In the US, could a flight ever be considered to be operating under Visual Flight Rules and Instrument Flight Rules at the same time?
If not, where in the FARs is this spelled out?
Consider the specific case of an IFR-rated pilot flying an IFR-equipped aircraft, but with no IFR or VFR flight plan filed and no IFR clearance requested or delivered, staying in Class G airspace and observing the VFR cloud clearance and visibility requirements spelled out in FAR 91.155. How can we determine whether this flight is operating under VFR or IFR? Why could it not be considered to be operating under both?
Bonus question-- is there any specific case, falling within the general conditions described above, where it would matter whether the flight were operating under VFR or IFR? Assuming that the pilot intends to, and is able to, remain in Class G airspace and observe the VFR cloud clearance and visibility requirements spelled out in FAR 91.155, would it be practical for the pilot to decide in advance that he will simply flip a coin to determine the answer if ATC, or anyone else, enquires whether he is operating under VFR or IFR? With the added proviso that if no one makes such an enquiry, he will leave the coin unflipped until after landing, at which point he will flip the coin to see whether the flight was conducted under VFR or IFR? Would such a plan present any practical difficulties?
Related, but not the same--
Can an IFR clearance be issued and flown through IMC in class G airspace? -- note that several of the answers, as well as many comments, to this question also address a different question (which probably ought to be asked as a separate ASE question), which is "May an IFR-rated pilot flying an IFR-rated aircraft in Class G airspace violate the VFR cloud clearance requirements without an IFR clearance?"