The clash between safety and airline profits has been resolved in favour of profits. The standard of toleration of airborne dust has been raised. The Guardian reports that airlines have been resisting the setting of a safe level until now.
There is one slight problem, in that the ash is not uniformly distributed in the atmosphere. It is stratified - arranged in layers. So while averaged concentrations of ash may be below the newly set safety limit, there may exist layers of more dense ash. A plane flying in one of these layers may experience difficulty. Even at low levels, the impact of particles on turbine blades will cause micro-pitting, which will reduce the aerodynamic and therefore fuel efficiency. Turbine maintenance intervals should be reduced, allowing more frequent inspections.
Ideally, planes should be fitted with a small radar or laser sensor that can estimate the ash concentration in the air ahead, to that the pilot can alter altitude if she finds she is in an ash-dense strata.
Let's all hope that the drive for airline profitability does not cause a crash.