If you use scenery add-ons in Flight Simulator X, especially airports built for prior versions, you may notice strange behavior with runways and other large, polygonal surfaces.
Because Flight Simulator X now models a round earth, and previous versions did not, corrections need to be applied to polygons above a certain size to ensure that they conform properly to the surface. The most common examples are runways - the ends of a long and uncorrected runway polygon will float above the surface and airplanes will appear to sink through as they approach the end points. Unfortunately, on polygons above a certain length, somewhere around 100 meters, forcing the correction causes what's known as z-fighting - a flashing effect that occurs when two polygons attempt to occupy the same space.
These issues only apply to legacy add-ons built for prior versions of Flight Simulator - content built using the Flight Simulator X SDK does not exhibit this behavior.
Our upcoming Service Pack for Flight Simulator X will resolve these issues by forcing the correction for polygons less than 100 meters, but content that requires longer polygons will need to be updated using the FSX SDK.