Yeah thats all well and good, until they have a engine our or a de-pressurisation then have to decend into the ash concentration !!!
and over what level of concentration would it be problem ? you can give it in ppm or gm/3 i dont mind...
so an engine out automatically means an aircraft cant maintain altitude?
better get on and revise those ETOPS regs !!
and an emergency descent isnt going to be a push the nose over and descend at flight thrust.. VmoMmo will be behind you.
yes in an explosive decompression (which are rare events) the objective is to get to a lower altitude, but you cant do that if you are over mountains.. the ground tends to get in the way.
also, we are not talking about prolonged flight over ash concentration of whatever level, and if an explosive deco happens at altitude you dont have long to get on supplimental O2 before its game over anyway.
if on a twin jet you loose both engines going over an ash concentration it's all rather moot as you are in a very expensive and heavy glider anyway.