Funny everybody always mentions the BA and KL encounters with vulcanic ashes but total silence on SQ. They encountered a similar engine out situation somewhere over Indonesia on a B742 and managed to get it safely down at Jakarta on not more than one and a bit despite zero forward visibility due to grit-blasting of cockpit windows, poor weather conditions (rain and moderate crosswind) and ILS u/s.
An other phenomenon that was attributed to volcanic eruptions was the crazing of the cabin windows. Thousands where having to be changed prematurity. There was a large study done in the 80's of the problem and was traced to the the acidity of the volcanic ash. Corrective action was to throughly wash windows of aircraft that flew through volcanic clouds.