Photo-Based Scenery Add-On Update
Certain highly-detailed scenery add-ons that make use of large satellite images may not display correctly in Flight Simulator X. Specifically, you?ll notice large rectangular areas that are black or dark blue, where the proper ground texture does not draw at all. Microsoft has confirmed this issue to be a bug in the Flight Simulator X terrain display engine. To resolve this issue, download and install the Flight Simulator X update for your language, linked below.
English version
Just replace original FSX terrain.dll with this updated, also original terrain.dll downloaded from Microsoft site