Based on my own experience. I was trying to understand how TheEngineer could fail to see the curvature on his own flights, then I remembered back to my last flight (about six years ago), and realized that, while the curvature was fairly distinguished, it was slight enough that to a biased observer it could easily seem straight. The human eye/brain combination is not very good at measuring angles. It's one thing to see it in a photograph, where the pixels don't move and you can study it from a single viewpoint, but seeing it in real life, where the eye can more easily play tricks on you and there is movement involved, you could easily believe it was flat, at that height. A chartered fighter is another story; there'd be no backing out of it at that height.