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

A plane’s travel is affected by gravity and the layers of air that they’re traveling through. The pilot maintains a consistent altitude and therefore doesn’t just fly off. At some point it wouldn’t be able to fly higher due to lack of air.

On the other hand, if that plane was flying level and had a very powerful laser pointing straight out of its nose that laser beam would travel off into space.