I see some ask whether or not overflight of the area can be carried out above FL350. However, ask yourselves, especially for twin engined aeroplanes, what would happen when you have an engine failure right over the ash area?
bfisk, yes I agree and then there are emergency descents to consider but I suppose it's a question of risk assessment - what about 4 engines a/c with higher stabilisation altitudes in the even of an engine failure?