does anyone saw this thing before or know what is it ?  red thing coming out from top of the tail 

Flight test package..or something along those lines in the FMC options.You need to turn it off.

You have the "flight test package" installed on the FMC.

Thanks people ! btw do you know what its purpose in real life ?

That's the YAW damper.... :Big Grin: Sorry, couldn't resist....

Thanks people ! btw do you know what its purpose in real life ?

 

I'm not sure exactly what data it collects, but it's a sensor that is used when testing a new aircraft model. In reality it is extended much farther behind the aircraft while in flight to collect various parameters. If you look at one of the side passenger windows, you'll also see a set of pitot tubes in place of one of the windows. This is also used for flight testing.

IIRC collects air parameters outside of area of influence by airplane and the data is then used to calibrate sensors on the airplane. i.e. the temperature sensor might show a degree or two higher on the airplane, or the static pressure shows a tiny bit lower owing to Venturi effect... such things.