Can anyone explain (Blurry 2)

what actually happens behind the scenes when the tiles in front of me switch from the "less detailed LOD" into the "more detailed LOD"?Let me explain: according to Phil, there is a ring of detail tiles that get rendered, and move with the aircraft. If I understand it correctly, LOD=4.5 means that 4.5 tiles are displayed at the highest level of detail. I can see this area ahead of me when I fly, and when things are good, it keeps up nicely. When I go into urban areas, however, this ring seems to contract.. ie the tiles that should be switching about 1/3 up the screen, stay at the lower LOD and move toward my airplane until I can no longer see any high detail tiles.For the tiles to switch to the higher level of detail, does a new tile have to be loaded from disk and sent to the video card (a), or does a tile in memory have to be recomputed and sent to the video card (:(, or does a texture tile in video memory need to be converted to a higher LOD by the video card alone ©?Reason I ask, is that this happens with fps nicely running at 20 fps, no disk activity, and the sim running smoothly. Where in FSX does the process get held up?P4 2.8, 1.5 GB PC3200 memory, 256 MB 7600GT video card.