Having read every page of this thread over the last six days (up to page 99 which is when I started typing this offering) it is clear that there is a wide variety of argument and counter argument, much of it centering around conjecture.
Having been trained in the industry as an engineer 50 years ago! (though I never "practised) and as a PPL of 20 years, from what I can deduce there are some things we know, some we don't know and some which we ought to know for future episodes of a similar nature.
In summary I think these include -
What we know -
1 A volcano erupted and due to the high inflow of glacial meltwater it's output consisted of high level of fine particulate ash which is relatively unusual.
2 An unseasonal slow moving high over the UK caused a southerly flow which caused the plume to move south before splitting and moving both east and west. This dispersal pattern is generated by computer models rather than direct empirical observation.
3 Volcanic ash (VA) with high levels of silica CAN cause severe problems both short and longer term to jet engines, both immediately suppressing combustion and damaging turbine blades.
4 An ICAO "directive" or advisory, issued after a major emergency in 1982 laid down "rules" to be followed in the event of VA clouds impinging on airspace.
5 The subsequent shutdown of UK and European airspace has caused economic damage to the industry, other industries, national economies and considerable discomfort and worse to large numbers of passengers.
6 A small number - very limited - of "research flights" have launched in the later stages of this situation and the results, if clear, have not been widely published.
7 Airframe, engine manufacturers, operators and national and international regulators have failed to undertake or insist upon research which would increase knowledge on which to base actions regarding such situations.
What we don't know -
1 The degree of accuracy of the models and therefore the actual dispersal of dust, either laterally or vertically, and there is little information as to the vertical thickness of layers.
2 Precise data about particulate size, or chemical composition once the dust has reached altitude, and even if we did -
3 There appear to be few if any advisories from engine manufacturers as to the chemical composition of dust, the particulate size, or the density of cloud (ie likely throughput) which engines can "tolerate" without damage, or continue to operate in while maintaining power while suffering damage.
There are many, many more knowns and unknowns but these seem to be the key ones.
Therefore given that the regulators and "safety authorities" do not know what's going on, or what is a safe situation in the first place it is entirely predictable that they will take the NIL RISK route, assuming the worst case scenario and avoiding it by putting aircraft on the ground.
Far more important than what is happening at the moment is what WILL happen in the coming months. The industry must undertake detailed research to find out what its equipment will "tolerate", while national and international authorities MUST put in place systems to ensure that any computer models of future events can be checked in detail by actual sampling to ensure that a vital industry is operated on the basis of knowledge rather than computer guesstimation.
Had such work been done on a regular basis since 1982, we would have 28 years worth of useful information instead of very little.
Until that happens I suspect that the authorities will insist that we sit this out until it goes away…..