It seems the regulators and forecasters have known that a problem was coming for quite some time. Found this paper last night:
Comparison of VAAC atmospheric dispersion models using the 1 November 2004 Grimsvotn eruption
METEOROLOGICAL APPLICATIONS, Meteorol. Appl. 14: 27–38 (2007)
Comparison of VAAC atmospheric dispersion models using the 1 November 2004 Grimsv[]tn eruption - Witham - 2007 - Meteorological Applications - Wiley Online Library
Just one quote from the paper:
"One of the most important differences between the models used in this comparison is the technique used for defining the reported ash cloud. The results suggest that the criteria used by the London VAAC leads to a greater forecast extent of a plume than the concentration contours used by the other VAACs, whereas ash forecasts from Washington are the most constrained in aerial extent. Of note here, is that the London results are not based on a threshold chosen by an operator at the time, hence they are reproducible and errors cannot be directed to an individual. The use of one threshold, rather than contours of concentration, also clarifies the data for the forecaster. Given that the current ICAO guidance is to avoid all ash, it is unclear how a forecaster should deal with low concentration contours."
So the London VAAC model is known to be more pessimistic than all the others in use around the world. Aircraft are almost certainly flying elsewhere quite happily in what is known as the "Red Zone" in Europe, due to differences in the model output and interpretation. The paper also points out that the Washington VAAC uses the same intial criteria for defining the outer edge of the cloud as London VAAC. However, the Washington forecaster is allowed to use judgement and sattelite imagery to (apparently) go to levels that are 10, 100 or even 1000 times higher than the initial number. Not playing darts, as Lomapaseo suggest but very flexible nonetheless. I wonder what the London VAAC view on this is? Is the 2 mg limit effectively a 10x judgement call, for the specific case of the 2010 eruption?
Lomapaseo - I wouldn't personally want anyone to consider going back to JT8D type fuel efficiency, to guard against unexpected encounters with very high density ash (way above 4 mg per m3). I thought we were discussing the possibilities of raising the limits at the margins, to make everything a bit more flexible.... Incidentally, another paper (puff.images.alaska.edu/classes/pdf/Prata_2001.pdf) suggests (without reference) that the "US military consider mass loadings >50 mg/m3 a potential hazard to their aircraft operations".