One of our moderators is a pilot and engineer. He has supplied us with his pilots license as well. He has testified that he has never witnessed any evidence that showed the Earth to be round.
I suggest you go on a popular travel flight - start to play with different locations for your destination and departure. I think you'll be surprised with what you find. You may be up all night trying to prove it wrong.
If this is an answer to my post, I see that you don't catch what is on my mind.
I wanted to show, that it is impossible to see anything (like an arch inclined vertically) except seeing straight line.
If you expect to see a vertically inclined arch as the globe horizon, I see the best explanation as just that: imagine. You must imagine, since in reality you will never see anything like that.
So, imagine that your dreams come true and you finally see the horizon as vertically inclined arch.
Now, take this arch/horizon into a picture frame. Next, turn yourself with the picture 90 degrees right or left: what would you expect to see? Another vertically inclined arch of horizon? What would you expect to see between those two pictures? Another arch? How they should be connected to create one long picture?