In last month's " WxWatch" we discussed the basics of icing, including the various types of icing formations and the conditions under which they form. In this second installment it's time to take action. Action, that is, to avoid icing in the first place, or escape it should you become inadvertently caught in icing conditions.
Icing effects
Let's get this straight: There's no such thing as "a little ice." Even small amounts—less than a fingernail's thickness—can cut climb rates by as much as 300 fpm. Stall speed goes up and lift characteristics can change radically. Stall speed increases, and an airplane with normally benign behavior in a stall can snap over into a spin with a layer of ice. It all depends on the type of airplane you're flying and the shape of the ice accretion.
Sometimes, the stall of an iced-up airplane is more of a high-sink-rate mush. One minute you're flying well, the next you're pegging the vertical velocity indicator. Obviously, this is a serious danger on approach, or any time you're close to terrain or obstacles.
Ice accretions affect both lift and drag. Ice forming on the top of a wing degrades lift. Ice forming beneath the wing is more of a drag-producer. Put them both together and you've got trouble.
As for the shape of the ice accretion, the double-horn formation can cause the most severe, abrupt losses of lift during a stall. Can because researchers have learned that even tame-looking, small ridges of ice can degrade airfoil performance much more than large, double-horn accretions. In wind-tunnel tests simulating the weather conditions in the ATR–72 crash, it was learned that a spanwise half-inch protrusion of ice aft of the leading edge deice boots caused greater loss of lift than much larger accretions.
So airplane behavior in ice is an intricate, highly idiosyncratic thing that depends on a wide range of variables: airfoil shape, temperature, droplet size, water content of clouds, and geographic location. Airplanes certified for flight in known icing have to fly in conditions that best reflect the icing environment as we know it. But, as we implied earlier, we probably don't know nearly as much as we should about the icing environment.
Avoidance
That's why avoidance of icing conditions is so vital. Even pilots of airplanes certified for flight in known icing must obey this rule. Yes, deice boots may buy you some time while you figure out an escape plan, but they won't let you mill around indefinitely in icing conditions. There's a saying: If you stay in light icing long enough, it will become severe. And severe icing is defined as icing that ice protection equipment can't handle.
Preflight strategies
There are several sources of information you should use to learn about any icing problems you could face on your proposed flight. Your briefer or other weather service provider should provide them up front, but in case they forget, here they are:
Airmet Zulu. These airmets focus solely on icing issues. They tell the location, intensity, and type of any expected icing conditions. Freezing levels are included, and if the freezing level is sloping or lowering that will be mentioned, too. You should also learn about the bases and tops of any ice-bearing clouds.
Sigmets. Any chance of severe icing is mentioned in sigmets.
METARs. These can provide information about freezing conditions at the surface, and tell of freezing rain, drizzle, or snow. If it's snowing, the liquid water content of the air mass is being drained from the clouds, so icing is less probable.
Area forecasts. These give a synoptic overview of the weather situation, mention any areas of icing, and refer you to any airmets or sigmets that could affect your route.
Significant weather prognosis charts. The "prog" charts can give you advance, graphical warning of the freezing levels, areas of cloud and instrument conditions, and the synoptic situation predicted for your route of flight.
Constant pressure charts. Station models on these charts can provide some information on the chance of icing. If a station model's central circle is darkened, this indicates a close temperature-dew point spread aloft, and therefore the chance of clouds. If freezing temperatures coincide with this symbol, icing may be a problem at the altitude depicted.
Pireps. Nice if you can get 'em, but light-airplane pilots who fly in icing conditions are reluctant to give reports of their experiences. There is a fear of an enforcement action in cases where the airplane isn't approved for flight in known icing conditions. That's too bad, because we need all the information we can get about flying in low-altitude airspace during the winter months. Pireps can verify if predicted icing really came true, and give all of us near-real-time (check the date and time of the pirep) icing reports.
The Web. The National Weather Service's Aviation Weather Center posts an experimental site that you can use for information purposes. It doesn't count as part of a formal, legal preflight weather briefing of the type you receive from Flight Service. Visit the Web site ( www.awc-kc.noaa.gov/awc/nnice.html).
In-flight strategies
Apart from avoidance—which is the prime directive, even if it means staying on the ground—it's important to bear the following in mind when you fly near icing, whether you're equipped with ice protection devices or not:
Keep an eye on the OAT gauge. Too often neglected in the scan, the outside air temperature gauge is a critical instrument. If the temperature is between zero and minus 20 degrees Celsius (or worse, right around zero degrees Celsius) and you're in cloud, then it's time to change altitude to warmer temperatures or clear air. Remember that the worst ice happens right around zero degrees C.
Watch small-radius objects. OAT probes, rivet heads, windshield wipers, antennas—all these and any other small protrusions into the relative wind will collect ice way before it shows up on the wings. See ice on these, and you've been warned: You're beginning to ice up.
Mind your MEA. You want plenty of margin between you and terra firma if you encounter ice. If your route has a high minimum en route altitude and you start picking up ice, you're stuck. If you're going to descend, you want to descend into above-freezing air and benign terrain.
Get on top if you can. The best strategy is to get on top of any cloud layers early in the flight and stay there as long as you can. For the descent to your destination, you've made sure that there are relatively thin cloud layers and high bases for the approach, right? For VFR-only pilots, it will be necessary, of course, to maintain VFR through any climbs or descents.
Don't press on into a well-defined front or low. You may be the best instrument pilot around, but that won't get you anywhere when the ice starts to build and your airspeed begins falling off. Play it safe and land or turn around if ice begins building. It will probably become much worse as you approach the front or low—especially if you're heading toward the northeast quadrant of a low-pressure system.
Night IMC? Forget it. You can't see clouds as well, and can't do as good a job at viewing any ice buildups. If a VFR landing or instrument approach is required, you'll have a tougher job in reduced visibility. With terrain clearance a factor, any ice-induced sink rates could be catastrophic.
Climb through freezing rain? Studies have shown that layers of freezing rain can be as thin as 500 to 1,000 feet, but don't count on a safe climb to clear conditions. You're probably in a warm front, and clouds and freezing temperatures may still be above you. You may get out of the freezing rain, but assuming you're lucky and are in above-freezing air, it could be a long time before the ice sublimates away or melts off. Climbing in freezing rain is one thing if you're flying a jet with bleed-air anti-ice—it's quite another if you're in a piston single or twin, as we're assuming in this article.
OK, you've got ice
Let's say you've inadvertently entered icing conditions. What now?
Turn on all your ice protection equipment, if you haven't already. Equipment such as pitot heat, propeller heat, carburetor heat, or alternate engine air should already be on prior to entering any clouds with icing potential. If you have deice boots and windshield heat, lucky you—but don't count your blessings until you're safely on the ground.
Talk about it. Tell ATC of your predicament and ask for help, even if it means declaring an emergency. You need help in the form of vectors to better weather or a landing at the nearest suitable airport. That would be one with long, wide runways; good lighting; and an ILS approach.
Turn around—if you've come from an ice-free area.
Have engine vibrations or a rough-running engine? It could be propeller ice, causing imbalances in the propeller. Cycling the propeller from high rpm to low and back again, several times, may cause ice to break free and may solve your problem—for now.
Warm air and good terrain clearance below? Then descend while you still have enough control to do so safely.
Don't try to climb to on-top conditions. Unless, that is, you're sure that tops are only a few feet above you. If you've got a thousand or more feet to go and have a load of ice, you might stall before getting on top.
Don't linger in cloud tops. The largest water droplets and highest liquid water contents are in the upper regions of cloud layers. If you're going to climb to a higher altitude, climb clear of cloud tops.
Carry extra airspeed and minimal—if any—flap settings for landing. Bear in mind the problems mentioned earlier concerning tailplane stalls. Minimize pitch changes, and keep power up and flaps retracted to help prevent any stalls. If your windshield is iced up and you don't have windshield heat, you can try opening your storm window or side window for a glimpse at the runway or its environs. In some airplanes, full defroster heat can also help melt some ice from the windshield. Slipping the airplane for a better view is questionable in an iced-up airplane—you could be setting yourself up for a stall.
So what's the final answer for dealing with ice? Avoidance is best, of course, but the prime directive if you enter icing conditions is to do everything you can to safely exit them, and land as soon—and as safely—as you can.
Links to additional articles on weather-related topics can be found on AOPA Online ( www.aopa.org/pilot/links/links0011.shtml). E-mail the author at [email protected].
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