By default, FlightGear will not use anti-aliasing during the simulation. This results in incorrect rendering of sharp straight lines and very small pbjects. In order to correct this, you can apply the anti-aliasing function.
Example
Left: Anti-aliasing disabled
Right: Anti-aliasing enabled
How to set anti-aliasing
Command line
When starting FlightGear using the command line, add the following two commands:
--prop:/sim/rendering/multi-sample-buffers=true
--prop:/sim/rendering/multi-samples=4
FGrun
When you use FGrun, add these commands under Advanced... > Properties.
Options
If preferred, you can choose another value for the number of samples. The number has to be greater than 1. The greater the number of samples, the greater will be a possible drop in framerate (frames per second).
Credits: Thanks to fredb for publishing this information in the FlightGear forum.