All of the engine manufacturers are advising not to fly, this extract is from the Pratt & Whitney web site:
Volcanic ash can damage aircraft and engines in several ways. P&WC encourages operators to refer to their airframe OEM’s guidance on a potential volcanic ash encounter for additional information.
While P&WC acknowledges that the Local Regulatory Authority has the final determination of whether flight operation is to be conducted, we want to inform you, our customers, of potential hazards.
P&WC does not recommend operation in conditions where volcanic ash is present. Let us explain why.
Volcanic ash may clog air filters of turbine engines, block cooling air passages, erode the gas path components, and erode the protective paint on casings. Volcanic ash entering the engine can also melt in the combustor and then re-solidify on the static turbine vanes, potentially choking the turbine airflow and leading to surging and an in-flight shut-down. It is also noted that there is a high level of acidity associated with volcanic ash, and this may also lead to deterioration of engine components.
Should you experience an encounter with volcanic ash, P&WC advises you follow the recommendations contained within the engine maintenance manual.
Peter Boyd
Chief Engineer, Customer Support