Yeah, at about 40,000 you can start to see curvature. I’ve been at about 55,000, but I have friends that fly the U2 at 80,000 plus... I know some flat earthers have said that is an optical illusion.... but all of our navigation calculations really wouldn’t make sense if the earth were flat. True north / magnetic north / grid north conversions, navaid radial intercepts, etc... I’ve flown around the world trips, literally. For me personally I don’t believe there is any way this is true. There is overwhelming evidence about the earth being round, and really only vague accusations on the flat earth side. Just because it is becoming apparent that so many things in our world have been lies, it doesn’t mean that every single concept is true.