To go back to the OP's original question, I don't see why you couldn't fly a glo, or gas powered model airplane @ 12,000 feet IF and that's IF it were launched (normal take off) from an 11,000 foot up mountain runway. If correctly needled before take off the mixture change at another 1,000 feet up would be no different than wherever altitude you fly from and then going up another 1,000 feet. Yes, the engine output would be less than at a lower altitude but if the model were well powered why not? The guys from Colorado should have a little first hand knowledge on what the limits would be. As we all know the thinner air would reduce the wing's lift and the engine power but heck a lot of the Rocky Mountains are taller than 12,000 feet and people fly RC out there!