Data required
As I see it the data required for a decision is:
1) The concentration of ash in the air at different locations and altitudes.
2) The allowable concentration for safe flight.
The first can be gathered by data collecting flights which appears to be being done.
For the second, is it practicable to run a test bed engine (on the ground) and feed known concentrations of ash through it to determine this limit?
What do you think ?