What happens when an airplane's pilots have to radio ATC to request an emergency landing, and the planes' sensors have failed so it can't safety land itself?
If the plane is able to communicate with ATC it is able to broadcast to the other planes that it is on an emergency landing, so this is not an issue. Even if it has lost all communication this is still not an issue because all the sensors of the other planes can see it. So if the disabled plane immediately goes for an emergency landing, all other planes in the area are able to see its position and that it is not responding to pings, and therefore set safe courses that avoid it. This really isn't a very difficult problem.
