Volcanic ash is particularly bad because it is so abrasive, having been freshly formed without any opportunity for erosion to smooth it down like regular dust.
That's not the only problem - volcanic ash also has a low enough melting point that it'll melt in the combustion chamber of a jet engine and leave glassy deposits on cooler components.
That's not the only problem - volcanic ash also has a low enough melting point that it'll melt in the combustion chamber of a jet engine and leave glassy deposits on cooler components.