Eli5: When a super fast plane like blackbird is going in a straight line why isn’t it constantly gaining altitude as the earth slopes away from it?

Question

In a debate with someone who thinks the earth could be flat, not smart enough to despute a point they are making plz help.

In: 1372

Answer 1

Aircraft in level flight fly parallel to the ground. Because the ground curves, the plane’s path curves. They do this with a device called an artificial horizon that uses a little gyroscope that stays level, while gravity makes the bottom of it point down at the middle of the earth. (Or just by looking at the real horizon if they can see it.)