I don't know about production aircraft, but NASA uses X-Plane pretty extensively for all kinds of research, including modeling flight dynamics for small UAVs. I

I don't know about production aircraft, but NASA uses X-Plane pretty extensively for all kinds of research, including modeling flight dynamics for small UAVs. It's used often enough that several years ago I developed (and released on NASA's GitHub) a tool to simplify programmatic interaction with the X-Plane simulation engine. It's not nearly popular enough to make maintaining it a full time job (thankfully), but I do get pretty regular comments from NASA researchers and members of the public. I have never heard of anyone using MS Flight Sim in the same way, although another fairly common use case for my library is to disable the X-Plane physics completely and just use if as a visualization tool. If someone developed a library that made that equally easy for MSFS, I can see it taking over that role. It certainly looks better the X-Plane IMO.