Wikipedia
Very fine volcanic ash particles (particularly glass-rich if from an eruption under ice) sucked into a jet engine melt at about 1,100 °C, fusing onto the blades and other parts of the turbine (which operates at about 1,400 °C). They can erode and destroy parts, and cause jams in rotating machinery. Ash can also “ blind” pilots by sandblasting the windscreen requiring an instrument landing, damage the fuselage, and coat the plane so much as to add significant weight and change balance. Propellor aircraft are also endangered.