What is the difference between VFR and IFR in class G airspace?

Question

What is the difference, and how would an outsider discern, whether a flight was conducted under Visual Flight Rules or Instrument Flight Rules in class G airspace? (United States, FAA jurisdiction, although I’d be interested in EASA or other answers)

To be clear, I am not asking about receiving an instrument approach clearance to transit and land at an airport within class G airspace, or picking up an IFR clearance before take off and flight into controlled portions of the National Airspace System. I am talking about flying an entire flight under IFR within uncontrolled airspace, without radar services, and without talking to ATC.

Presume that the aircraft is certified for IFR flight, and the pilot is instrument rated and current. The do-nothing default would be VFR, so:

• What is the actual trigger for enabling the pilot to make the flight under IFR if no clearance is issued?
• I.e. what specific action must be taken to be considered operating under IFR, and not VFR?

And here’s the kicker to this scenario… the weather is clear and a million. That’s right - VMC, not IMC!

Yes, the pilot could make the flight under VFR, and without radar and ATC service there is no compelling reason not to, he/she just wants to prove to Aviation SE readers that it can be done. You can file and fly IFR under VMC in controlled airspace so you should be able to do the same in class G, right?

Is this what would make the flight IFR then, the filing of an IFR flight plan? What would be the response of ATC in such a situation if they are not required to issue a clearance, and cannot “control” the flight?

Answer 1

Well, I would say that you simply conduct the flight under instrument flight rules. Assuming all of the conditions above were true, and you were conducting a flight under instrument flight rules from untowered departure and destination airports, both in Class G airspace, and along a route completely in Class G airspace, and you can descend and land at your destination in VFR weather minimums ie no instrument approaches, it would be very similar to conducting that flight under VFR. The only difference is that you would controlling the aircraft by solely reference to instruments (but practicing see-and-avoid if in VMC), and flying a course defined by electronic instruments, most probably RNAV systems using GNSS guidance. Since you’re flying under instrument flight rules, you could depart in zero-zero weather conditions (under Part 91), but you must be able to descend and land at your destinations airport in visual meteorological conditions (no available instrument approach). Filing a flight plan, thence opening it with flight service when airborne and making periodic position reports with flight service enroute is highly recommended, but not required under law.

That also assumes that you physically could execute such a flight, as there is very little, if any, class G airspace above 1200 ft AGL left in the continental United States, and it’s rapidly shrinking up in Alaska, too!

Answer 2

What is the difference [between a flight] conducted under Visual Flight Rules or Instrument Flight Rules in class G airspace?

The difference is that the VFR flight would be conducted in accordance with Visual Flight Rules, and the IFR flight would be conducted in accordance with Instrument Flight Rules. Some examples:

• The VFR pilot would navigate by pilotage, dead reckoning, or use of a non-IFR-certified GNSS display. The IFR pilot would navigate by their instruments (conventional NAVAIDs or an IFR-certified GNSS display).
• Should IFR conditions be encountered, the VFR pilot would have to deviate from their planned route in order to remain in VFR conditions. The IFR pilot would be able to continue as planned.
• The VFR pilot would comply with VFR fuel reserve planning (14 CFR 91.151). The IFR pilot, having departed in VFR conditions, would similarly be bound by 91.151 but would be prudent to plan fuel in accordance with 91.167 instead—in case they encounter unforecast IFR conditions along their route.
• The VFR pilot would cruise at a cardinal-altitude-or-flight-level plus 500', as required by 91.159, provided they were more than 3000' above ground level. The IFR pilot would cruise at a cardinal-altitude-or-flight-level exactly, as required by 91.179.

I believe that last point is the only way an outside observer would be able to definitively tell which flight was which. Of course there could be other evidence, like the VFR pilot flying circles and loops that would be unlikely for the IFR pilot. But assuming each one simply flew direct via GNSS, altitude would be the observable difference.

One very important similarity is that (when weather conditions permit) 91.113 requires both pilots, VFR or IFR, to see-and-avoid other aircraft. Both types of pilot must, if necessary, alter their course to avoid a collision and to comply with the right-of-way hierarchy.

What is the actual trigger for enabling the pilot to make the flight under IFR if no clearance is issued? I.e. what specific action must be taken to be considered operating under IFR, and not VFR?

The actual trigger, besides the requirement that the pilot and aircraft be legal to fly IFR, would be the pilot performing the necessary IFR flight planning actions. That's all. No clearance is required to operate under IFR in uncontrolled airspace. Even a filing an IFR flight plan, while certainly a useful piece of insurance, is not a requirement unless operating in controlled airspace.

In other words, IFR in uncontrolled airspace is a state of mind.