If the Earth were truly a sphere 25,000 miles in circumference, airplane pilots would have to constantly correct their altitudes downwards so as to not fly straight off into outer space.
This is a mistake, it is not a proof the earth is flat. Airline pilots have an altimeter which they use to keep the plane at a fixed altitude [distance from the earth] and also they aim at the horizon. So if the earth is a globe and you keep your airplane at a fixed distance from the earth then automatically you are correcting your course to remain at a fixed distance from the earth, in other words you are going around the curve of the earth. Also by aiming at the horizon that will cause you to constantly follow the curve of the earth.
So airline pilots already have two perfectly good systems in place that ensure they will follow the curve of the earth if it is a globe and these systems will work equally well to enable them to stay a fixed distance from a flat plane if the earth is flat.
The conclusion is airline pilots do adjust for the curve, if it exists, so this does not prove or disprove the flat earth or the globe earth.