Pictures have surfaced of turbine blades from Finnish air force F-18s which were in the air when the ash drifted over Finland. They're damaged, but not dangerously so. BUT - these were F404 military turbofans, with a bypass ratio of ca 0.35:1, meaning most of the air (& ash) going into them went through the core. A civil turbofan has a much higher bypass ratio, typically over 5:1, & most of the air - and even more the ash (I'm informed, I think reliably, that the ash, particularly the heavier particles, will tend to go into the bypass air rather than through the core) will therefore not go into the core. This is important: the main source of damage to jet engines from volcanic ash is it melting in the hot core, & being deposited on critical parts such as turbine blades. Ash in bypass air is relatively harmless.
My informant, a former Rolls-Royce engineer (proper one: Masters degree in it 'n all), thinks that the grounding is an overreaction. He doesn't believe the ash cloud is dense enough to be dangerous. Mildly damaging, possibly causing long-term costs to airliners in engines with degraded efficiency unless they have expensive rebuilds, but not dangerous, not like the dense ash clouds which have shut down airliner engines in the past. He thinks it's 'Elf 'n Safety gorn mad . . . (not his choice of words).
