Whether at sea-level, the top of Mount Everest, or flying over a hundred thousand feet in the air, the always horizontal horizon line always rises up to meet the eye-level of the observer and remains perfectly flat.
You can test for yourself on a beach or hilltop, in a large field or desert, aboard a hot-air balloon or helicopter; you will see the panoramic horizon ascend with you and remain completely level all around.
If the Earth were actually a big ball, however, the horizon should sink as you ascend, not rise to your eye-level, and it would dip at each end of your periphery, not remain flat all around.
Standing in a rising balloon, you would have to look downwards to the horizon; the highest point of the ball-Earth would be directly beneath you and declining on each side.
Check out this excellent amateur balloon footage of our flat, motionless Earth! You can clearly see the Sun is NOT 93 million miles away as we're told. This is evidenced by the hot-spot seen on the clouds directly underneath the Sun as it moves over the Earth. Over 20 miles high and the horizon remains perfectly flat and rises to the eye-level of the observer all the way up. If the Earth were a ball, no matter how big, the horizon could not rise with the observer like this. On a ball-Earth the horizon would stay where it was and you would have to look DOWN to the horizon further and further the higher you rose.