Hi, I'm been help by an helpful support guy but issue not fixed so far.
What I have tried since my last post :
- replaced Flaming Cliffs\bin with "working" files,
- deactivated FC in admin mode (fc3_protect.exe), launched the game as admin, prompted to activate FC back again.
Curiously in the same mission, I can enter other aircrafts (such as A-10C for example) but when I try to enter an F-15C (R-Alt + J) the plane behaviour is altered for a fraction of second (as if I'd managed to take control of if) but it quickly returns back to its previous course.
My guess is that there is a bug with the last Steam version. Maybe the game is now looking for modules named after aircraft rather than module named "Flaming Cliff" ? It would be interesting to hear if other users are still able to use FC3 planes with Steam version.