Seedo, I'm not trying to make a joke out of this because it's really pretty serious subject. But if your plane flies out of range it's going to crash. It's just that simple. How far it continues to fly before that crash depends on a lot of factors. It's possible that the plane could travel many miles before it crashes, or at other times it will just spiral in quickly and crash. The easiest way to tell that you plane is out of range is that it will not longer respond to the control inputs you give it.

Ken