Plane & Pilot
Tuesday, October 5, 2010

Managing Risk: VFR Versus VMC

Are you prepared for when the weather deteriorates?

It’s Legal, But Is It Safe?
In the regulations, visual flight rules are a set of operating rules that require reported ceilings to be at least 1,000 feet and visibility to be three miles or more for operation under VFR. These criteria change, depending on the class of airspace in which we’re operating, but the salient point here is that the regulations simply provide minimum criteria, which must be met to operate legally. Just because the weather meets these criteria doesn’t suggest that it’s safe to fly under VFR.

What VFR pilots really care about is whether we can remain clear of clouds and see ahead well enough to maintain a wings-level attitude and navigate by visual references. That’ll depend on ceiling and visibility, of course, but also to some degree, it’ll depend on the kind of terrain over which we’re operating. A better term for these conditions is VMC, or visual meteorological conditions. We obviously care about the regulatory criteria, but what’s equally important is whether we can see well enough to operate safely. When someone uses the term “VFR weather,” what comes to your mind? I would bet that most pilots don’t visualize three miles’ visibility and overcast at 1,100 feet. Yet those are the conditions in which we can legally operate throughout much of the U.S. airspace.

Unfortunately, most pilots are never exposed to true marginal weather during their flight training. Flight in three or four miles’ visibility can be very disorienting, and the perspective from 500 or 600 feet AGL presents a very different view and a different set of challenges than flight at 5,000 feet AGL.

Consider the conditions encountered flying on the East Coast in the heat of summer, or Southern California, when visibilities often are less than three or four miles. Whether they know it or not, most pilots are relying much more heavily on their instruments than they may realize. Even though conditions may be above basic VFR, the failure of a GPS, a vacuum failure or electrical failure could easily precipitate a very real emergency, or at least some uncomfortable moments. The development of powerful GPS systems has made pilots perhaps braver than they might be if they were navigating by pilotage. All we have to do is follow that little pink line, right? But GPS units do fail.

Managing Weather
What can the prudent pilot do to improve his or her weather savvy? First, pilots need to better understand the definitions used in weather forecasts. For example, in a forecast, what does the term “occasional” really mean, as opposed to "frequent" or "intermittent?" An understanding of the definitions and basic forecast terminology is essential to successfully understanding the available weather products. Know how to accurately read and interpret forecasts, because you’re going to fly in the weather they describe.

Take advantage of the many excellent weather tools available via the Internet. The National Weather Service offers powerful weather-evaluation tools on its website Aviation Digital Data Service, at There are many other free and pay-as-you-go weather tools out there. Find one you like and use it regularly. Learn everything you can about meteorology and forecasting, so that you can become a better evaluator of forecasts and a better forecaster in your own right.

When planning for a flight, determine what the current conditions are, and what they’re most likely to become, over the route of flight and the time frame covered by the flight. What are the weather trends? What’s the next significant weather feature headed that way? When is that next system expected to arrive over the route?


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