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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?"

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    $\begingroup$ Sure, if the pilot's name is Schrodinger... $\endgroup$ – Michael Hall Jun 9 at 2:18
  • $\begingroup$ P.S. I downvoted specifically for the ridiculous bonus question about flipping a coin after the flight to determine whether it was VFR or IFR. $\endgroup$ – Michael Hall Jun 9 at 5:50
  • $\begingroup$ @MichaelHall -- the "ridiculous bonus question" was meant to make people stop and think twice about the assumed exclusivity of VFR vs IFR $\endgroup$ – quiet flyer Jun 9 at 13:38
  • $\begingroup$ @MichaelHall -- ... and also to mirror the ridiculousness of the FAA sanctioning a pilot for flying under IFR in IMC in Class G airspace, while routinely condoning IFR flight in IMC in Class G airspace under circumstances that are really only slightly different and still offer no protection against collision with some poor guy legally flying an aircraft with no electrical sytem, radio, or transponder just outside the edge of a cloud. For more see aviation.stackexchange.com/questions/21888/… $\endgroup$ – quiet flyer Jun 9 at 13:40
  • $\begingroup$ @MichaelHall -- the ridiculous bonus question was meant to make people stop and think twice about the assumed exclusivity of VFR vs IFR $\endgroup$ – quiet flyer Jun 9 at 13:40
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No, you can't be both VFR and IFR at the same time. It would be like saying that a ball is both red and blue at the same time. You are either one or the other, never both.

If the pilot hasn't received an IFR clearance from flight services, then they are operating under visual flight rules, regardless of their intentions, capabilities, or flight plan. The sole exception is in class G airspace, specifically when they're flying a pre-set departure or arrival procedure from an airport. If a VFR pilot enters a cloud (and there's no emergency in progress), then they are breaking the law, and they can't just decide later that "oh, I was actually IFR at that time".

ATC is never going to inquire about which rule set you're operating under unless there's some kind of administrative foul-up. They're the ones that either did or did not issue you the clearance, so they already know which rule set you're operating under.

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    $\begingroup$ One can operate under IFR in class G without a clearance or even a flight plan. So, unless/until you enter a cloud, it can't be established which rules you are using. And, if you don't do so, it is theoretically possible to comply with both at the same time. $\endgroup$ – StephenS Jun 9 at 3:33
  • $\begingroup$ @StephenS, are you an instrument rated pilot or air traffic controller? $\endgroup$ – Michael Hall Jun 9 at 5:52
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    $\begingroup$ @StephenS Yes, I did mention that exception in my answer. And, yes, it's possible to comply with both sets of rules, but that doesn't mean you're actually operating under both sets of rules. Some highways in this country have a speed limit of 60mph, and some have a limit of 70mph. If I drive 60, I'm technically complying with both sets of rules, but that doesn't mean that the speed limit is somehow both 60 and 70 at the same time. $\endgroup$ – HiddenWindshield Jun 9 at 13:20
  • $\begingroup$ @StephenS, if you were to operate IFR in class G without a clearance, what squawk code would you use? $\endgroup$ – Michael Hall Jun 10 at 1:41
  • $\begingroup$ @MichaelHall If you're taking off IFR from an airport in class G airspace, you wouldn't get a squawk code until you get in contact with ATC. So it doesn't matter what you put in your transponder. $\endgroup$ – HiddenWindshield Jun 10 at 17:20
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VFR on Top is flying VFR while on an IFR flight plan:

What is VFR-On-Top? First things first: it’s ONLY for pilots operating under an IFR flight plan.

VFR-On-Top is a request to fly at a VFR altitudes in lieu of an assigned altitude. You must ask permission from ATC to operate VFR-On-Top.

Why would you want to operate VFR-On-Top? The most common reason is to avoid bad weather. You may need to avoid turbulence, ice or some other unsavory condition, but the altitude you want is tied up by another aircraft.

When the conditions permit (ie VFR conditions) you can ask to fly above the clouds or in between layers.

What are the rules governing VFR-On-Top? A VFR-On-Top clearance has its limitations. Here are a few you need to follow:

Pilots may not fly below minimum en route IFR altitudes because you could crash into the terrain. Minimum en route altitudes exist regardless of IFR or VFR weather conditions and weather assigned a specific altitude or VFR conditions on top. VFR-on-top is not authorized in Class A airspace. Altitudes as prescribed by 14 CFR Part 91, Section 91.159 must be maintained. Don’t remember 91.159? Here it is from Cornell Law School’s website:

CFR 91.159 VFR cruising altitudes or flight level

Any VFR cruising altitude appropriate to the direction of flight between the MEA and 18,000 feet MSL may be selected that allows the flight to remain in VFR conditions. Any change in altitude must be reported to ATC, and pilots must comply with all other IFR reporting procedures. The pilot is responsible for traffic avoidance VFR-On-Top is NOT a cancellation of IFR flight plan. You must still adhere to ATC directions. If you can’t stay VFR you must request an IFR altitude from ATC.

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From my 2-year experience working on flight plans in ATM software, the answer is no. The flightplan must either be VFR or IFR. If the pilot, plane and route qualifies for both, the pilot may flip a coin before submitting the flight plan, but he/she must choose one rule set.

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    $\begingroup$ Not all aircraft are required to be on a flight plan. $\endgroup$ – StephenS Jun 9 at 3:31
  • $\begingroup$ @StephenS, correct for VFR, but you need to file a flight plan if you are going to fly under IFR. At least in the US... $\endgroup$ – Michael Hall Jun 10 at 1:49
  • $\begingroup$ @MichaelHall There is no regulation requiring an IFR flight plan entirely within uncontrolled airspace. Doing so in CONUS without running afoul of 91.13 may be difficult, but it is at least theoretically possible—and it is completely normal outside CONUS. $\endgroup$ – StephenS Jun 10 at 2:48
  • $\begingroup$ No, it is possible to fly in instrument meteorological conditions in uncontrolled airspace without a flight plan, (this is different than an IFR flight with a terminal point in class G...) but it Is not correct to say that doing so complies with instrument flight rules. $\endgroup$ – Michael Hall Jun 10 at 4:29
  • $\begingroup$ @MichaelHall -- re " it is possible to fly in instrument meteorological conditions in uncontrolled airspace without a flight plan" -- did you mean legally? If so, are you meaning to say there is a way to fly legally in IMC but not under IFR? Interested to know whether this is your position. $\endgroup$ – quiet flyer Jun 10 at 17:53
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I can't find any regulation that says "every flight is either VFR or IFR", maybe because it's such a fundamental thing. 14 CFR 1.1 defines IFR and SVFR as differing from VFR, but doesn't actually define VFR itself.

The only explicit statement I can see is 91.101:

This subpart prescribes flight rules governing the operation of aircraft within the United States and within 12 nautical miles from the coast of the United States.

In other words, 91 Subpart B applies to all aircraft in US airspace. I'd take that to mean that where it just says "do this" then it applies to all flights (e.g. 91.119); if it says "do this under VFR" then it's a VFR-only rule (e.g. 91.159); if it says "do this under IFR" then it's IFR-only (e.g. 91.179).

If you're looking for 'evidence' that VFR/IFR applies to every flight, one place is the FAA flight plan format, which has (had) only three options: VFR, IFR and DVFR. Or the PHAK Glossary, which says "if you can't operate VFR, you must operate IFR":

Visual flight rules (VFR). Flight rules adopted by the FAA governing aircraft flight using visual references. VFR operations specify the amount of ceiling and the visibility the pilot must have in order to operate according to these rules. When the weather conditions are such that the pilot cannot operate according to VFR, he or she must use instrument flight rules (IFR)

Or the many sections of the AIM, ATC Orders etc. that talk about the differences between VFR and IFR procedures. Maybe someone else will be able to find something more explicit.

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It's not possible to simultaneously comply with the IFR and VFR cruising altitude requirements, unless perhaps your aircraft is 500 ft tall.

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    $\begingroup$ Those don't apply at lower altitudes. $\endgroup$ – quiet flyer Jun 9 at 1:14

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