Speedbird 009 to Australia piloted by Cpt Eric Moody (This was in 1982 I think) - flying over Jakarta (spelling?) at 32000 feet and hit the invisible ash.

Result of this was the engines became clogged with this ash after it ebing compressed and melted into a gloopy gel substance. After the engines had flamed out and the aircraft dropped in altitude the gel cooled, solidified and broke off and allowed the engines to "breathe" again and thus being restarted After this the plane landed safely at Jakarta airport but with all facing surfaces having the paint stripped off them and the windscreens shotblasted so the pilots could not see out of them

The Aircrash Investigation series on the National Geographic channel covered this incident in great detail - well worth a watch and is being shown at the moment (part of Season 5 of the program) on Nat Geo