as a U2 pilot in the USAF Mostly VMC then It is legal in the US to fly IMC in Class G without a flight plan or talking to anyone, but since there isn’t any, it’s not really relevant. Recalling some previous posts from long ago, there have been US pilot busts for illegal IFR in Class G (basically, taking off straight into IMC, below the Class E base) but the pilot got busted not for IFR...