NASA's Unexpected Volcanic Ash Encounter Flight.

Ten years ago, a NASA DC-8 unintentionally flew through a diffuse ash cloud generated by Helka, a volcano in Iceland, and upon first inspection showed no damage -- key words "unintentionally" and "upon first inspection." What the event showed is that the most up-to-date information on ash cloud location could be misinterpreted and significant damage could be incurred in spite of a vigilant, well-briefed flight crew. What's more, that damage can be very expensive and hard to detect. Then NASA propulsion engineer Tom Grindle worked the case and co-authored a paper on his findings

http://www.avweb.com/pdf/volcanic_ash_cloud_encounter_nasa_grindle.pdf

source:avweb.com