Yes i understand and i knew that, but why, in NAV mode, does the HUD show the correct HDG of the aircraft if the INS (or whatever it's called in the Su25) isn't aligned ? And you can test, if you select AG mode, and the HDG changes on the HSI (that was pointing the correct bearing before switching modes) and go back to NAV, the HSI is still pointing the wrong HDG but the HUD shows you the correct heading. That's without the aircraft being aligned or at least not at 100%.