Because the particles can still damage the plane engines, that's why.
As the ash is like sand blasting media, it also scours the surfaces, including the windscreens. An attractive frosted look on the windows isn't such fun when you're trying to see the runway :-)
As the ash is like sand blasting media, it also scours the surfaces, including the windscreens. An attractive frosted look on the windows isn't such fun when you're trying to see the runway :-)