What are the visibility and cloud clearance requirements for VFR flight in Class E airspace assume below 10000 ft MSL )?

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?

To most pilots, VFR means three statute miles' visibility and far enough away from the clouds that they don't hassle you, you don't interfere with aircraft flying under instrument rules that are coming out of the clouds, and you can see where you're going and spot other airplanes. That's pretty close--but there's more.

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?"

So what does VFR mean?

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 versus VMC

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.

How airspace affects VFR

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.

Class G: You can remember Class G (uncontrolled airspace) because it's just like the good old days at the dawn of aviation. Very few rules. One mile visibility and "clear of clouds" is the daytime requirement. At night, requirements jump to three miles visibility and from merely "clear of clouds" to 500 feet below, 2,000 feet horizontal, and 1,000 feet above clouds. Some identify it as "G for general aviation."
Class E: Any time you cross the boundary from Class G into Class E airspace, the visibility requirement to remain VFR automatically jumps from one mile to three miles. Why? Because you have crossed over onto a federal airway, entered an instrument approach zone (which could start at the surface, 700 feet agl, or 1,200 feet agl), or ventured someplace else where other airplanes are likely to be operating.
Class D: This is airspace directly above and around an operating control tower. Cloud clearance and visibility requirements for Class D airspace are three statute miles, 500 feet below clouds, 2,000 feet horizontal, and 1,000 feet above.
Class C: Although there is radar control within the airspace surrounding Class C airports, it's usually much busier than Class D. A transponder and radio contact are required before entering Class C airspace. Still, three miles and cloud separation required.
Class B: This is controlled airspace surrounding the very busiest U.S. airports with the highest traffic counts. But why is only three miles of visibility and clear of clouds required to fly VFR here? Specifically, it's because all aircraft must be under positive ATC control.
Class A: This requires (almost) all airplanes to have IFR clearances before they climb above Flight Level 180 (around 18,000 feet)--unless you have special permission. Gliders sometimes get waivers to fly VFR in Class A airspace.

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?

Ground visibility

Here are some techniques to determine ground visibility:

Get a good briefing from Flight Service or check official sources on the Internet. More importantly, listen or read carefully to specifically note cloud and visibility information before you fly.
Listen to the automated surface observation system, automated weather observation system, or automatic terminal information service. Visibility on the airfield is always noted. As a practical matter, I always call ahead and get actual airport conditions before beginning my one-hour drive to the airport.
Using your sectional chart, pick easily identified, prominent landmarks--towers, buildings and terrain features--near the airport. Measure how far away they are. Use them to evaluate pretakeoff visibility. Sectional chart mileage scales are depicted for both nautical and statute miles, but keep in mind that most visibility requirements are measured in statute miles. When visibility is reduced, use those predetermined checkpoints.
Pay attention to runway length(s) and use the info to estimate visibility. "That checkpoint is four runway lengths (of 6,000 feet, for example) away, so visibility must be about four and one-half statute miles."

In-flight visibility

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.

Pilot's tip of the week

Featuring John Krug

“Let’s review briefly the cloud clearance requirements and reasons for each. For the altitudes most of us fly, it is simple.

In general, when in Class E, D or C, you must remain 500 feet below, 2,000 feet laterally or 1,000 feet above any clouds. This is to allow an IFR aircraft popping out of the clouds sufficient time to see and avoid a VFR aircraft.

In Class B airspace, we merely have to remain clear of clouds. Because in a Class B, all traffic (both IFR and VFR) is under positive control of ATC.

None of this allows a VFR pilot to enter a cloud. If an ATC vector or altitude assignment will cause you to enter a cloud, you must promptly advise ATC that you are Unable due to weather and request an alternate clearance.”