Thinking of the situation where fine-grained VA gets into the cooling channels inside turbine blades and / or forms a glassy coating on blades or stator vanes, therefore blocking leading-edge or other cooling holes, what's actually going to remove it? Therefore, further ash encounters must result in accumulation in the same area(s). If not, why not?
I think you are dead right. It wouldn't suprise me at all if there are incidents of blade shrouds departing the back end or vane trailing edges disappearing due to a very slow deposition rate of ash deposits inside the components.
Just to add some balance to this. I believe that the glass deposits will flake off with engine thermal cycles (takeoffs and shutdowns) and that any deposits in the cooling air holes will eventually work they way out over time. The cooling air is an open system depending on flow pressure where the air comes in at some compressor station, picks up the heat in the front end of the turbine and then flows back out into the turbine gas path. Meanwhile the reduced, by ash, holes will cause the metal temeprature to rise and the metal will begin to ablate away (like ice melting) over many engine takeoff cycles. It's typical for this process to be seen by borescope inspection but if left unchecked over will lead to lowered EGT margin on hot day takeoffs and reduced in-service life.
In other words history will be repeated in the numerous events from Mt Pinatuba as documented in the
Thomas Casavadell report
Casadevall
Now what was the question again?
