I've read most of this thread, and have previously decided just to hold my tongue for fear of getting flamed, or moderated.
It seemed to me all along that the response to this (or indeed) any volcano's eruption should have followed the following, fairly simple, train of thought:
1) Does flight through the affected airspace pose an unacceptable risk of damage sufficient to cause an imminent loss of life? In this context, "imminent" means that the loss is likely to occur during either the current flight or any subsequent flight(s) prior to it being possible to adequately inspect and repair said aircraft. In this case there is a clear case for airspace being closed to traffic.
2) If (1) doesn't apply, then the event is not an immediate safety of flight issue and airspace should not be closed. Decisions then become commercial. It must be an operator's prerogative whether or not it is to its benefit to ground its aircraft and avoid increased repair costs or to fly and accept that there will be increased wear and tear. That is not to say that there is no place for the regulator in that process. It is entirely proper, and indeed should be expected, that the regulator impose a more stringent inspection regime on aircraft that are known to have been exposed to unusual atmospheric contaminants.
We now seem to have arrived at a modus operandi that reflects that train of thought. Unfortunately, getting there has been a painful process which has cast no credit on government, the regulators, or indeed the aviation industry:
a) Government, because it sat back for too long allowing the aviation authorities to take decisions with major impacts outside their field of competence. Like it or not, a decision to impose a prolonged closure of airspace over an entire continent should not be taken purely in an aviation context, without regard to other effects. In the modern interconnected world, such a closure has financial, social and economic consequences far beyond that restricted regime. Most directly, as any closure extends, we should expect that people will die in accidents using alternative transport or for lack of medication or treatment, but there will be bankruptcies, economic disruption and so on. It is explicitly the role of government to balance conflicting interests in such cases.
b) The regulators, because their preparations for such events had almost certainly not been adequately stress tested. I will be amazed, and rather disturbed, if it subsequently emerges that the ICAO and others had realised that the procedures they put in place could lead to the shutting down of air traffic over one of the world's most economically active regions for nearly a week. Especially as a result of the eruption of a volcano smaller than the one they used as an example in their policy document! The Met Office hardly emerges with any credit from this process either. How can the VAAC be satisfied that it has discharged its role adequately when it has issued charts showing the “boundaries” of contaminated regions accompanied by the clearly contradictory statement that concentrations within those regions were unknown? And why had it not established any methodology for actually checking its predictions against measured data? Or against the predictions of other forecasters?
c) The aviation industry, because there seems to have been so little prior investigation of the physical effects of this phenomenon. How can airframe and engine manufacturers have issued documents stating that should be no flight through ash-contaminated airspace with a straight face? Surely that was a prohibition honoured more in the breach than the observance, since a zero concentration of ash is clearly never possible, mathematically or practically, in the atmosphere of a planet whose geology is driven by plate tectonics.
Despite newspaper terminology, this has not been a "shambles" or "chaos". And I am very glad that I haven't been caught up in it personally. But it has been woeful to watch. We weren't struck by an asteroid, folks! A routine, well-understood, geological event occurred in one of the richest, most developed, most sophisticated and technically capable parts of the world, and the various authorities' best response was, in essence: "well, we don't know what the actual effects are!"
And this in a field where the maxim: "prior preparation prevents p**s poor performance" is supposed to rule. Spare us....