what is this? it starts as a small square white dot in the left upper corner and when i put the mouse over it i can enlarge it.I start a flight in full screen mode and the small white dot appearce every time.If i start a new flight with for example the level-d 767, nothing's wrong.

it looks like a grey square Daniel

It has been reported before, just don't click on or resize the small square in the top left corner. Mark.

maybe a sort of technical window.... Regardsczarek

I could easily be wrong on this, but I believe it's well documented on some of the FSX graphics websites that it's best graphics wise to not to use full screen with FSX, especially when third party aircrash (whoops, aircraft). Rather the best method is to always boot in to the Windowed mode. I can tell you that using windowed mode made all the difference to setup. Dave

That square in the top left corner is where you should hide the mouse-pointer when not using it, when running in Fullscreen.You then will have some more FPS. / Leffe

it's a light sensor gauge thing.