I was googling and found an interesting discussion. Here is a quote:

"This is a subject for some debate.

The true curvature of the earth would be the actual edge of the disk, a great circle of exactly the same diameter as the earth. What you can see from any airplane altitude is a horizon circling you at a few hundred miles away. A small circle.

From 35,000' the horizon is about 230 nautical miles away. So 230nm is the radius of a small circle of the earth's surface that you can see. To see the actual curvature, which is vastly greater than this circle you would have to be able to see the entire earth.

Actual earth-disk circumference is 21,600 nautical miles.

Earth's surface disk visible from FL350 is about 1446 nautical miles.

To claim you can see "the curvature of the earth" from low altitude, like a hundred thousand feet or so, is like putting your eye a couple of thousandths of an inch from a large beachball and believing that you can see its curvature. The math is about the same.

What looks curved is that the horizon is about 230nm away at your twelve o'clock but it is also 230 nm away at your eleven, and your one, and your ten, and your two, and your nine, and your three, and so on. If you can see all the way around (as in, from the basket of a balloon) the effect sort of goes away and you again get the sense that you are just hanging above a big (possibly curved) surface. The line of the horizon curves all the way around you, so hanging above a circle it will look curved.

Again, to see the true curvature, you'd have to be able to see the entire near side of the earth."