GarageYears,
The NASA DC-8 incident has been cited in this thread several times.
Allow me to quote from the official report, which can be downloaded
here
Top of page 11:
The flight crew noted no change in cockpit readings, no St. Elmo’s fire, no odor or smoke, and no
change in engine instruments. They did notice that no stars were visible, but this is typical of flight
through high cirrus clouds. After seven minutes the crew noticed that the stars had reappeared, and at about this time the
scientists reported that the research instrument readings had returned to normal. There was still no change
in engine or airplane instrument readings.
So, this was a flight at night through an ash cloud that was so thick that it was able to obscure the stars. Conclusion: this ash cloud would have been easily distinguishable by daylight VMC.
The research instrument readings returned to normal as soon as the stars had reappeared. This would suggest to me that no visible ashes, also means that no significant ash concentrations are likely to be present or even measurable.
This incident further shows that even flying through an ash cloud so thick that it can obscure the stars did not cause anything catastrophic to happen to the flight. (apart from the bill afterwards)
So allow me to use to use this incident to reinforce my ideas that keeping a flight clear of any visible ash concentration will keep the flight safe.
Therefore closing the entire airspace at is was done was a terrible overreaction.
Best regards,
Sabenaboy