To put things into some perspective on the risk of flying into ash here is a presentation Snecma (CFM) made following the European Ash clouds.
It looks at past events and associated aircraft flying into ash clouds.
http://en.keilir.net/static/files/co...cfm-snecma.pdf
To summarize.
There was found to be NO immediate operational effect at distances greater than 1000km from the Volcano.
The time after eruption is the determining factor as the larger damaging particles fall to the earth or disperse.
No Damage has occurred with clouds older the 35 hours. Most engine damage occurs within a few hours of en eruption.
- CFM recommends avoiding flight through visible ash clouds
- Positive industry experience when operators avoid visible volcanic ash, defined as 2mg/m3.
- Flight into predicted ash concentrations of higher than 2 mg/m3 may be undertaken at operators’ discretion, provided flight into visible ash clouds is avoided
- Follow current AMM maintenance requirements when operating in areas of potential volcanic ash encounters .Report findings to be provided to CFM
Considering the this ash cloud was much greater than 48 hours old and last weeks cloud was then weeks old. And the fact that the cloud wasn't even really visible. I think AirNZ,
VB and all the others where quite justified to continue flying and probably could have flown and normal flight levels.