I am sure it was proved on this forum that the at the standard flight that planes fly, if the world is round, the curvature is insignificant anyway - so either way, from a plane, there is no noticeable curvature. Do we really need to work out all those calculations again? Go to google earth - I know that is popular program amongst REers and see what height you need to be above the google Earth to see that the Earth is round. Now imagine an Earth a few millions times larger and think about how far away you need to be to see that the Earth is round (assuming for the sake of the argument that it is round, which it isn't). I'm sure from this simple and easy experiment you will be able to see that you actually can't tell if the Earth is round or flat from a plane. And even if you could see a curvature, there is nothing to say that the Earth is perfectly flat - it could well have a slight curve to its shape - like a contact lens instead of a lens cap.