Playing Devils advocate
Now that airlines are allowed to fly through some of the ash clouds providing the engine manufacturer has certified their engines are up to the task, where will the liability lie if there is (perish the thought) an incident?
The engine manufacturer for stating their engines can cope in the ash?
The airline for taking the engine manufacturers word?
The regulator for accepting the idea?
Hmmm the lawyers must be rubbing their hands in glee and anticipation!