The aircraft manufacturers and engine manufacturers.
Thus far, all report none to absolute minimal potential or real damage when flying in conditions as they exist over Europe. Over 1,000 flights have been conducted with Boeing aircraft alone, with no passengers, and they all come back no damage.
Alas, we are not talking about the capability of the engines but instead the ability of a cloud of volcanic ash to damage them.
For all anyone knows the Germans actually took their aircraft AROUND the clouds and not through them. The Belgians probably imagined they were giant sausages in the sky and barged on through the clouds.