I said nothing about reported visibility; neither RVR nor tower-reported visibility in statue miles (or meters, as the case may be). This more recent part of this discussion is very specifically about flight visibility, which is what the pilot sees. The present discussion regards only flight visibility, and is restricted to what the pilot can see from the cockpit, while flying the approach.

What I'm describing isn't a "reasonable way" of determining flight visibility. It's the only way. It's also regulatory. We'll explore that (again).

Note that we're talking about flight visibility, which is not a specific value. It's defined as an average value, an important distinction. Unlike runway visual range flight visibility does not require that we pin down an exact distance value, nor is that possible, most of the time. By definition, we either see a prominent object, or we don't, and it's an "average" distance, especially given that our seeing at that range is subject to individual determination. What's a half-mile to one person may appear so to another. We're moving. Perception of distance varies with visibility: close objects look farther away in lower visibility. Our distance to the object is changeing continuously. It's an average value, and one we must judge based on what we see, looking out the window. We reference average distance to a prominent object, and that prominent object, for our purposes when flying an approach, is the approach lighting system, and then the runway environment, as spelled out in 14 CFR 91.175(c) and (d).

With few exceptions, visibility is controlling for an approach, not ceiling. Minimum altitudes are not connected with ceiling, but with terrain clearance. The requirement for the procedure is a minimum altitude to which we can descend, and a minimum visibility with which we can see the runway, or at which we must find the approach lights or runway, in order to continue below minimums and subsequently land. This visibility is what we see out the window. If the tower tells us visibility is ten miles, but we arrive at minimums and can find neither approach lights, nor the runway environment (as spelled out in 14 CFR 91.175(c) and (d)), then we must execute a missed approach, regardless of what is reported by the airport RVR transmissiometer, or by the tower. The guy in the tower cab might be able to see just fine, but he isn't looking for the runway from minimums on the approach. It is for this reason, and this reason ONLY, that the regulation specifies "flight visibility."

Many of us have flown approaches when we could see the airport and runway overhead, but not on the approach, or when reported visibility was fine, but on the procedure, at minimums, was insufficient. Many of us have arrived at minimums at some point or another, to find actual conditions not the same as reported, and of course there are those days when it's quite clear below a low overcast, but we see nothing at minimums, or a fog bank is clear on one side, and pea soup on the other, sometimes half-way across the airport, or runway. What we concern ourselves with in discussing flight visibility, is what we see out the window.

At NO TIME is a pilot expected to determine flight visibility to the nth degree, be it an exact half-mile or some specific eighth-mile fraction, based on a cockpit guestimate. Instead, the pilot is given a simple requirement: see the approach lights, or see the runway environment. The pilot is not required to guess what part of the approach lights can be seen: the regulation simply states "approach light system," and then notes that the red terminating row bars, or the red side row bars must be "distinctly" visible AND identifiable.

Nowhere in the regulation, aeronautical information manual, or any recognized reference, will there be found any requirement to calculate flight visibility while in the cockpit on an approach, based on the number of lights seen, or runway markings, in order to make the decision to continue beyond minimums. The pilot must have the flight visibility prescribed for the procedure. Prior to starting the approach, the pilot requires reported visibility, because the pilot isn't there to see it for himself. At minimums, the only thing he needs to see is the runway environment, or to continue below minimums, the approach lights, and then subsequently the runway environment.

If the pilot can see the approach lights, that constitutes having the necessary flight visibility to continue the approach. If the pilot arrives at 100' above the touchdown zone elevation, having descended below minimums based on the necessary flight visibility to see the approach lighting system and the requisite terminating red row bars or side row bars, then the pilot has a specific list of items that he must see in order to continue below that 100' threshold. These visual references are clearly specified to ensure the pilot is identifying the runway, and not something else, which is why the regulation doesn't stipulate "a big dark spot" or "something that looks a lot like a runway." The regulation cites items that make it clear that the pilot has found the end of the runway and what lies beyond it. These, then, become the metric for determining flight visibility. To be clear, at minimums, either the approach lighting system, or the runway sighting, constitute adequate flight visibility to continue. Flight visibility is the distance at which a prominent unlighted object is seen in daylight, or a lighted object at night; the approach lighting system, and the runway environment are the prominent objects used to determine flight visibility; either we see them, or we don't.

Threshold, threshold markings, threshold lights, or runway end identifier lights identify the first thing we see of the runway: the end of the runway. Additionally, the regulation specifies references beyond the end of the runway: a visual glideslope indicator (which, while not on the runway or a part of it, is a dedicated guidance device to the runway and not something that is part of something that might look like a runway, but not actually be a runway); touchdown zone or touchdown zone markings, touchdown zone lights, runway or runway markings, or runway lights. That's it.

There are two parts to the flight visibility question: what we see to continue below minimums and subseuquently below 100' above the touchdown zone elevation, and a second requirement that we have adequate flight visility to land, each spelled out in separate sub paragraphs of 91.175. The former is specified by 91.175(c), and the latter by 91.175(d). Specifically, for a 121 operation, 91.175(d)(2) states that to land, one ust have the flight visility prescribed by the instrument aproach procedure being used. Just as one does not arrive at minimums and count lights to calculate flight visibility before making a decision to continue, one does not land based on counting runway lights or stripes to calculate flight visibility and then land. If anyone is doing this, they're an idiot. The necessary flight visibility prescribed for that procedure, is what's required: what the pilot sees out the window.

There are additional requirements that apply to the descend from minimums to the runway, and to land on the runway, of course; normal descend, normal rate, normal maneuvers, landing in the touchdown zone, and so forth (or as one arrogant airline retiree once told me "what we like to call, the airline touchdown zone"). This discussion is about flight visibility. We know the distance between runway lights, centerline markings, and other standard features of the runway environment. We don't count them or calculate them to determine if we have the requisite flight visibility to continue below minimums, below 100', or to land. We are required to have the flight visibility prescribed for the instrument approach in use. We determine that by looking out the window at a prominent object that's unlighted in the daylight, or lighted at night, and that prominent object is, for the purpose of landing, the runway. If one is taking the time to count runway stripes or lights while landing the airplane, then one is what we like to call "a god damn idiot," and an unsafe one, at that.

We don't count these elements and calculate them at minimums. We don't count or calculate them at 100' above the touchdown zone, and we don't count or calculate them when we're landing.

We are not required to see all of the approach lighting system and calculate the distance as a half-mile. The regulation is very clear about what we must see with that approach lighting system, or runway environment, to constitute the requisite flight visibility, or in other words, that golden prominent object. Not every procedure is a half-mile visibility. The PANC ILS 7R uses 1/2 mile. Does one count anything to arrive at that number? No. One sees the approach lights and then the runway, and for operations when approach lights or other features are out of service, adjusts accordingly. If the approach lighting system is out, visibility jumps to 3/4 miles. Counting anything or calculating? No. However, at minimums if one can't see a prominent object (the runway, as specified by 91.175), then one is going missed. If the glideslope is out and one is flying the localizer-only procedure to 7R, visibility is 5/8 mile. is one using any metric to calculate 5/8 mile? No. How does one determine that he has that additional 1/8 mile above the standard half-mile visibility? He looks out the window and sees the runway, threshold, runway end identifier lights, touchdown zone, yada, yada, yada. If (s)he's doing the ILS 7R loc-only procedure with no appraoch lights, then it's 1 1/4 miles flight visibility needed, and the same thing is used to determine the flight visibility is there. Are we looking out the window and counting objects? NO. We are looking out the window for the only lighted/unlighted prominent object we need to see, or care about, and that's the runway and the specific features of the runway or runway environment that the regulation spells out.

The question of this thread (original post) casts aside landing from a precision approach, and asks specificall about determining visibility on a non-precision approach, and refers to the "decision bar" in approach lights. First, there is no "decision bar," nor does the regulation refer to one. Secondly, the requirement for visual reference remains the same regardless of whether the approach procedure is precision, nor non-precision. In neither case is one attempting to divine visibility down to the fractional eighth of a mile. One is looking out the window for the necessary visual references: first, the approach lighting system, then for the runway environment, as specified in the regulation (91.175).

https://www.ecfr.gov/current/title-1...section-91.175

Flight visibility, again, is the distance that a prominent object can be seen, from the cockpit, and the definition of flight visibility is regulatory.

https://www.ecfr.gov/current/title-1...t%20visibility)