Actually it seems to be ok on your screenshot, look closer, it have straight line parallel to horizont, part of this line simply covered by shadow.

Agreed with OP
Left horizon rolls in the opposite side

Hello.

As I understand, this straight line shows the plane(or helicopter) but not the horizon.

I've just attached a screenshot from DCS:Ka-50 simulator(ScreenShot_045). In this screenshot my helicopter is in exactly the same condition. Have a look at its's air horizon indicator. Here the line indicates my helicopter's orientation and the blue and black background doesn's rotate but it is stable. In Arma's screenshot it is the same but it shows opposite angle.

Yes Sparker, you're correct. DCS:Black Shark works as it should, arma works the opposite way for that particular gauge

The same is true for a bank angle indicator on the helmet mounted display. Now it mimics the horizon, but it should display aircraft bank instead, just like on DCS screenshot (on the HUD).

Poor screenshot, had to see the attitude on the left attitude indicator.

Now it's fixed