the earlier comment about how close to the erupting volcano an aircraft passes is meaningless.
In correct. The further you are downwind the more dispersed it is, not only horizontally but also vertically. This is why test flights have not crashed with engines out.
The flight that had engines out had just crossed a mountain range and could not safely return. They also did not know what the problem was, they thought it was St Elmo's fire that they could see, but it was dust abrassing the aircraft. They were in the Pacific and a long way from available emergency landing sites. They were flying at night time and couldnt see any cloud.