And why would you call that a stupid question? When flying a plane, your eyes will be on it and no place else. It would be very difficult to determine how high a plane is even with telemetry. To look at your radio, you still have to take your eyes off the plane which, and I'm guilty of this one, means it will take you a few seconds to find and re-orient yourself to the plane. I don't know if this is required or not but you would almost have to have a spotter that could be your "altimeter", as well as a second set of eyes to watch for potential problems. Even with a spotter, I crashed a boat due to that one or two seconds of not having my eyes on it.