for those seriously interested in the impact, here is a section from the US governments volcano site 
http://volcanoes.usgs.gov/ash/trans/index.php
Life-threatening and costly damages can occur to aircraft that fly through an eruption cloud. Based on reported damages from ash encounters, the hazard posed to aircraft may extend 2,000 km (3,200 miles) from an erupting volcano. The actual effects of ash on aircraft depend on several factors, including the concentration of volcanic ash and gas aerosols in the cloud, the length of time the aircraft actually spends in the cloud, and the actions taken by the pilots while in the eruption cloud.
Numerous instances of jet aircraft flying into volcanic ash clouds have demonstrated the serious damage that can be sustained. Ash particles are angular fragments having the hardness of a pocket-knife blade and, upon impact with aircraft traveling at speeds of several hundred knots, cause abrasion damage to forward-facing surfaces, including windscreens, fuselage surfaces, and compressor fan blades. Moreover, the melting temperature of the glassy silicate rock material that comprises an ash cloud is lower than the operating temperatures of modern jet engines; consequently, ingested ash particles can melt and then accumulate as re-solidified deposits in the engine. The overall result of an aircraft's flying into an ash cloud can be degraded engine performance (including flame out), loss of visibility, and failure of critical navigational and operational instruments.
Experimental tests (Dunn and Wade, 1994) determined the following mechanisms that can affect aircraft performance due to exposure to a volcanic ash cloud:
(a) Deposition of material on hot-section components.
(b) Erosion of compressor blades and rotor-path components.
Blockage of fuel nozzles and cooling passages.
(d) Contamination of the oil system and bleed-air supply.
(e) Opacity of windscreen and landing lights.
(f) Contamination of electronics.
(g) Erosion of antenna surfaces.
(h) Plugging of the pitot-static system which indicates the airspeed of the aircraft.
An ash cloud eventually dissipates in the atmosphere, and ash concentrations drop. However, the threshold concentration at which ash poses no harm to aircraft is not known, and indeed, may never fully be characterized for all situations involving aircraft. It is usually assumed that ash identifiable on satellite images continues to present a hazard to aircraft. Accordingly, the consensus of the aviation community is that if an ash cloud can be discerned, it should be avoided.