really what should they have done??? Firstly talked to airlines who are used to dealing with ash in active volcano areas and use their operational advice.
Second use satellite real time visible ash plume monitoring. Stick a safety margin around those and make those dense ash areas NO GO.
Fly in daylight and if possible avoid flying in clouds especially pollution coloured clouds.
The mathematical computer generated low ash areas should have no ash limits at all ( better than choosing a number between one and ten)
but should have been purely precautionary areas. I say that because even with millions of tests you have no way of telling whether half a mile from the test spot you dont have denser areas.
Operate in those predicted low ash areas and be prepared for more inspections to monitor longer term ash damage if any.
Lastly eliminate all the expensive quangos and committees involved in the descision making process on the basis that committees can never agree about anything (No one ever built a statue to a committee
and have one body to make descisions
Thats it NO more NO less
Pace