so your general theory is that if a plane is sitting on the ground facing you at 500 yards you could sight it in but if it was 15 to 50 feet higher up somehow it would be to difficult for a gunner to see?
and of course the commander couldn't "Kentucky windage" a shot from his perch outside by just telling the gunner to fire the darn round at 180 degrees plus 10 degrees elevation?
and the gunner probably wouldn't be able to see the plane thru the sight because of this big blue thing blocking the view of the plane,, it looks like a large plane,,, as soon as it passes I'll try to find "your" plane in the viewfinder
I have created a top secret picture to help you understand

now explain to me why a round coming out of that tank, at that moment, wouldn't hit that plane?
also from the picture we can see why the gunner wouldn't be able to see the plane,,, because the plane was in the way!!!
luckily, this top secret prototype tank has 3000 HP and does 0 to light speed even in reverse
you like my new photo shop ? Send me a self addressed stamped envelope with three dollars U.S. for my secret to great pictures,, yes you too can create cartoon events that never really happened using the latest gadgets and stuff!!!