Of Virgil and Ice
Virgil was at his wits' end. Gone was
his confidence, his euphoria, his peace of mind, and most of
his options for keeping his ice-laden airplane flying
through the cloudy and turbulent summertime air.
In his attempt to get home for the weekend–for he was a dedicated husband and father–he had skimmed rapidly through the weather reports. He noticed, but did not spend much time contemplating the forecast for icing conditions. He knew weather forecasters were able to identify regions or even large geographic areas in which icing is possible, and realized also they cannot define the exact localities or altitudes; not declaring there will be icing, only making a disclaimer that conditions are conducive to airframe icing.
He knew too, whenever there are clouds in the sky and the temperature is near or below freezing, icing will be forecast. Many times, with these same conditions, he ventured out and hadn't encountered a trace of the forecast ice.
Now here he was over the rough and unforgiving mountains of western Wyoming–flying at the minimum en route altitude of 14,000 feet–and because of his apprehension, gasping for oxygen from a mask impressing grooves in the side of his face.
Hearing the sickening sound of wet snow slapping against and adhering to the windshield, he sucked in an extra deep breath and become momentarily dizzy and disoriented. The engine too, commenced snorting and wheezing, being choked by the iced-over air filter. Without an alternate air system, carburetor heat provided only a minimum amount of unfiltered air from around the engine compartment. Even so, the enriched mixture and loss of ram-air effect reduced the power output to the critical point.
The ice accretion caused the airspeed to slowly deteriorate while he struggled to maintain altitude. Finally, to avoid stalling, when the airspeed reached 80 knots indicated, Virgil began a slow drift-down from his assigned altitude.
Virgil was not a religious man, but he began to pray. And, someone must have heard him, for he unexpectedly broke out of the cumulus clouds and saw the airport dead ahead. With a sign of relief he called Salt Lake center and cancelled IFR, not wanting to risk a further encounter with any other clouds while executing the instrument approach.
It was a sleepless night for Virgil. Thoughts of what could have happened tortured his mind. He was enough of a realist to castigate himself, and not the weather briefer, for getting into this defenseless situation from which he barely escaped. He cussed himself for not maintaining documentation of the weather briefing so he could compare what happened to what he thought would happen.
Early the next morning, filled with trepidation, he drove to the airport. It was his hope that he could catch Tom, his flight instructor for many years, during a slack period in his schedule and talk about what happened.
He found Tom—a young, dedicated man who chose flight instruction as his career—in the pilot lounge nursing his morning coffee before starting work. Virgil approached and began, "Tom, you've known me for years. And you know I don't gamble when it comes to flying, but yesterday I had a frightening experience." Tom listened to Virgil's recount of his harrowing flight, and said:
"Virgil, you learned in ground school that icing forecasts are found in area forecasts and in-flight advisories; but these are subjective with a tendency of being on the conservative side. The only way you can obtain evidence that icing conditions really exist is to check the PIREPs. But you have to be careful with these too, because they are ambiguous. For example, what might be moderate ice to a pilot of one type airplane may only be a trace to another. Or it could be heavy ice to yet another ..."
Virgil interrupted, "I know that, I haven't forgotten everything you taught me. But, what did I do wrong? Or, more importantly, what could I have done differently?"
Tom pitched in, "Your evaluation of the potential for ice begins with the weather briefing. It should concentrate on finding the altitudes containing moisture, the temperature at those altitudes, and the stability of the air. Ice accretion is caused by and depends on liquid water content in the cloud, water droplet size, temperature, airfoil shape and aircraft speed ..."
Virgil was becoming impatient. "Tom, could you run through the way you would go about checking the weather? I remember you telling me all this before, but something is missing. Maybe I could pick it out if you were to check the weather and talk out loud to yourself during your evaluation."
"That's a good idea. Let's run over to flight service. The old method of calling them by telephone or even using my computer to access DUATS, which seemed adequate in the past, makes it difficult to make an intelligent evaluation of icing potential based on a bunch of METARs, a handful of TAFs and a description of the surface weather map. You really need access to those newfangled weather charts to compare the icing potential to the aircraft performance and anti-icing or deicing equipment capability."
Walking to the FSS building, Virgil commented, "Looking at these heavy, dark, moisture-laden cumulus clouds, I wouldn't venture out today. I know well-defined edges like these indicate a high liquid-moisture content in the cloud; whereas soft, fuzzy edges show the cloud is mostly composed of ice crystals. These seem to reek of supercooled water droplets and ice. What do you think?"
Tom said, "Visual appearances are often deceiving," and began checking the surface weather analysis and weather depiction chart. "It is important to see what is happening, where it is occurring, and what is causing it. Two fundamental requirements exist for structural ice formation; visible water and temperature.
Visible water can take the form of rain, wet snow or droplets within the cloud. When these liquid droplets strike the airplane, their temperature or the temperature of the aircraft must be at or below freezing for ice to form. The type of icing will depend on water-droplet size and temperature. Most of the icing you encounter will occur between 0 and –20 degrees Celsius, but the worst icing, statistically speaking, is between 0 and –10 degrees Celsius. Lifting action can intensify the amount of moisture, so I look for something which would cause this lifting action, either meteorological, caused by weather systems, or orographic, as a result of the wind pushing air over topographical features.
Next I look at the constant pressure charts to pin point the location of moisture and the temperature where this moisture is concentrated. We know there will be moisture at those altitudes having a dew point depression fewer than four degrees and an icing problem at temperatures between 0 and –20."
Virgil recalled being told about the constant pressure charts and exclaimed, "Now I remember. You look for the station circles. Whenever the temperature-dew point spread is five degrees Celsius or less, these circles are shaded. Boy! I completely ignored this chart yesterday."
"When you see those shaded circles you know the possibility of cloudiness and precipitation exists at this altitude. If the temperature is on target, you are able to accurately predict ice from this information. I have always considered the constant pressure charts to be among the most important to look at. They are observed data, not forecast conditions. And, often a large scale wind flow around a low aloft may spread cloudiness, low ceilings, and precipitation far more extensively than indicated by the surface map alone.
I don't want you to think you can ignore the other reports and forecasts. Use these constant pressure charts in your evaluation, but don't bet your life on them; they report weather that has already happened, and the weather can change. It's important to compare the synopsis, turbulence and icing sections of the area forecast to these charts. And compare the winds aloft forecast to relate the forecast winds and temperatures to the actual weather that has occurred on the constant pressure charts. Look, too, at the surface aviation weather reports (hourly sequence or METARs), not just for your en route, destination and alternate airports; but at stations in the direction of any incoming weather. Obtain TAFs and check tops from area forecasts and PIREPs. And finally, check the AIRMETS AND SIGMETS."
Back at the FBO, sipping hot coffee, Virgil said, "Tom, I've got a pretty good idea of how to detect the potential for icing now, but what else should I be aware of?"
"Virgil," signed Tom, "you're not unlike a lot of pilots who have heard the cry of "wolf" so many times that you begin to disbelieve or ignore the icing forecasts. Having recalled now how to check the weather you should also keep in mind that you have to compare the MEA to the aircraft performance capabilities. It is preferable to fly when the forecast freezing level is at least 3,000 feet above the MEA. Forecasters aren't accurate enough for you to hang all your hopes of survival on their guess of the freezing level. A good technique is to file a couple of thousand feet above the MEA, with the freezing level above you. Then, if you encounter ice you have some maneuvering room below you. If you fly the MEA and pick up ice, you've run out of options.
With your normally aspirated Cessna 180, find low-altitude routes that detour areas with icing potential. On departure, ask ATC for a VFR climb or a deviation to avoid clouds if you suspect ice. Realize that over mountainous terrain the heaviest icing will be within 5,000 feet of the ridge tops. Expect heavy ceiling when penetrating any mountain wave that contains moisture. And when the temperature is near or below freezing, you will get glaze or rime ice within 1,000 feet of the top of cumulus clouds.
Another thing, never takeoff if sleet (ice pellets) occur at the surface. It is a guarantee that you will run into freezing rain aloft. In your little airplane, on those days when the forecast is for light-to-moderate icing and your weather check confirms this possibility, do not depart unless VFR conditions exist under the clouds along the entire route of flight and the freezing level is at least 2,000 feet above the surface along the entire route."
That night Virgil slept like a log.