Wx Watch: Icing facts and myths

Truths and consequences

December 1, 2008

Most of us have been around enough wintertime bull sessions to have heard some hair-raising stories of icing encounters. You know, landing with full power. A desperate nighttime descent in an attempt to both preserve airspeed and make it to a runway. A full-power setting that results in a mushing descent instead of a climb. Landing with your vision impaired by an iced-up windshield. Those sorts of experiences can be educational, but there are other sorts of yarns that continue to fuel debates about icing. Call them persistent urban legends of the icing kind. Addressing a few of can help us all become more ice-savvy. So which of the following is myth, and which is fact?

A little rime ice is OK, and actually boosts airfoil performance. Fact, but quickly turns to myth. One expert at NASA’s Glenn Research Center, home of an icing wind tunnel and many icing studies, says that small rime accretions can act like leading-edge slats at low angles of attack, increasing lift and drag. But this minimal benefit is short-lived. Depending on the airfoil and conditions, within two minutes of rime accretion, drag can almost double. Maximum coefficient of lift can decrease by around 30 percent. And the stalling angle of attack can decrease by about six degrees. As for short—say, two-minute—exposures, there is little difference between the effects of clear and rime ice. So the idea that a small ridge of rime ice can delay airflow separation along the chord of a wing, and thus improve lift, is nothing to hang your hat on. Much depends on the airfoil and the nature of the icing conditions, but the blanket advice we’ve all heard holds true: Any amount of ice, however small, is bad for your health.

You should wait until one-quarter-inch of ice (or more) has accumulated before inflating deice boots. Myth. You should inflate deice boots at the first sign of ice. The idea that too-early or too-frequent boot inflation can lead to ice accretions that build over the boots—rendering them useless—has been debunked. In fact, a conference on ice bridging was held in 1997. Surveys were held, case studies were explored, and the many pilots and representatives from the educational and research communities agreed: There have been no actual, documented cases of ice bridging. True, ice can bridge over a failed boot, or boots that have leaks large enough to prevent full inflation. But in cases where functional boots were used in attempts to create ice bridging, ice was shed instead of accumulated. Flight manuals for newer aircraft certified for flight into known icing advise pilots to inflate de-ice boots at the first sign of ice. Some manuals even say to put the boots on an automatic cycle rather than activating them on a one-shot basis.

The origin of the ice-bridging myth is often traced to Ernest K. Gann’s wonderful (and scary) book, Fate Is The Hunter. In it, Gann writes about an experience flying a DC-2 over the Appalachians when the deice boots failed. “Yes, the boots are working,” he writes. “But they are expanding and contracting beneath the sheath of ice and consequently useless! The ice has accumulated too fast for them.”

Icing experts now explain Gann’s predicament by claiming that the inflation pressures and pneumatic pump capacities of the DC-2 are far less than the inflation pressures used today. And besides, Gann says he was flying in freezing rain—a situation in which all bets are off in the ice-shedding game. No airplane is certified for flight in freezing rain, which is classified as severe icing.

You can safely take off with frost on the wings, as long as it’s polished to a smooth surface. Myth. You should only take off with wings and other lifting surfaces completely free of frost. Though perhaps little known, since 1960 the FAA allowed airplanes operated under FAR parts 135 and 125—and certain airplanes operated under FAR Part 91—to take off with frost accumulations that were polished. This could be done via several means. A large swatch of burlap can take the fuzz off of frost, as can a credit card, or even a rope slid along the wing. Makeshift, yes, but it works.

But as of August 2008, the FAA changed its mind about polished ice, and now it’s illegal. The reasoning behind the decision is in a May 8 Notice of Proposed Rulemaking (docket number FAA-2007-29281; Notice No. 08-06). The NPRM cites 11 known accidents in which individuals attempted to polish frost smooth, yet crashed shortly after takeoff. From now on, it’s clean wings and tails for takeoff, period.

NASA Glenn researchers say that it’s leading-edge roughness that dominates frost effects on lift degradation, while roughness on the rest of the wing causes drag increases. Losses in lift can be as high as 30 percent, and 40-percent drag increases can occur at the same time. NASA Glenn also quotes Cessna Aircraft Company as saying its Caravans can experience a 35-percent increase in stall speed with 0.1 inch of frost accumulation.

Climb if you encounter freezing rain. There are above-freezing temperatures above. Dangerous fact! True, warmer temperatures are aloft. That’s how freezing rain happens—it falls as warmer-than-freezing rain from (usually) a warm frontal surface aloft, and then turns into supercooled droplets as it falls into the freezing air beneath. When your airplane comes along, this freezing rain rapidly accumulates as the worst sort of clear icing.

But how far above your altitude is that warmer air? And can your airplane manage a safe climb to reach it? These are critical questions, and answers are hard to come by. If that warm frontal surface is 500 feet above you, and you’re flying a high-performance airplane, you might just make it. But 2,000 feet above, and flying a two-seat trainer? Don’t even think about it. In an F-16, and warmer air 5,000 feet above? Go for it. But the general rule is a good one: Make a 180-degree turn or other maneuver to escape freezing rain. This may even involve a precautionary, off-airport landing. Quickly. Anything except allowing your airplane’s performance to degrade.

You can’t ice up in visual meteorological conditions. Myth. Actually, the worst ice—freezing rain—can occur in clear air. This is a corollary to the dangerous fact we just discussed. Rain may fall from clouds at the frontal surface aloft, but the freezing air below—in the retreating cold air mass being overtaken by the warm front—is frequently cloud free, or populated with scattered to broken cloud layers. So you can fly along VFR, perfectly legal in excellent weather, and still be affected by supercooled rainfall. It’s something to think about whenever a winter warm front is predicted for your route of flight. Be sure to check temperatures aloft, and correlate them with any cloud masses. If you see a temperature inversion accompanied by supersaturated clouds (confirm this by checking satellite and radar imagery), watch out. A layer of subfreezing temperatures and visual conditions beneath (check METARs and TAFs for cloud levels and any mention of precipitation) is a setup for freezing rain in “visual” conditions.

Finally, here are some tried-and-true icing facts that will always hold you in good stead: Stay away from ice where it’s predicted, avoid suspect areas, and escape ice the moment it’s encountered.

PROB, TEMPO, and BECMG

My August 2008 article (“Wx Watch: TAF Tune-up”) used an example of a TAF bearing a “PROB 50” forecast element. Larry Burch, deputy director at the Aviation Weather Center, says this can’t be. PROB 30 (a 30-percent forecast probability) is the highest a PROB group can go; a PROB 40 forecast category was struck in 2004, though it may still appear in certain military and foreign TAFs. “TEMPO implies a 50-percent chance of a weather condition occurring,” says Burch. “And if there is no PROB or TEMPO prefix in a TAF group, then that amounts to a 50- to 100-percent chance of the predicted weather.” In addition, PROB will not be used in the first nine hours of a TAF’s valid time—unless it’s requested by the airlines or the military to make fuel loading decisions in critical weather situations. As for BECMG, it hasn’t been on a U.S. TAF since 2004, when the National Weather Service did away with it. However, the term can appear in foreign and military TAFs—and, I understand, certain pilot knowledge exams.

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

Web site of the month

Model analyses and forecasts

Those wanting a look at several weather variables with forecasts as far as 16 days away can check out this site (www.nco.ncep.noaa.gov/pmb/nwprod/analysis). The site may be over the top for many, what with its many products. But still, it can be a great educational and long-range planning tool. Looping imagery is available for many products, and this alone lets you watch each model’s ideas of what the weather will be. Across the top of the page you’ll see each model’s initial times and dates, with 00Z, 06Z, 12Z, or 18Z being common times. Pick the most recent time, and then select the model and product you want. The models are listed on the left in abbreviated form. “NAM” stands for the North American Mesoscale model, “GFS” means the Global Forecast System model, then comes the “GEFS” (Global Ensemble Forecast System) and other models, including those specific to hurricanes (the “HWRF”—Hurricane Weather Research and Forecasting—being one of them), and one—the “RUC” (Rapid Update Cycle)—which updates at short, one- to three-hour intervals. Medium resolution works best with most browsers. Surf away to check out everything from thickness levels to forecast precipitation amounts to relative humidities aloft, and much, much more. Many products let you choose between surface and upper-level products. —TAH