[DrEvil]

Nav meshes are 3d. They can have overlapping surfaces and all that. They are basically an optimized copy of the world geometry minus the non walkable surfaces. Personally I would treat flying differently anyways. Flyers have a vast empty space to navigate, they don't really need to use A*. Ground vehicles and infantry are restricted to walkable surfaces for the most part, and a nav mesh is perfect to represent that. The air above the ground is typically empty enough where you don't need to perform searches on it.