Would it be possible to align the shadow with the ground normal to prevent wing clipping? Is that something that should be done XML model side?
It is trivial to tilt the shadow with the ground normal (that's what I mean when I use the words 'co-planar with the triangle') - the tree shadows exemplify that. What's not trivial is getting the normal in the first place, because you don't need it underneath the plane, you need it at the position of the shadow - but that in turn depends on terrain elevation at that position.
So it's a ray intersection problem between aircraft and terrain along the light to find the position at which you want the normal - and they're not cheap to do. In any case, yes, this needs to be done model-side, because the shader doesn't have the information to do the ray intersection problem because it only knows the local mesh (current vertex or pixel).