In the history of the world a plane has most likely never been shot down by a tanks main gun. If a plane was even shot down by a tank it was an accidents and never reported or the Russians lied and said they did.
I am retired Army with one of my MOS's being 19D30 ( Cavalry Scout) We were cross trained in the M-48A5 and the M-60A3 tanks as drivers, loaders and gunners. Trust me when I say you can not track an aircraft with the main gun optics much less shoot at one.
Cool! Someone who's actually done this stuff. I'm not asking this sarcastically, but seriously.
From your tank, forget about a plane circling. Assume something the size of a B24 or C47 that's making straight passes on you, back and forth straight over top of you, or even passing say, 20 yards to the side at 200 feet alt. You know he's there, you know he's coming, he's not maneuvering. You're also sitting on a mountain of ammunition that your life doesn't depend on. For all intents and purposes, as he's approaching, until he gets right on top of you, he's stationary.
Why couldn't you hit that? I don't doubt it didn't happen in the real world because it's such a low percentage shot and they'd need those rounds for more important things. However, why is it not possible, given unlimited ammo and a relatively stationary target?
Wiley.