From the official blog:
The idea for Flight Gear was born out of a dissatisfaction with current commercial PC flight simulators. A big problem with these simulators is their proprietariness and lack of extensibility. There are so many people across the world with great ideas for enhancing the currently available simulators who have the ability to write code, and who have a desire to learn and contribute. Many people involved in education and research could use a spiffy flight simulator framework on which to build their own projects; however, commercial simulators do not lend themselves to modification and enhancement. The Flight Gear project is striving to fill these gaps.
There is a wide range of people interested and participating in this project. This is truly a global effort with contributors from just about every continent. Interests range from building a realistic home simulator out old airplane parts, to university research and instructional use, to simply having a viable alternative to commercial PC simulators.
Flight Gear and its source code have intentionally been kept open, available, and free. In doing so, we are able to take advantage of the efforts of tremendously talented people from around the world. Contrast this with the traditional approach of commercial software vendors, who are limited by the collective ability of the people they can hire and pay. Our approach brings its own unique challenges and difficulties, but we are confident (and other similarly structured projects have demonstrated) that in the long run, we can outclass the commercial “competition.”
You can fly around 400 aircraft from various airports around the world in flightgear. At the time of this article writing, FlightGear 2017.3.1 is available for download.
Also, available in the Archlinux AUR too.