First if all you can see curvature from 30,000 feet, secondly how do you know you can only see it from 60,000 feet if you say it isn't there. Also how do you explain gravity if the earth isn't round, but there is weaker gravity on the top of mountains and lower air pressure the higher up you go. If it was just the flat earth accelerating at 9.81 ms^-2 then the pressure would be the same higher up