i read the summary of the WMO meeting 2007.
In its final paragraph, it says that clear limit - data on acceptable
ash concentrations in the athmosphere are required from manufacturers
and authorities, as ash-clouds will become indetectable after a while....
but some ash residue will still be around. So, does anyone know of limitations published since then ?
No. Work may proceed more quickly now! But in fairness to them, Airbus started talking in earnest before the current crisis.
As has been noted many times here, volcanic ash pervades the atmosphere and so the warning has to stop somewhere. The limit of damage appears to be something a bit below concentrations that can be remotely sensed in normal conditions (i.e. when the cloud can't be remotely sensed it might still be dangerous for a little while longer. There have been some examples of aircraft encountering clouds that have turned out to be mostly sulphates (hey, and we love flying through sulphuric acid, don't we?) without much ash, and some controversy around how dangerous these are (Google: Hekla 2000 NASA Grindle - a 35 hour old cloud from Hekla, Iceland)