Flying In The Margins

Understanding and flying marginal VFR weather involves more than just playing the game of ceiling and visibility numbers.


In some ways VFR flying can be more challenging than flying under instrument flight rules. Apart from achieving the skill of being able to read and understand the instruments, controlling an aircraft solely by reference to them, talking to and taking notes from ATC and planning your next move-all at the same time-IFR flying is almost entirely built upon procedures. Compared to VFR flying, its about as intuitively challenging as rendering a picture by the use of numbered colored pencils. When the weather is marginal and youre flying under visual flight rules, the challenge comes from knowing how to interpret a different set of numbers.

A challenge greater than and perhaps more important is staying ahead of changing weather, and knowing when to quit when its closing in. Staying ahead of it implies you understand it, and knowing when to give up implies that you have (and adhere to) margins of safety. In the old days, our airspace was friendlier and less crowded; there were fewer tall structures reaching up to grab airplanes that were also slower (and in which an off-airport landing was less of an event, anyway). If someone suddenly decided that he were getting in over his head and wanted out, “down” was almost always a viable option-but not any more! Many more areas are populated, and speaking of running…for anyone scud running, as I learned during my first helicopter dual cross-country, at 500 feet, its incredibly easy to get very lost or disoriented at low altitude. I shudder to think about trying it at 200 feet. Also, there is little time to avoid any of the many man-made structures looming up out of the murk, theres less time to deal with any problem or emergency, and equally awful are much higher stress levels and the inevitable degradation in decision-making and flying skills when a VFR pilot is confronting marginal weather.

Defining Marginal VFR

First, what is “marginal” weather, anyway? Well, theres the official version, and then there are (and should be) personal minimums, which are based on the pilot, the mission, the machine, the local geography and familiarity with it. Also of course, basic VFR weather minimums regarding visibility and cloud clearance criteria change depending upon what airspace youre in, at what altitude, the time of day, what category of aircraft youre flying and in what environment (such as in close proximity to a runway).

One easy way to know once and forever what “marginal” VFR officially means (as well as what IFR and VFR mean) is simply by remembering the two numerical ranges defined by these words: “one to three and three to five.” That is, marginal VFR is anything in which ceilings are from 1000 to 3000 feet, inclusive, or the visibility is between three to five miles, inclusive. (The language says “and/or” so, of course, if the ceiling and visibility are both lousy, and not just one of them, thats marginal, too.)

This also means IFR weather is anything involving a ceiling less than 1000 feet and/or visibility of less than three miles. Similarly, VFR conditions are anything where there either is no ceiling, or its more than 3000 feet, and the visibility is greater than five miles. (Hint: it has to be both.)

Theres also the concept of “low IFR” by the way, wherein ceilings are below 500 feet and/or visibility is less than a mile. And if you have to quibble about whether those are statute or nautical miles, the standard is actually the shorter one; its statute. Also, you can have VFR numbers in haze that might as well be considered marginal VFR. Finally, for those of us who do much of our flying in controlled airspace, theres another type of VFR to remember: Special VFR (see the sidebar on page 14).

If you want hard numbers, the AOPA Air Safety Foundation has come up with some, and in a very appropriate setting. Just as influential upon go/no-go decision-making as the business pilots meeting the next morning are the mission imperatives felt by a pilot who is transporting a patient for urgently needed medial care. In their publication for volunteer pilots titled “Recommendations for Enhanced Safety,” they came up with numbers for departure, en route and arrival (as well as up to one hour afterwards), both during the day as well as at night. They were: 2000-foot ceilings and five miles visibility (with 5000 and 10 miles for all phases of flight in mountainous terrain).

The most important-and tricky-stuff, however, isnt just ceiling and visibility. Just as significant are other quantitative weather attributes in this numbers game-things like the spread between temperature and dew point, which way the altimeter is headed, as well as the wind (and by how much). We also need to understand the big picture, too, as in the synoptic picture.

Other Considerations

But going back to just the ceiling and visibility for a moment, an important consideration is obviously local terrain. As far as ceilings go, 1500-foot ceilings in the Ozarks are a good deal more serious than in central Indiana. And, as for visibility, airports located near rivers and other bodies of water are often adversely influenced either by low-lying elevations or terrain, or both. Plain and simple, having water around usually means more moisture.

Around San Francisco, as well as northern California in places like Eureka, you can go from good VFR to zero-zero in minutes. If you can be sure ceilings are more than 1000 feet above the tallest thing within five miles of your planned route, there arent any Metars reporting obstructions to visibility (typically mentioned when its less than six miles) and the forecasts are for improving weather, youre probably good to go.

But if the forecast calls for rain, or radar reports show it moving in, watch out. Although it takes a veritable downpour to bring visibility down to IFR levels, rain adds moisture to the air and can result in either lowered ceilings or an entirely new lower layer of clouds altogether. Also, if the temperature-dewpoint spread is narrow, beware. If theyre exactly the same, I wouldnt even take off unless I was otherwise convinced it will get better before I get back (or perhaps if I was staying in the pattern), because thats the recipe for fog.

Also, if the spread was small and the sun was about to go down (or less likely for me to be there, come up), I would advise caution. Once the sun rises, things usually improve (but not always); either the temperature rises to well above the dewpoint, or the fog burns off. A frontal passage might also clear things up. The main point is: somethings got to happen to change it.

Which Way the Wind Blows

Wind is one way this might happen, of course. It pays to look “upwind” to see whats headed your way. If its VFR, but the spread is only four degrees (which is generally a minimum value for VFR weather) you should have your feelers out. If airports 60 miles to the west show better weather and westerly winds, things will probably improve. (If theres a large body of water in between, or your airport is on higher terrain, then maybe not.)

Forecast winds aloft, of course, represent valuable numbers. Theyre usually in the right ballpark, but when theyre not, you should consider all bets off as far as the forecast goes. Often, a westerly wind is good, and southerly winds, less so.

Back down on the runway, some learn rather quickly, hopefully after a rip-snorter of a landing with a CFI on board, just what gust factor or crosswind limits might be wise to stick with. Some of us (not me, so far, though Ive come close) have learned colorful lessons about turbulence tolerances, in deference to our passengers, if not our upholstery.

As far as fronts go, the safest rule is to be a pessimist. In the winter, dry cold fronts could cause you little more than a bumpy flight, and cold fronts are usually less of a barricade to VFR flying than warm fronts, or worse, stationary ones. Warm fronts dont move as rapidly and stationary ones, well, arent moving at all. Clouds are rarely a problem when theyre widely scattered, and both the clouds and any area of rain beneath should be avoided if its bottom looks dark and agitated; it could be a thunderstorm.

Since a disproportionate number of fatal VFR weather-related accidents happen after dark (about three times as many as occur during the day) when few pilots fly cross-country VFR, and when you cant see clouds, always be extra cautious about reported ceilings and visibility; at night, your minimums should consequently be higher.

There are, of course, many other numbers: recency of experience, how much sleep youve gotten, your fuel reserves, time in type, density altitude, runway length, lifted index, etc. There are no single sets of minimum values that can be universally imposed on every one of us. Just remember: Very few of us ever get chastised for staying on the ground, and its better to be late in this world than early in the next.

Jeff Pardo is a freelance writer and editor who holds a Commercial certificate for airplanes, helicopters and sailplanes.


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