Challenge? YOU said that ships don't go over the horizon. I proved they do. Challenge won already! Or rather, your claim was rebutted, since the 'challenge' has been won already for the past 2000 years that we've known it's a globe and the thousands of observations supporting that. Facts are not a 'challenge'. There are claims and facts. Flat earthers have claims, and globers have facts that rebut those claims.
What would it matter how far away they are? It's OVER the horizon and can't be brought back with a telescope. You DID say that ships don't go over the horizon, right? Oh, but they do. Even with a magical p900. So, challenge already won, or rather, your claim was rebutted, since there is no 'challenge' at all and it's absolutely 100% provable that ships do go over the horizon. Plus, how would one provide the exact distance to a random boat? Moving the goalposts is not a honest debate technique. Here's a children's video on this particular logical fallacy.
However, I CAN provide this with measurements that I do know though. This plane does does go down below the horizon.. like you say nothing can do. And since it's coming towards us, not away, you can't claim that it's disappearing due to perspective, or that you can bring it back with a telescope since it's not too far away to be seen before it disappears.
Here are the specs from observer to the airport. The planes are landing at the airport.
RE: Question : At what height do we see curvature