There is no curvature to the earth, certainly not visible from any airplane. All I ever see is a flat expanse stretching out to the limits of my vision, often around 200 miles. The funny thing, at that distance the earth is supposed to have curved around 3 miles downwards, something that should be immediately and strikingly apparent. Seeing as I, nor anyone else, has ever seen curvature of this nature... I can pretty confidently say the earth is flat.
The far distance view is proof on its own. You are not able to look the same distance from the ground, because was lies beyond your vision is beneath the horizon. As soon as you start to ascend, you are able to see much further, because you can see 'over' what was previously your horizon. You can't look around corners either, can you?
Edit: Those 3 miles is seen over a distance of 200 miles. That means for every mile you look, the earth is curved by only 3/200 = 0.015 miles, which is the equivalent of 24 meters or about 79 feet. This is a rather small curvature to see clearly. Not impossible, but if clouds cover your view, it becomes a lot harder. Hence we ("Round Earthers") say, in general you will not be able to see it from an airplane, but only from higher altitudes.
But as you can see on the pictures I posted before in this topic, with almost clear blue skies, you are able to see and capture the curvature. And the second photo, from the same person, flight, etc, shows you a straight horizon. Which would be impossible if there were any distortion on the window or camera.