Per the About page on the FlightGear website:
"FlightGear is an open-source flight simulator. It supports a variety of popular platforms (Windows, Mac, Linux, etc.) and is developed by skilled volunteers from around the world. Source code for the entire project is available and licensed under the GNU General Public License.
The goal of the FlightGear project is to create a sophisticated and open flight simulator framework for use in research or academic environments, pilot training, as an industry engineering tool, for DIY-ers to pursue their favorite interesting flight simulation idea, and last but certainly not least as a fun, realistic, and challenging desktop flight simulator. We are developing a sophisticated, open simulation framework that can be expanded and improved upon by anyone interested in contributing.
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 frame work 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."
Posted March 31, 2012