Wx Watch: A Perfect Ice Flight

Why on-top rules

February 1, 2006

With a title like this, you're certainly wondering if anything to do with flying around ice can be called "perfect." And strictly speaking, you'd be right. A safety-conscious pilot would never knowingly fly in icing conditions, and we all know why: Even small ice accretions can cause significant decreases in lift, compromised handling characteristics, and increases in stall speed, as well as ruin your ability to climb. But if you grounded yourself every time you learned of an icing airmet, chances are you'd never fly on any cloudy day this winter. There are ways of dealing with ice, however, and in-flight strategies that will allow you to make the most of your winter flying objectives. All of them hinge on having the right set of weather conditions, the right pilot qualifications and experience level, the right attitude toward the preflight briefing, and the right in-flight decision-making savvy. As you examine all these variables, bear one overriding objective in mind — avoid icing conditions. Notice that I didn't say to stop flying the moment a layer of ice-laden clouds crops up. Instead, I'm saying to be persnickety about how you plan to fly around any icing situations. If you can do that — voilà — you have your "perfect" ice flight. We're basically talking about staying in visual meteorological conditions — even if you're planning a flight under instrument flight rules.

It all starts with the preflight briefing. Here you need to be able to identify weather setups that are definitely not conducive to ice-free cross-country flying, and those that are. Then you need to factor in the geographic elements of the flight, as well as your airplane's capabilities.

To keep your distance from icing clouds, you want high cloud bases and low cloud tops. What constitutes "high" and "low"? It all depends on the terrain — and your airplane. The combination of low clouds, a freezing level at the surface, and high terrain is a definite no-no. You don't want to attempt a low-level flight weaving around obstacles while all the time an icy overcast prevents you from climbing. As for tops, you want them, well, toppable. And here's where your airplane comes into the picture. Are you flying an airplane with 100 or 300 horsepower? Normally aspirated or turbocharged? (For the purpose of this article, we're assuming your airplane is not certified for flight into known icing.) More power means faster climbs to altitude, so the more power the better.

As you scan METARs, TAFs, airmets, sigmets, pireps, and the area forecast, bear in mind that a perfect flight demands that any cloud layers be either few or scattered in coverage. Few, as you recall from ground school, means that less than one-eighth to one-fourth of the sky is covered; scattered means coverage of three-eighths to one-half of the sky.

Another requisite is a solid forecast of good VFR weather at your departure airport — no less than a 3,000-foot scattered lower layer with visibilities no less than five statute miles. This is your out. You don't want to fly halfway through your trip, encounter rising tops or other deteriorating weather, and not have a nearly cloud-free airport waiting for you after you do a 180-degree turn.

Lots of blue sky is a must. Any METARs or TAFs mentioning broken (five-eighth to seven-eighth coverage) skies — anywhere along the route of flight — mean a no-go decision. There may not be enough margin for a cloud-free climb or descent through these kinds of layers. Of course, if you're just planning a local flight, you can ease up a bit on the sky-cover restriction, as long as you have good basic VMC weather.

Although considered a supplemental source of preflight information, it's a good idea to check the Aviation Digital Data Service (ADDS) for its icing information. By clicking on the "bases" or "tops" icons you can quickly see where trouble begins and ends along your proposed route of flight. Freezing-level graphics and icing airmets also appear on the same Web page. Just remember to double-check the valid times of any imagery.

Other preflight considerations call for an evaluation of the terrain you'll be flying over. Mountain ranges and nearby large bodies of water generate large areas of deep icing layers, and must be carefully evaluated before attempting to fly over them, especially if any fronts or lows are close by. Lifting is always prevalent over mountainous areas, so any clouds with icing potential are apt to have tops that can rise to your cruising altitude.

Higher minimum safe altitudes (MSAs) or minimum en route altitudes (MEAs) are also the rule over high terrain, and this is another mitigating factor. With cloud bases at or near the mountaintops and minimum altitudes thousands of feet higher, you lose the option of descending to cloud- or ice-free conditions because rocks await you. There are fewer suitable airports that could serve as safe havens, too.

New England, the Great Lakes, and the Pacific Northwest are other trouble spots. These areas produce clouds that soak up a lot of moisture and generate dangerous clear and large-droplet icing. That's all the more reason for sticking with the few-to-scattered formula advanced here.

Finally, there's the human factor. If you're a VFR-only pilot, have you flown cross-countries in on-top conditions? Do you feel comfortable about it? If push comes to shove, could you call air traffic control for help and perform an instrument letdown through rising clouds to clearer conditions below? If you answered yes to these questions, then you're a better candidate for this kind of ice-avoidance flying. If not, get some experience with someone who has more time flying in and/or among the clouds.

Instrument-rated and -current pilots have more options if the weather falls apart and a descent on instruments — or an instrument approach — is the only way out of an inadvertent icing encounter. Just remember that the airplane doesn't know whether you're instrument-rated or not, and even the best of pilots, and the most capable of airplanes, have been felled by dwelling too long in icing.

That's why all pilots of light piston singles and twins should always try to fly in "perfect" winter weather. With high cloud bases and low tops, plenty of space between clouds, no angry fronts, and lots of altitude, you can nearly always safely transit areas singled out in icing airmets.


E-mail the author at tom.horne@aopa.org.


Links to additional information about winter flying may be found on AOPA Online.

Thomas A. Horne

Thomas A. Horne | AOPA Pilot Editor at Large, AOPA

AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.