[KolmogorovComp]

There is also FlightGear[0] which is FOSS[1].

I cannot judge how realistic its physics engine is, but it is marketed as a ‘professional’ flight simulator.

[0]: https://www.flightgear.org/ [1]: https://sourceforge.net/projects/flightgear/

[Koshkin]

“Too realistic,” I’d say - it’s the only FS where I was never able to take off in a Cessna 172. (Even after having flown a real one.)

[app4soft]

FlightGear is not "simple".