Shadow problem

I've been working on a couple of edits today, and I keep running into a problem with image shadowing.
What causes this?
It is most prominent around the wheels, but can, and usually does, envelope the entire outline of the plane. :-/
Example of the shadow problem around the wheels:

What causes this?
It is most prominent around the wheels, but can, and usually does, envelope the entire outline of the plane. :-/
Example of the shadow problem around the wheels:
