brooksjg – the problem in the EU is that there are 27 different Nation States, each with undisputed sovereignty over their airspace. Plus, just about every one has a hub airline; if you close down your major airport (for reasons well-justified or otherwise) you close down one of the drivers of your economy. The more refined the European prediction model becomes, and the more it shrinks the no-fly zone, the more likely it is that future events will impact just one or two countries/economies/hub airlines. At least Apr 15-22 hit just about everybody equally.
The current EU procedure establishes a no-fly zone based on a concentration of 2g per m3. There is general consensus that this is ‘safe’ but countries are still closing airspace and airports which are outside (but close to) this contour + its precautionary buffer zone. At the same time, there is little enthusiasm amongst the engine-makers to define a more accurate (i.e. higher) threshold, because that would move them out of their 2gm comfort zone and bring them into the realm of accountability.