I agree that Jim's comment would make a better, (at least more concise) answer, but I am going to risk DVs and TLDRs to make a point and honor what I hinted at in a comment above.
This is not so much a direct answer to the question, but is an example intended to emphasize that while it is important to know and understand the regulations we fly under as pilots, it is at least as important to discern intent so that we can exercise sound aeronautical decision making. Regulations simply cannot cover ever possible scenario, (nor do we want them to, really...) but that doesn’t mean that we cannot, or should not, make every attempt to comply with the spirit when there are gray areas or corner cases.
To illustrate my point, let me offer a hypothetical question, and then outline a real life scenario by which you could violate the letter of the regulation while complying with the spirit.
Question: If the primary airport served by class C airspace is reporting weather less than published instrument minimums, can I land there under visual flight rules?
The obvious answer from everyone here would be a resounding NO! (and legally I agree…) However, let’s examine a scenario where doing so would be no more risky than a normal landing on a CAVU day, and in fact is the preferred course of action.
Naval Air Station Whidbey Island in Washington state is located at the Northern end of the island, in a low flat area adjacent to Dugualla Bay. Late summer and early fall weather patterns often consist of lingering high pressure which makes for the clearest flying of the year. However, the cold waters of the Salish sea are very efficient at producing advection fog, which blankets the water as well as low lying areas persistently, especially in the morning hours.
As the sun rises it will typically burn off overland, but it isn’t uncommon for a pattern to set up for days where mid-morning the land is clear for some brief period of time until a low layer fog creeps in from the water again. It can go on for most of the day, with fog cycling in and out like slow motion waves, from the shoreline to about the midfield intersection, and back out again.
If you note the tower location, they are on the shore side of the intersection, and there are plenty of times where the tower is completely socked in while a majority of the field is CAVU. When the tower is socked in the weather is reported as below minimums because that's where the weather observers make their observations.
The EA-6B Prowler based at Whidbey is a single piloted aircraft, but it operated under a waiver which considered it multi piloted for USN instrument approach criteria. This was because the side by side seating allowed the NFO to function as a copilot, monitoring instruments and assisting the pilot in acquiring the runway visually. (Provided the NFO is qualified and current…)
Navy regs allowed multi piloted aircraft to commence an instrument approach if the weather was below reported minimums provided they had sufficient fuel to proceed to their alternate with reserves. Single piloted aircraft were prohibited from even commencing an approach below mins. (unless it was practice only?)
Let’s get to our scenario: An EA-6B is coming back to the field from one of the MOAs in the Eastern half of the state on a day when you can see all the way to Mount Jefferson in Oregon. (maybe even Shasta in CA?!) ATIS reports the ceiling is zero with visibility of a quarter mile. Below minimums.
No problem most of the time, just stay on the IFR clearance you filed for coming home and request vectors to a visual. Or fly an instrument procedure if you like. Either way you already have the field in sight before you contact approach control, and you can see from over 20 miles away that it is just the far end of the runway that has shallow layer of fog sneaking in from the shoreline. You will be at taxi speed by the time you enter it, and if you go around for any reason you will be well above it by 200’.
The problem occurred when you are an instructor pilot… Because your right seat student NFO is not yet qualified in the aircraft you have to operate as a single piloted aircraft. This means that you cannot commence an instrument approach when tower weather is below mins. So, what are the legal options?
You could divert to McChord AFB and wait for the weather to report clear, launch to come home, and by the time you get there risk having the fog roll in again. Or…
You could hold overhead a mostly clear field waiting until the weather is technically “legal”.
Both of these options felt ridiculously silly on such a nice day when most of the field is completely clear. Obviously the intent of the single pilot restriction is to prevent a weary solo fighter pilot from pressing decision height in nasty weather at the end of a long day, and being without an extra set of eyeballs to provide another margin of safety. But options # 1 & 2 would have brought our training schedule to a screeching halt for weeks at a time during the best weather the PNW has to offer. So, us instructor pilots would routinely exercise option #3 by calling “VMC, field in sight, Cancel IFR” and just land.
I tried numerous times to find a resolution, speaking with tower supervisors, the weather office, local FAA rep, and Navy leadership. All had valid reasons why it was too difficult to change things to explicitly allow this, so it was just accepted.
That’s not to say the practice was understood by all, or approved. If fact often there would be a pregnant pause on the radio, and a quizzical response from the controller. i.e. “Confirm you have information Charlie? Current weather is zero and a quarter, say again approach request?”
I could have simply accepted an instrument approach, because controllers were unlikely understand the limitations that bound us, or to know and care about crew qualifications. Certain things are assumed, and not policed.
However, doing so would be an acknowledgement that I understood and agreed that the weather was IMC, and if any incident occurred it could be demonstrated that I knowingly violated the regs by commencing an approach below minimums with an unqualified crewmember in my right seat. I felt my actions were more defensible by cancelling IFR. By getting it on ATC audio tape that I called VMC field in sight at 20 miles I was in effect throwing the weather observation right back at them, implying “the ground conditions you report are not the actual in-flight conditions I observe directly. (I override thee...)” And with no prohibition against landing VFR with a student NFO, I haven’t actually violated either the letter or the spirit of any rule.
Anyway, we were taught early on to evaluate how we could justify our flying decisions if ever called on the carpet, and this one would be easy to defend. Decisions like this should factor in probability, severity, and detectability. I wouldn’t hesitate for a minute to take off in CAVU weather from a non-towered airport simply because another non-towered airport nearby reported some fog through a robotic system. And I severely doubt that any regulation expressly prohibits doing so.
