Volcanic ash can pose a severe threat to aircraft, and although passengers will definitely know when they're inside an ash cloud, they may not know how dangerous it can be.
One pilot on Reddit was inadvertently directed into an ash cloud spewed by Mount Etna and remained in it for 20 minutes before making an emergency descent.
Volcanic ash is composed of rock and glass particles, and while the jet engines are hot enough to melt the glass, it can resolidify once inside, eroding the blades of the compressor, blocking air flow, and even stalling the engine.
The electrostatic charge carried by volcanic ash can also cause an electrical failure, which is an immediate hazard.
You can read more about how volcanic ash can affect aircraft on Wired.