Does freezing rain really warrant all this concern? You bet it does, although accident statistics don't paint an absolutely grim picture. A quick search of the National Transportation Safety Board's database from 1983 to 1999 shows a total of 79 accident briefs mentioning freezing rain. Of that number, 20 accidents involved fatalities. Though some of these accident summaries are preliminary, that's still about a 25 percent fatality rate-and unacceptable odds by any measure.
By contrast, plain-Jane airframe icing was a factor in five percent of the 5,894 total weather-related accidents between 1983 and 1992, but was mentioned as a factor in 43 percent of the 1,750 fatal weather-related accidents in that time frame.
So if freezing rain is so deadly, why isn't it more heavily represented in the statistics? Maybe it's because the accident reports were incomplete in the weather department, or the reports lumped freezing rain in with "airframe icing," without any further elaboration. Of course, part of the reason is that the deceased pilots obviously weren't able to provide any details of their icing encounters. Another reason is that accident investigators often are limited to using surface reports-er, sorry, METARs-and indirect observations (i.e., radar, balloon soundings) or distant pireps as sources of weather information near the accident site. So we often don't know the exact weather conditions aloft when an airplane runs into icing conditions-or other adverse weather-and an accident ensues.
There are two other big reasons why freezing rain may not crop up in accidenLocked within the deep recesses of our pilot brains reside all sorts of weather-related fears. For some, those fears are strong enough to prevent a takeoff-or bring about a landing-when instrument meteorological conditions, marginal VFR conditions, or other types of lousy weather are lurking. For noninstrument-rated and/or low-time pilots that kind of fear can amount to a survival instinct.
But for those of us who expect to deal with weather this coming winter, there's a more specific fear-a fear that somehow we'll blunder into freezing rain. As we've been taught from our earliest weather classes, freezing rain is the absolute worst type of icing condition. It can cover an airplane in seconds, robbing wings of lift, covering windshields, blocking airflow to engines, and adding gobs of weight to the airframe. Worst of all, we're told, freezing rain can strike in cloud-free air.
Does freezing rain really warrant all this concern? You bet it does, although accident statistics don't paint an absolutely grim picture. A quick search of the National Transportation Safety Board's database from 1983 to 1999 shows a total of 79 accident briefs mentioning freezing rain. Of that number, 20 accidents involved fatalities. Though some of these accident summaries are preliminary, that's still about a 25 percent fatality rate-and unacceptable odds by any measure.
By contrast, plain-Jane airframe icing was a factor in five percent of the 5,894 total weather-related accidents between 1983 and 1992, but was mentioned as a factor in 43 percent of the 1,750 fatal weather-related accidents in that time frame.
So if freezing rain is so deadly, why isn't it more heavily represented in the statistics? Maybe it's because the accident reports were incomplete in the weather department, or the reports lumped freezing rain in with "airframe icing," without any further elaboration. Of course, part of the reason is that the deceased pilots obviously weren't able to provide any details of their icing encounters. Another reason is that accident investigators often are limited to using surface reports-er, sorry, METARs-and indirect observations (i.e., radar, balloon soundings) or distant pireps as sources of weather information near the accident site. So we often don't know the exact weather conditions aloft when an airplane runs into icing conditions-or other adverse weather-and an accident ensues.
There are two other big reasons why freezing rain may not crop up in accident reports. One is that most pilots have the good sense to stay on the ground when it's forecast. The other is its comparative rarity, due to its extreme sensitivity to atmospheric variables.
Making ZR
What causes freezing rain? To be brutally brief, the cause is precipitation falling from a layer of warm air, then being chilled to supercooled status as it falls through another layer of air at or below 0 degrees Celsius.
Ask yourself: What weather phenomenon features warm, moist air aloft, dropping rain through colder air? If you answered "a warm front," then you're correct. As winter warm fronts ride up and over the retreating colder air masses ahead of them, they become perfect setups for freezing rain scenarios. What starts as snow, ice crystals, or a cold rain falling from a warm front's advancing surface aloft can become nice, huge, supercooled raindrops by the time they fall from that surface and into the cold front's subfreezing air mass.
These drops are at freezing temperatures, but they're still in liquid form, which is the definition of supercooling. The drops remain liquid until they strike an object with a subfreezing temperature-like the skin of your airplane.
That's when the trouble begins. Those giant supercooled drops splatter, run back a great distance, then instantly freeze, coating large portions of the airframe with a tenacious clear ice. Even airplanes with deice boots and certification for flight into known icing conditions can have a difficult time breaking this kind of ice from their leading edges.
It's important to emphasize that freezing rain is not the same as the more common clear icing conditions. With garden-variety clear ice, there is not as much runback, and while its accretion rate can be alarmingly quick, it's nothing like a bout with freezing rain. Yes, freezing rain is a type of clear ice, but its occurrence and buildup rate make it a much different-and more dangerous-animal.
Where it occurs
Most pilots have mental pictures of fronts that are heavily influenced by textbook drawings of frontal cross-sections. So we tend to think of warm fronts as having these nice, unwavering, gradual slopes. That may help us to understand the basics of frontal structure, but in nature things aren't so well-ordered. The reality is that frontal surfaces aren't always as well-defined as the ones shown in the textbooks.
For example, a closeup view of an advancing warm front may not show a completely smooth surface. Instead, it can advance as blobs or tendrils of warm air aloft. (It's the same thing with stationary fronts, which have mini-cold and -warm fronts along their length.) Think of these as layers or projections of warm air intruding into the retreating cold air mass ahead of them. It's these layers that can generate the precipitation that falls into the cold air below them.
Because these projecting layers can be small-scale and very localized, so is freezing rain. An airplane in one area of a frontal zone may not experience any precipitation at all. Another airplane two miles away could run into freezing rain. It all depends on whether any precipitation is falling from a particular area.
Temperature is a critical variable. In many cases, a difference of just one or two degrees can mean the difference between ice pellets and freezing rain. Freezing rain is most likely to occur at outside air temperatures around the freezing mark- minus 5 to 0 degrees Celsius. Any colder, and the rain droplets can be frozen solid, creating ice pellets. In fact, minor temperature fluctuations are a major reason that the onset of freezing rain is so difficult to predict accurately.
In clear air?
Yes, freezing rain can occur in clear air. This might come as quite a shock to VFR-only pilots out flying on a nice winter day. If that winter day includes a warm or stationary front with an overcast layer, then freezing rain is a possibility. The air may be clear beneath that overcast, and you may be flying in perfectly legal VFR conditions, but if that overcast is shedding rain, and the OAT is hovering close to 0 Celsius, don't be surprised if you run into freezing rain. You may encounter it only for a brief time, but it doesn't take long for freezing rain to wreak havoc.
Survival strategies
The standard, timeworn advice is to climb if you encounter freezing rain. The theory is that you'll reach warmer air-the air of the warm front aloft. That's where the rainfall that causes all this trouble begins, and it is where balmier temperatures will surely melt any ice accretions.
This advice is good in theory alone. I suppose that if you were in an F-16 you could maintain the performance necessary to climb several thousand feet. But in a light general aviation airplane? I don't think so. Above-freezing temperatures may be too far away. You might begin a sickly climb, but as ice builds, airspeed and lift will surely bleed off in a big, big hurry.
Is it possible that warmer air could be just 200 feet above you? Of course. As we've mentioned earlier, freezing rain can be extremely localized in extent-both horizontally and vertically. But what are the odds? Even if recent pireps from your area promise nearby warm air, conditions may have changed by the time your airplane passes through.
The only real alternative is to alter your course and prepare for a landing at the nearest suitable airport. That, or perform a precautionary landing before an ice-induced loss of control occurs. As with any other type of icing emergency or precautionary landing, you'll want to do the following:
- Advise air traffic control of your position and intentions. Depending on the situation, make an emergency call on 121.5 MHz and squawk 7700 on your transponder.
- To prevent a loss of engine power, immediately activate carburetor heat (carburetor-equipped engines) or alternate sources of engine air (fuel-injected engines).
- Freezing rain can block air filters and screens in short order.
- On approach, keep your airspeed up. This can help provide a margin above your now-increased stall speed.
- Do not extend the flaps. This could blanket the airflow over the elevator and cause a tailplane stall.
- Be aware of any elevator control pulsing, abnormal nose-down trim changes, loss of elevator effectiveness, or sudden elevator force changes. These are signs of a tailplane stall. To combat a tailplane stall, reduce your flap setting (if you extended the flaps), increase power and airspeed, and make nose-down pitch changes slowly. If the airplane is equipped with deice boots, inflate them several times to try to clear ice from the horizontal stabilizers' leading edges. If the windshield is obscured by ice, open a side window or storm window to help you see the runway or landing area.
- Keep the power and airspeed up right to touchdown. Do not attempt to flare or a stall may occur.
Forewarnings
What should trigger a freezing rain alert? METARs, TAFs, area forecasts, sigmets, airmets, pireps, and low-level significant weather prognosis (prog) charts can help to identify areas likely to be hit by freezing rain. Flight service station briefers and DUATS providers can also give warnings of freezing rain as well as information on general icing conditions.
Even more information is available on the Internet. The National Weather Service's National Center for Environmental Prediction (NCEP) provides the Aviation Weather Center with numerous icing-related products, including graphically presented pireps (http://adds.awc-kc.noaa.gov/projects/adds/index.html), airmets and sigmets on icing and other conditions (www.awc-kc.noaa.gov/awc/awc-airmets.html), and graphic icing forecasts for many layers of the atmosphere, based on numerical forecasting models (www.awc-kc.noaa.gov/awc/nnice.html). An experimental icing forecast model under development at the National Center for Atmospheric Research (NCAR) goes as far as to predict the horizontal and vertical extent of any super-large-droplet icing conditions (www.rap.ucar.edu/largedrop/integrated/index.html). This is of interest because large-droplet icing conditions are conducive to freezing rain.
Surface analysis and winds and temperature aloft charts can also help in predicting freezing rain. Look to the north and east of any surface warm front, or along any stationary fronts, to begin some rudimentary forecasting of your own.
Then, check the temperatures aloft at several altitudes in these levels. If above-freezing temperatures are stacked atop altitudes with temperatures near 0 Celsius and there are cloudy skies, that's fair warning.
What about inflight warnings? Here, your best friend is the outside air temperature (OAT) gauge. If it's at-or just below-the freezing mark, be prepared to execute your escape plan should ice form. Of course, you've planned your flight so well that you know above-freezing temperatures are below you, and above any minimum enroute altitudes. You also know that you can make a 180-degree turn to more favorable conditions. If you don't have these assurances, then maybe you'll sample some primal fear.
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