Bigpants:
But what makes me most sceptical is personal experience. When flying the GR1 over Iraq c 1993/4 the vis around Dharan was poor and full of desert blown dust.
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So is desert blown sand and dust that different from volcanic ash?
Maybe, but at the high temps found in the back end of a gas turbine both form a glassy material that blocks cooling ducts in blades. Not convinced its ok to fly in desert dust and a few km vis but not ok to enter dilute levels of volcanic that are not visible to pilots.
seems you may have missed some important parts of the thread form the beginning. Just in two short sentences:
(1) dessert sand is COMPLETELY different than volcanic ash: material, size, particle shape, melting temp
(2) sand does not usually melt since it has higher melting temp then the engines, volcanic ash unfortunately is melting in engine temps
details are available via search button in this very thread