Many thanks for prompt reply and comment ! No, the plane was not powered,only loaded. Will check and hope on problem solving. ADDED Thanks, it works as advised by you. Indeed, the plane is to be powered what enables to move the PL below Flight Idle detent on the ground.
I read all messages above but still not able to move the PL below flight idle detent ON THE GROUND after loading the plane. The throttle of the joystick (X52Pro) in full back position, pop-up TQ window shows that the throttle is in full reverse position (green rectangles, picture 1) but actually the throttle are still in the idle flight detent (picture 2). The User Preference window (picture 3) is attached as well. Any comments and suggestions are very appreciated. PS. XP 11.52 (Vulcan) + SAAB 340A 1.6.5 hot fix.