By default, FlightGear will not use during the simulation. This results in incorrect rendering of sharp straight lines and very small objects. In order to correct this, you can apply the anti-aliasing function; however, this will slow down rendering and is not guaranteed to work on all graphics cards.
How to set anti-aliasing
When starting FlightGear via the command line, add the following two arguments:
When you use FGRun, add these commands under Advanced > Properties.
If you are using the Integrated Qt5 Launcher, then you can easily enable anti-aliasing by toggling the "Enable Multi-sample anti-aliasing" option from the "Settings" tab.
Anti-aliasing does not work with Rembrandt enabled due to GLSL limitations, but some graphics cards have hardware anti-aliasing which may work.