WX Watch: Runway in sight?

The Aeronautical Information Manual (AIM) defines visibility as the ability to see and identify prominent unlighted objects by day and prominent lighted objects by night. That makes it a subjective measure that can vary from pilot to pilot. Bear that in mind when looking over the federal aviation regulations for VFR or IFR weather minimums.

These flight regulations mention visibility of varying values. Under VFR, the visibility minimums are either one, three, or five statute miles, depending on the airspace (or half a mile for you rotorcraft types). There are also minimums for maintaining horizontal and vertical distances from clouds. But how can you be certain you’re flying, say, 500 feet below, or 1,000 feet above, or 2,000 feet horizontally from any clouds? The same way you judge visibility—by guessing. There are no distance markers in the sky! And one pilot’s determination of three-mile visibility might be another’s judging it to be closer to one mile. Either way, that’s poor visibility—the kind that tempts or tricks some pilots into flying inadvertently from visual to instrument meteorological conditions.

Technically speaking, we’re talking about flight visibility here, which is defined in the AIM as the forward visibility from the cockpit in flight.

When weather deteriorates, other forms of visibility come into play, such as ground visibility. This is the prevailing visibility near the surface as reported by the National Weather Service or an accredited observer. No one can take off or land under special VFR unless the ground visibility is at least one statute mile, according to FAR 91.157. If ground visibility isn’t officially reported, then there must be flight visibility of at least one mile. In this case, flight visibility is considered the visibility from the cockpit of an aircraft in position to take off. This assessment, by the way, does not count as an official weather or ground visibility report. Basically, with a one-mile special VFR takeoff you’re on your own once the wheels lift off—at which time you’re probably in instrument conditions anyway. It’s a bad place to be without an instrument rating, or an instrument arrival, approach, or departure procedure. And yet another reason special VFR should be approached cautiously.

In IFR operations, visibility values have huge effects when the weather is low. The fuel requirements for IFR flight state that an alternate airport must be named if a destination airport is forecast to have ceilings at or below 2,000 feet agl and visibilities below three statute miles from one hour before to two hours after the estimated time of arrival. There are forecast visibility standards for naming alternate airports, too: those with precision approaches must have ceilings and visibilities equal to, or better than, 600 feet and two statute miles, and those with nonprecision approaches must be forecasting ceilings and visibilities equal or better than 800 feet and two statute miles. Those values pertain to the ETA at the alternate, but in a real-world situation the published minimums for alternate approach procedures apply. In other words, if you fly to your destination and have to perform a missed approach, and then fly to your alternate, those 800-and-2 and 600-and-2 alternate minimums go away. The new minimums are now the published minimums.

For determining forecast minimums, the regulations simply say to use “weather reports or forecasts, or a combination of them.” Ideally, TAFs would be published for both the destination and the alternate. If not, the regs are rather murky. Presumably a combination including the Aviation Weather Center’s G-Airmets and the Graphical Forecasts for Aviation (GFA) tool would suit the intent of the regulations (FAR 91.167 and 91.169).

Larger airports served by instrument approaches often rely on runway visual range (RVR) as a means of reporting visibility to determine landing minimums for IFR traffic. RVR is measured by transmissometers that use sensors along runways to report horizontal visibility near the ground. RVR reports appear in metars and are expressed in hundreds of feet. A report of R19LVR20 translates to “Runway 19L visual range 2,000 feet.” A report of varying RVR might read “R19LVR20V40,” meaning the visual range varies between 2,000 and 4,000 feet.

Similarly, airports with automated surface observing system (ASOS) stations also can issue visibility reports. ASOS detects visibility with a scatterometer, which measures the scattering of a beam of light sent between a pair of lenses mounted close to each other. The amount of scattering caused by fog, rain, snow, or other airborne particles is translated by an algorithm into a visibility report.

Older automated weather observing system (AWOS) instrument arrays designated as AWOS II and higher can also automatically generate visibility reports, but at longer 20-minute intervals. ASOS can give minute-by-minute updates in rapidly changing weather conditions.

These automated reports have their limitations. Ceiling sensors—ceilometers—only see the sky directly above their beams. Passing scattered and broken cloud layers may trigger false reports. It’s possible that the location of an ASOS or AWOS array may be shrouded by fog, when the runway environment is clear as a bell. And scatterometers sense only a small parcel of air, and extrapolate that to include a wider area. But automated reports are better than no reports at all, and there are plenty of them serving 24/7/365 at airports that wouldn’t otherwise have any weather reporting capability.

Trained observers at airports with control towers are often able to make manual reports of prevailing visibility, which is the greatest horizontal visibility through at least half of the horizon. With these, you have the benefit of knowing that there’s ground fog, say, south of the airport, or that eight-tenths of the sky is obscured by ground fog. Tower visibility is another means of cross-checking any automated reports. With tower visibility, observers make reports based on nearby landmarks—a tree line, a billboard, a McDonald’s sign—of a known distance from the tower.

Slant-range visibility is another type of visibility, one that doesn’t appear in the regulations but has great importance during instrument approaches in weather near minimums. Slant-range visibility is the visibility from the cockpit to the ground and features ahead; it’s the view down the approach path. Slant-range visibility is critical to seeing approach lights, runway end identifier lights, and the entire runway environment in the final stages of an approach. Even so, it’s a subjective variable, and of course it’s not published anywhere. This means the pilot must make the call.

It can be a tough one. Visibility is the governing factor in the decision to land, but what’s published and what you’re able to see may be two different things. After all, you can see through thin clouds. Passing bands of ground fog may afford a glimpse of the runway environment. A look downward may show ground features, but the view ahead may be opaque. If the view to the runway at decision height/decision altitude (or at the designated elapsed time) meets published minimum visibility values, then go ahead and land. But slant-range visibility can vary from moment to moment as you descend.

FAR Part 91 gives us the “look-see” privilege of legally shooting an approach even if the reported visibility is below minimums. But don’t press your luck. Slant-range visibility may let you see through a layer of ground fog, tempting you to land. But the ground visibility near the runway surface may be so bad that you end up facing a flare in zero-zero conditions. That’s one reason why a close watch of RVR can prepare you for a missed approach procedure. Too bad more airports don’t have this vital tool for knowing your “last few seconds” type of visibility.

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