To amplify another posting re. different engines and different air intake volumes....
A massive high bypass engine needs much more air than a small low bypass one.
...this disregards the fact that the higher the bypass ratio, the more air (with entrained ash particles) will go ROUND rather than through the hot section. (OK - there is some risk of compressor blade damage but compared with destruction of turbine blades due to loss of cooling air, this has to be less- or insignificant.
There's also the question of what percentage of ash particles will get 'centrifuged' into the fan duct and therefore not cause any problems in the core of the engine. Clearly, the smaller the particle, the less energy it will gain as it goes through the fan and therefore the lower the probability it will make it into the duct (assuming it started its journey near the centre of the fan). Particles already towards the circumference will definitely go straight into the duct.
.. this is all stuff for the engine builders to measure and calculate when deciding how much ash of what average particle size a particular engine will tolerate. Presumably a somewhat new experience for them now that 'zero ash' has been declared an unachievable aspiration in European airspace.
And potentially at least, REALLY fine ash will pass through the cooling plenum, through the blade passages and out the cooling holes without causing a blockage???
Tricky stuff!
The average ash particle size is also determined by the age of the plume and, crucially, by how much water / ice is left in the caldera of the volcano. If a lot of water gets in amongst the outflowing lava and flashes to steam, this produces a semi-explosive eruption (the special name was mentioned earlier but I can't relocate it) and much finer ash. Now the eruption has been going for a while, presumably the ash particles are bigger on average and therefore will fall out of the plume faster?.... Someone who knows about volcanoes, please put me right on this.....