Single Integrated Air Picture (SIAP) was the first project I worked on at BA; a great idea to have all participants share information on aircraft location and identification. Our task was testing the SIAP software, and there were some nice simulation tools running on an open source HLA RTI. But there was no visualization, nor were there any accurate complex aircraft models. Airforce guys would come in and ask how do we know if the we can follow an intercept, or how will it look to the AWACS and so on.
Enter X-Plane.
Source X-Plane Website
I had looked for a flight sim that supported an external API which X-Plane did. This allowed me to connect X-Plane to the HLA RTI sim and provide completely realistic flight parameters for aircraft like the F-4 and F-15 and F-22, as well as some tankers and sensor platforms.
I could pick a plane and fly the mission and either inject the data live, or record it for playback latter.
Even better I could take generated aircraft data and visualize the flight with X-Plane. Nothing like checking your rotation matrices by watching a plane bank and fly.
Using X-Plane also added the wow factor to some of our demos, and completely stopped the question of our using valid flight models.