Ok, dear friend, I have to apologies, I have made a terrible mistake !
The airbrake is between the both vertical stabilizer, and I had the feeling it was on the back of the aircraft, like the F/-15 or the Su-27! I was not able to see it from the angle of the extarnel view.
Very sorry obout that, this is not a bug. After another test, the airbrake is working correctly.
I suppose I can count on you for a punishement!
Happy Christmas and have a good flight,
Vincent