Pete, tried to reply to your last post, but the thread has been locked.

As usual you are confusing level and straight, they are two different things.
Straight is the line that follows the shortest path between two points on a flat surface.
Level is a line equidistant to the centre of gravity.

So on a spherical world, flying level maintains altitude, flying straight will increase altitude if the plane started off flying level..

On a disk World, flying straight and parallel maintains altitude but flying level decreases it.

A disk world with a flat plain, will have a low point at it's centre and rise uphill towards the perimeter, so unless it's spinning fast enough to maintain the disc shape, it will eventually collapse into a ball, under the effects of weather and gravity.