LOD_RADIUS under [TERRAIN] determines the size of the coverage area to be refreshed around the aircraft.   It has a default value of 4.5.

[TERRAIN]
LOD_RADIUS=4.500000

However, a number of other aspects are also influenced by this value in addition to refreshing the coverage:

• visual quality   –   higher value = wider & clearer the coverage
• frame-rate   –   higher value = lower frame-rate
• stutter   –   higher value = more stutters
• memory usage   –   higher value = more memory consumed  (higher risk to OOM crash)
• session load-up time   –   higher value = longer session load-up time

Nowadays, using a value higher than the default one is recommended as most video cards are equipped with 512MB or more memory.

Common values used range from 6 to 9.    The best value for one’s system can only be achieved by trial and error; and different scenery set-up may encounter diverse results.   For example, LOD_RADIUS in my various flying zones ranges from 5.5 to 8.

NOTE

1. Despite this setting, certain autogen objects still won’t appear until certain distance is reached or will disappear when the aircraft is away from the viewable range.
2. Any changes made through SETTINGS inside FSX will revert the value of LOD_RADIUS in fsx.CFG to its default value (4.5).    It needs to be changed again manually if required.