AC3D and X-Plane
AC3D is widely used to create screnery and aircraft for the X-Plane flight simulator system.
You will need to download and install the X-Plane/AC3D plugin.
As well as modeling for X-Plane, the X-Plane plugin adds animation capability for X-Plane models - see a tutorial.
Discuss and find more info on X-Plane and AC3D on the forum.