Being in an airplane at 35,000 is not high enough nor do you have a big enough field of view to detect the curvature of the earth or detect a sense of height above the horizon. We are dealing with something quite massive, the earth is 25,000 miles around, so if your puny human eyes are going to detect height above or curvature of the horizon your gonna have to be at least 60,000 ft up. This has been documented in interviews with U2 and SR-71 pilots and even passengers on the Concorde.