Jet engine failure modes with ash ingestion
Understanding why a jet engine should flame out when heavy concentrations of ash go in the front end (compressor and combustion chambers) is just common sense. We also know from experience of this that it is possible to glide down for 15 mins into some cleaner air where the engines can be restarted.
However, there is another failure mode which is much more of a problem and potentially terminal for the engine.
If very fine material that the compressor and combustion stages can happily swallow enters the engine, the risk is to the turbine which in normal operation must be cooled. This risk arises because the airflow past the turbine is sometimes several hundred degrees above the melting point of the turbine material. The cooling air exits through a myriad of small holes in the surface of the blade ensuring that the blades are encased in a cocoon of cool air.
Clearly quite small particles could affect this cooling flow but even worse, depending on the composition of the particles, the temperature can cause some of the particles to change their nature and form a glass like material that can build up in and on the blades. In the worst case such a process, when the engine is running normally, could destroy the turbine in a minute or so.
Apologies to all those who understand these matters.