Ash is very different from sand. Ash has a very high silica content and is soft but very abrasive. IE. jagged shape. Silica's melting point is the same as the internal temp within a jets combustion chamber. So it melts and clogs things up. not good, as this will generally make a plane fall out of the sky. Sand is rounded in shape, very hard and will cause impact abrasion but will not melt at the jet engines temp's and will pass through. Will need a rebuild though.
I can tell you from personal Mt. St. Helens experience. If the ash lowers to ground level. DO NOT touch your or any one else's car.
Best Regards
Jerry