Eli5: why is volcanic ash dangerous to aircraft, and yet they can fly through forest fire smoke?

Question

Thank you!

In: 32

Answer 1

Smoke particles are mostly small enough and light enough to stay airborne. Volcanic “ash”, by contrast, is basically lumps of stone and glass of assorted sizes – hard, heavy and sometimes rather large (over 2 inches – so-called “volcanic bombs”). It’s only up in the air at all because it’s been blasted up there by the force of the eruption. And there’s a LOT of it – the ash that fell when Vesuvius erupted, for example, buried the coastal town of Heculaneum near Pompeii under a layer of rock **80ft** deep.

Aircraft engines don’t tend to work well with tonnes and tonnes of stone dust blasting through them. Few things do. As for the possibility of the airframe colliding at high speed with some of the larger lumps…