I hate to suggest that your dad might be a danger to his passengers and other innocent people on the ground, but do you think that might be the case? Personally, I don't know if I would want to fly with someone like that.
In my experience, flat-earthers can deny what their eyes are telling them, and what their brains ought to be telling them, all to hold onto that whacky belief of theirs.
In their obstinacy, they could conceivably fly their plane into the ground rather than admit what they are seeing is the curvature of the earth up ahead.
I didn’t mention that my Dad is a theological scholar...Bob Jones University, SC in the latter 1950’s.
....same place (and period) where Fred Phelps went to seminary (but never graduated).
The place was co-ed....but people would get in trouble for romantically holding hands. Dad told me how guys and gals would ‘pet’ between the rows of shelves in the library.
Weird culture...