Running the Numbers

In some ways VFR flying can be more challenging than flying under instrument flight rules. Apart from the subtle logistics of reading instruments, knowing where you are, and controlling where you’re going, IFR flying is almost entirely built upon procedures and doing what you’re told. Compared to VFR flying, it’s about as intuitively challenging as rendering a picture by the use of numbered colored pencils. When the weather is marginal, and you’re flying under visual flight rules, the challenge comes from knowing how to interpret a different set of numbers.

The greater challenge (perhaps the more important one) is staying ahead of changing weather … and knowing when to quit, when it’s closing in. Staying ahead of it implies that 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. Plus, if someone suddenly decided that he were getting in over his head and wanted out, “down” was almost always a viable option. Not anymore! Many more areas are populated than in days past and for anyone scud running — as I learned during my first helicopter dual cross-country, at 500 feet — it’s incredibly easy to get very lost or disoriented at low altitude. I shudder to think about trying it at 100 feet. Also, there is little time to avoid any of the many man-made structures looming up out of the murk, there’s less time to deal with any problem or emergency and equally bad, a much higher stress level and the inevitable degradation in decision-making and flying skills.

VFR is anything where there either is no ceiling, or it’s more than 3000 feet, and the visibility is greater than five miles. (Here, it has to be both.) So, what is “marginal” weather, anyway? Well, there’s the official version, and then there is (and should be) personal minimums consistent with pilot aptitude, 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 you’re in, at what altitude, the time of day, what category of aircraft you’re flying, and in what environment (such as in proximity to a runway). One easy way to know what “marginal” VFR officially means (as well as what IFR and VFR mean) is by remembering “one to three and/or 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 if the ceiling and visibility are both lousy, and not just one of them, that’s marginal, too.)

By extension, IFR weather is anything where the ceiling is less than 1000 feet, and/or the visibility is less than three miles; There’s also a “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 more pessimistic one; it’s 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, there’s another type of VFR to remember: special VFR.

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 pilot’s 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), for 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).

Important: The most important things however aren’t just ceiling and visibility. This is where the tricky stuff comes in. Just as significant are some other quantitative weather attributes in this number 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 the big picture, too — as in the synoptic picture.

But going back to just the ceiling and visibility for a moment, an important consideration is obviously local terrain. As far as ceilings go, fifteen hundred foot ceilings are a good deal more serious in the Ozarks than 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 that ceilings are more than 1000 feet above the tallest thing within five miles of your planned route, there aren’t any METARS that mention obstructions to visibility (typically mentioned when it’s less than six miles), and the forecasts are for improving weather, that’s great. But if there is a forecast 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-dew point spread is narrow, beware. If they’re exactly the same, I wouldn’t 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 that’s a recipe for fog. Also, if the spread was small and the sun was about to go down (or come up) I would advise caution. Once the sun rises, things usually improve (but not always); either the temperature rises to well above the dew point, or the fog burns off. A frontal passage might also clear things up. The main point is: something’s got to happen to change it. Wind is one way this might happen, of course. It pays to look “upwind” to see what’s headed your way. If it’s VFR, but the temperature/dewpoint 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 there’s 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. They’re usually in the right ballpark, but when they’re not, you should consider all bets off as far as the forecast goes. Often, a westerly wind is good — southerly winds, less so. Back down on the runway, we 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 I’ve 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. The worst are stationary ones. Warm fronts don’t move as fast as cold fronts, and stationary ones, well, they might not move at all. Clouds are rarely a problem when they’re widely scattered, and both the cloud and any area of rain beneath it should be avoided if you can see the cloud’s bottom and it looks dark and agitated; it could be a thunderstorm.

A disproportionate number of fatal VFR weather-related accidents happen after dark (about three times as many as occur during the day), and fewer pilots fly cross-country VFR at night. When you can’t see clouds, always be extra cautious about reported ceilings and visibility; at night, your minimums should consequently be substantially higher.

THE BOTTOM LINE: There are, of course, many other “numbers“: currency, experience, how much sleep you’ve gotten, your fuel reserves, time in type, weight and balance, the density altitude, runway length, lifted index, etc. You should be aware of all these things and what they mean in the scope of your own experience. 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 it’s better to be late in this world than early in the next.