Lomapaseo
I think that the operating temperature of the engines would not be a factor.
If an aircraft were to fly through a cloud of volcanic ash at 500kts it would be equivalent to flying through a giant gritblaster...
The entire aircraft would be thoroughly abraded, including the turbine blades
...
Have a look at this;
Airline captain recalls flying through volcanic ash - Telegraph
and this;
The story of BA flight 009 and the words every passenger dreads ... | Mail Online
CJ