machinery and jam it — the silica melts at about 1,100C and fuses on to the turbine blades and nozzle guide vanes that feed fuel, which in modern aircraft operate at 1,400C.
This was discovered by the crew of two aircraft, a Singapore Airlines airplane and a British Airways Boeing 747, in 1982 which flew through an ash cloud from the Galunggung volcano in Indonesia. On both planes, all four engines stopped; they dived over 20,000 feet before they could restart them and make emergency landings.
Ash particles are razor sharp and can pit the windscreens of the pilot’s cabin, damage the fuselage and light cover, and even coat the plane so much that it becomes tail-heavy. At runways, ash creates an extra problem because takeoffs and landings will throw it into the air again — where the engines can suck it in and it will create horrific damage to moving parts that suddenly find themselves in contact.
The problem with such ash is that it is extremely fine — less than 2 mm in diameter, and in the case of fine ash only 6 microns in diameter — which means that it is easily carried by the wind. Because it is ejected by hot air from a volcano it will often be thrown high into the jetstream at exactly the height that aircraft like to fly.
Once ash has got into an engine, it is impossible to remove as it is so fine. It pollutes filtration systems, electrical and avionic units — and the accompanying sulphuric acid aerosol can eat into rubber parts.