Volcanic ash is not just “smoke” you can create by burning a tree. The ash contains dozens of different particles of heavy metals (no pun intended with the music) and pieces of pulverized rock and possibly glass.

This smoke’s glass particles, tend to melt at a temperature that can be found inside Jet Engines (think a Jet operates around 1400-1500ºc) and these particles can melt sometimes below 1000ºC. When the engine sucks the ash and particles, these can melt in the blade and other parts of the engine, eventually causing the engine flameout due to different reasons; it can erode parts of the engine which would jam it due to being out-of-balance, or it can simply clog it because it blocks the cooling channels in the blades, effectively making it fail.

Outside of the engine, there are other problems, like Pitot Tubes being completely blocked by the ash, which is not something funny.

It’s important to know that you don’t necessarily have to fly through the cloud to suffer its effects. The air in the vicinity of the cloud can and will contain particles too, to a lesser extent of course. It would be safer to simply avoid flying until the cloud is less concentrated and hopefully taken away by the wind.

It’s often recommended that, in case of “unexpected” volcanic ash in the atmosphere, descend to or below 10.000ft and get out of the cloud as fast as possible. At least that’ what the Brits did in 1992 in Galuggung, Indonesia.

You can find more information about this (and a better english) in the beloved Wikipedia page for Volcanic Ash, which I just read and has some graphics and it points some more accurate numbers.

In the specific case of Europe right now, the amount of ash is immense and until the wind changes, it’s simply impossible and unsafe to fly. At the time of writing, the cloud is moving towards the north pole, and some airports are operating again, although with limited routes.

P.S.: as a general recommendation, I’d say that it’s never safe to fly through any kind of smoke, you never know what’s burning inside, and engines usually prefer air ;)