Is ash worse than Harmattan?
I have flown in Nigeria for many years, and for 6 months of the year, up to about FL100 there is constant Harmattan dust in the air. Sometimes so thick that the visibility can be reduced to 400 M. Yet everyone keeps flying in this dust/sand from the Sahara. Yes, over time it erodes the turbine blades and things, but engines do not stop.
I understand that the BA Aircraft that had all engines stop flew through the thick plume created by the volcano near Indonesia.
So why is invisible ash deemed to be so dangerous?