Couple of things not yet discussed much here but especially relevant if any testing involving military (eg. Nimrod or fighter aircraft) is proposed.
What difference in ash vulnerability between various turbine types (straight-through, low-bypass, high-bypass)? Point being that the front fan MIGHT act as a centrifugal separator to get dust particles away from the core of the engine and outwards into the bypass area where it would do nothing. Anyone know if this happens in practice, with dust about the consistency of talcum powder?
Most of the discussion here (and news) has focused on turbine damage on the outside of the turbine / stator blades. What is the actual risk of ash getting into the cooling passages inside the blades when there is insufficient ash ppm to cause stall or failure for other reasons? In particular, is the position of the cooling plenum inlet(s) bad or good considering likely paths of heavier-than-air ash particles through the compressor stages? Clearly (?), any cooling air blockage is more likely to cause catastrophic blade failure than deposits of volcanic glass on the outside surface of the blades. Or not
. I obviously don't know enough about turbine designs.
And another thing. How long before the volcano vapourises all the ice / water present in the caldera? Once this happens, apparently the character of the eruption will change and the average ash particle size increase a lot. When THAT happens, the ash-cloud will (obviously?) sink to ground / sea much faster and therefore pose less / no threat south of Iceland, whatever the wind conditions.