Once again, you are not taking into account relativity. From your explanation, the plane will fly because it is moving forward relative to a fixed point in space. By that method, the treadmill is irrelevant to the question. It simply becomes will the plane fly once it reaches a certain speed as observed form a fixed point in space. Of course it will. Now, if observed from the treadmill, which is where I take it the speed is measured from, then the plane is sitting still in space. The plane is maintain its speed relative to the belt, NOT to the surrounding air and space. If it accelerates, then it has moved beyond the speed of the belt beneath it and thus moved forward in space.