Unless you're just getting started, you're familiar with the term VFR--visual flight rules. But what do VFR conditions look like? How do you know whether that scene out the hangar door equals visual meteorological conditions, or the one through your windscreen allows flight under VFR rules?
VFR means very different things at different times and places, and it's important to know that. Knowing the rules is more than just an academic exercise.
Until you draw a line on a sectional chart and check to see how many different kinds of airspace you will fly through while following that line, you can't appreciate how often your visibility and cloud clearance requirements will change to remain under VFR.
Pull out your local sectional. Draw a straight line (or any other reasonable route). Imagine you're flying at 1,000 feet above ground level (agl); look at various points along your route, note the required visibility and clearance from clouds to remain VFR, and then think about what you've discovered.
Every time you cross the boundary into a new category of airspace, ask yourself, "What visibility and clearance from clouds must I need here to remain under VFR?"
Let's start with the visibility part of VFR. Below 10,000 feet mean sea level, basic VFR visibility is three statute miles. I say "basic" because, under certain conditions (in Class G airspace during the day, for instance), you only need one mile of visibility to operate under VFR. But if you're flying above 10,000 feet msl, you need five (not three) statute miles visibility to be able to fly VFR.
Why is that? You need two additional miles of visibility above 10,000 feet because airplanes at the same indicated airspeed are actually flying faster above 10,000 feet than they are at lower altitudes. So the rules do make a difference. It's true airspeed that counts, and true airspeed increases over indicated airspeed at about 2 percent per 1,000 feet as you climb in standard atmospheric conditions. So you're actually flying about 20 percent faster at 10,000 feet than you are at sea level for the same indicated airspeed. That 20 percent could mean the difference between whether or not you can see an airplane or obstacle in time to avoid it.
Now for cloud clearance. Check out Federal Aviation Regulation 91.155, which outlines the basic VFR weather minimums. Airspace is designed with safety in mind. The type of airspace you're flying in determines the visibility and clearance from clouds you must have to fly under VFR.
VFR is an abbreviation for visual flight rules (as opposed to VMC, which stands for visual meteorological conditions). Expanded VFR info is found in FARs 91.151 through 159, under the heading "Visual Flight Rules." They specify weather minimums, fuel required, flight plans, when it's legal to fly VFR, and when you are able to fly under VMC.
To determine what VFR is, you have to know the existing conditions, whether it's day or night, your altitude, the airspace you'll encounter, and the airspace you are in. Airspace is designated Class A, B, C, D, E, or G (there is no F in the United States). Let's consider them in reverse order, because VFR pilots fly in a lot more in Class G airspace than they do in Class A.
It doesn't do any good for you to know what the VFR rules are unless you can apply them. To do that, you've got to know the visibility. How do you determine that?
Here are some techniques to determine ground visibility:
Resources for VFR operations
To learn more about visual flight rules (VFR) operations and additional tips, consider these resources.
Once you're airborne, the challenge then becomes to know the rules, apply them, maintain adequate separation from clouds --and be able to estimate in-flight visibility.
As you progress along your planned route, compare your airplane's position with landmarks to estimate flight visibility.
Here's another way to estimate flight visibility: Look over the nose of your airplane at normal cruise speed. The point on the ground that you can barely see over the nose of the aircraft as it disappears is about the same distance ahead as your airplane is above the ground. At an altitude of 5,000 feet agl, for instance, the "vanishing point" just over the nose of the airplane is about 5,000 feet--or slightly less than a mile--ahead.
If you have runways of known length available, take time to really look at them. Extrapolate from where you are to estimate what the visibility is. Just over three times the length of a 5,000-foot runway would be about three miles--legal VFR.
You can also use normal traffic pattern spacing as a guide. Downwind should be three-quarters of a mile to a mile away from the runway. That can be used as a guide as well.
A simple term like VFR has a lot of different meanings. If you know them and practice estimating, you'll be able to have fun while simultaneously staying legal.
But here's the real payoff for thinking about VFR: When you're flying VFR and the clouds start closing in or the visibility decreases uncomfortably, think about what you're doing; call time out--and check it out. If you don't like what you see, do a one-eighty and get out of there.
Wally Miller is president of an aviation training, consulting, and marketing firm in Monument, Colorado. He is a Gold Seal CFI who has been instructing for more than 30 years and flying for more than 40.
Want to know more?
Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online.