barbiesboyfriend:
I think this is all a bit overdone.
We know that the ash that BA009 encountered was too much.
Is that all we know?
How much, for example, is of no significance? (in ppm)
How much is the level over, say, London today? in ppm or some other unit.
I'm sorry to have to tell you that it is not overdone.
First stage turbine blades in jet engines are operating in a gas stream well above their melting point.
They are cooled by an internal flow of cooling air that exhausts through hundreds of very tiny holes, providing what is called "film cooling".
Volcanic ash in the cooling air melts and clogs these holes. The result is blades overheating and failing. Failure = no thrust.
As far as I know, the only data on "How much" ash will cause this condition is supplied by Two or Three accidental flights through ash.
I don't believe anyone wants to risk aircraft or passengers in test flights to prove some theory about acceptable levels of airborne ash, or others about ash concentrations or flow patterns. As far as we currently know, the acceptable level is zero ash.
The blades look something like this, and they are very, very expensive.