As mentioned last month, I nearly always flight-plan for higher altitude in winter for a number of reasons. Navigational radio range is extended at taller heights, and that may be important when a blanket of snow obscures VFR ground reference points. For the same reason, higher provides a better pad of altitude in case you need to look for a place to park it in an emergency. Much winter weather is low-lying, so flying tall also gives you a better chance of clearing the clouds.
In-flight icing is a hazard all its own, one that has generated a plethora of articles, so we can hardly hope to fully cover the subject here. Icing is perhaps the most publicized hazard of winter flight, but it’s less common than you might imagine. Temperature, pressure, humidity and ionization all have to be right to produce airframe ice. When they are, though, the result isn’t fun.
Snow may look nasty to fly in, and it does reduce visibility, but it’s generally not much of a problem in flight, as it rarely adheres to aircraft surfaces enough to distort an airfoil. Conversely, true icing may occur over a much wider range of temperatures than you might imagine. While most icing happens between about zero and -10 degrees C, I’ve seen it at -20 degrees C. Colder temperatures generally don’t allow icing since there’s little humidity present and, therefore, poor adhesion.
There are two types of ice—clear and rime. The first normally is associated with vertical clouds such as cumulus. It’s a denser form of ice, so it’s tougher to break loose, and its temperature is most likely between about zero and -5 degrees C. Rime is the more common, garden-variety form—a chalky, more aerated, less cohesive collection of frozen water that’s found in flatter, stratus clouds over a wider range of temperature. Because of its airy composition, rime is easier to sublimate or break away.
Freezing rain is a phenomenon you’d better hope you never encounter, as it can accrete at an amazing rate. As the name implies, freezing rain results from an inversion when rain falls through sub-zero air below. Freezing rain isn’t limited to winter. I’ve seen it in all seasons, including in August over Libreville, Gabon, hard by the equator.
Remember that your primary instrument in predicting icing, the ever-popular, refrigerator-style outside air temperature (OAT) gauge, is probably the second least accurate instrument in the cockpit, right behind the airspeed indicator. Even if OAT is correct, the temperature inside a cloud may be dramatically different from that in the clear.
If you challenge the clouds in winter and encounter airframe icing, the best escape route may be up rather than down, as climbing may take you above the clouds completely. Also, remember that if you climb and things only get worse, you can always come back down. If you descend first and pick up more ice, you may lose the option of climbing out the top.
Keep in mind, however, that the tops of cumulus clouds are where the ice gods hold their parties. Some of the highest rates of ice accretion are found in cloud tops. I once had an Aerostar turn into an instant popsicle over the Gulf of Mexico at -20 degrees C and 19,000 feet when I was too stupid/lazy to circumnavigate a cumulus top. Unless you just naturally enjoy having the windshield become totally opaque in about five seconds, always elect to deviate around cloud tops rather than flying bravely straight through them.
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