Create simple collision detection in an existing WPF C# project

I have a simple flight simulator with a 3D airplane model flying around. What I need is collision detection to be implemented so that the airplane does not "fall" through the terrain or goes through the skybox. Basically, the airplane must be able to stay within the visible scene. It should be able to stay on top of the terrain when landing and stop at the edge of the skybox instead of flying through it. Please see the attached video.