Claire
I'n not a pilot. Of course in you view that makes anything I say invalid. So be it.
Just a few things you ought to know. You use the term empirical evidence. It means specifically evidence obtained via senses. Capt Moody has a lot of empirical evdience that flying through ash is bad for prolonged flight and engine function. As empirical evidence goes its very convincing. I'm sure Capt Moody had all the empirical evidence he needed when all four engines on his 747 went out. He wasnt the only pilot who experienced empirical evidence that volcanic ash and engines do not go well together. So empirical evidence means that flying into ash clouds can upset your afternoon somewhat. apologies for the sarcasm.
OK so what is needed is quantitiave analysis - we know that aircraft can do some very scary things in volcanic ash. If I have to nail home my point, that is empirical evidence. The next step from empirical evidence is that a problem is known, so how severe is the problem? Quantitive analysis means to evaluate how much of the nasty stuff which coats turbine blades with glass and blocks pitot tubes makes the aircraft unsafe and secondarily, makes the components of the aircraft liable to failure earlier than design specs. My concern is that the quanititive analysis should not have economic pressures applied on it. I'm all ears to hear construcitve quanititive analysis of the volcoanic ash problem.