December 1, 2002
By Thomas A. Horne
Planning to fly this winter? Of course you are. There's just one problem, though, and it's a big one — icing. Those needing a quick and reliable guide to flying around ice need to keep three main issues in mind before takeoff: the pilot, the weather environment, and the airplane.
VFR-only pilots facing deep and extensive cloud masses along their proposed route of flight are severely limited. By regulation, noninstrument-rated pilots must avoid clouds by prescribed margins. And common sense dictates that no pilot ever knowingly fly into a cloud with icing potential. So if you don't have an instrument rating — or if you do and your instrument flying skills are rusty — then tackling winter weather is pretty much out of the question.
This doesn't mean that VFR-only pilots are grounded whenever there's a cloud in the sky. If you can keep your distance, then flying around winter clouds — even ice-producing ones — is OK. As long as conditions don't deteriorate.
One trap that awaits unwary VFR pilots lurks ahead of warm fronts. There, in the cloud-free, subfreezing skies beneath the advancing warmer air, rain can fall from the clouds above. This rain becomes supercooled on the way down, setting the stage for an encounter with clear-icing conditions in freezing rain. To the pilot it may look like plain old rain — and you might even have good visibility through it — but it can coat your airplane with ice in seconds. So all pilots should sit up and take note if there's a warm front affecting a flight.
Instrument-rated and -current pilots have many more options in dealing with potential icing situations. That doesn't mean instrument-rated pilots have carte blanche to mill around in icing conditions. It simply means they have the qualifications to fly solely by reference to instruments. Knowing this, the remaining issues center on whether to fly in clouds, and how to minimize exposure to icing conditions should they be encountered. This is where the weather-environment and airplane issues surface.
Some winter weather setups should be avoided, period. In addition to the other items you'd normally scan, here are some warning flags you should look for during your preflight briefing:
If you must cope with potential icing situations ("potential" because the microphysics of icing are not fully understood; one cloud may cause icing, the next may not), here is the ideal situation. Anything varying from this model puts you at greater risk should things go wrong:
What you're flying has a huge bearing on your approach to flying in the icing season. Here are some nice-to-have features:
It's easy to see that minimally equipped general aviation airplanes are not cut out for extended bouts in icing. Maybe not even short bouts. Pitot heat and alternate engine air may be all you've got in your ice-fighting arsenal. That's fighting with a pretty small stick.
Got a well-used instrument rating, halfway decent weather setups, 5nd a well-equipped airplane? Then you've got a fighting chance at working your way over, under, around, or through a cloud system that's an advertised ice-maker. But no matter what kind of pilot you are, or what you fly, the same two cardinal rules hold: Avoid ice at all costs, and escape it at the first sign.
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The four main types of icing are four big reasons why you should always include the outside air temperature (OAT) gauge in your instrument scan. What follows are brutally short descriptions of their characteristics and the temperature ranges they typically occur in. Temperatures are in Celsius because on most OAT gauges these are the primary markings, printed on the outer margins of the scale. Also, the icing ranges (zero to minus 20 degrees C) are easier to remember when you think in Celsius.
Rime ice appears as a narrow, milky line across the airplane's leading edges and builds forward, making a sharp-edged protrusion into the relative wind. It happens mostly in the minus 10-to-minus 20-degree C temperature range, and in stratus clouds. It's a product of small, supercooled (liquid, yet below freezing — until it hits the airframe) cloud droplets.
Clear ice is created by larger droplets of the kind found in cumulus clouds. As the name implies, it's transparent. It occurs in the zero-to-minus 10-degree C range.
Mixed icing is a blend of clear and rime. It can look lumpy and protrude in a double-horn shape. Large-droplet icing is most often the problem in temperatures hovering around the zero-degree C mark.
Freezing rain occurs when rain falls through a subfreezing air mass. Typically, it happens ahead of a warm front, in temperatures just below zero degrees C. — TAH
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.
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