Andrew, set 'Masks' to NO in FSET.
I'm not too sure why you are using masks here, Aerofly reads the imagery files without masks.
The only changes that you should be making with the image tiles would be to add alpha to make those areas transparent, mainly for water areas that there is no images or for blending into the Aerofly base scenery.
Please follow the tutorial on the wiki for this as it explains it clearly.
Thanks. I have been having the same issues others have with the scenery tiles I made popping up what seems to be way late. Some of them I am right on them before they pop in. There was discussion here that using masks seemed to load larger areas, at a greater distance. Not pretending to know how it all works, LOL, just trying it to see if it makes a difference.