Wx Watch: Winterwise

Icing tips and traps

December 1, 2002

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.

Warning flags

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:

  • Lows and their attendant fronts entering the Pacific Northwest, the Great Lakes, the areas east of the Rockies and the Appalachians, and the North Carolina coast. These fronts can quickly intensify into major winter storms. Large-droplet icing (the worst kind, because of icing runback aft of airframe leading edges) is common around the Great Lakes and Pacific Northwest.
  • The northeast "corner" of a low pressure system. This is where easterly flows can create clouds with the highest moisture content and the fastest-building ice.
  • Mountainous areas. If your route crosses mountains, watch out. Orographic lifting forces make icing much more likely over mountainous terrain.
  • Airmet Zulu. These airmets pertain to light- to moderate icing conditions and can be found posted alone or in area forecasts. Some feel that these airmets cry wolf too often, but they are still useful as a heads-up. Besides, they're frequently correct. Sigmets warn of severe icing.
  • The Aviation Weather Center's current icing potential (CIP) Web site. This was discussed in last month's "WxWatch" (see " WxWatch: Icing on the Internet," November Pilot, and it provides useful guidance for determining the vertical and horizontal dimensions of conventional and large-droplet icing — even though it's intended for use by meteorologists and dispatchers only. Visit the Web site ( http://cdm.aviationweather.noaa.gov/cip/) to have a look.

What you want

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:

  • Low cloud tops. If you can climb to on-top conditions — and stay that way during the en route cruise phase of flight, bravo. You're ice-free.
  • High ceilings. With low tops and high ceilings, your exposure to potential icing conditions is greatly reduced. Climbs or descents through cloud layers like this take just a few minutes at most, and there won't be any tense instrument approaches.
  • High freezing levels. You want plenty of above-freezing air below you. That way, you know you have an out if you need to descend or if you have to fly an instrument approach in low ceilings. Always remember: You want to lose ice as you descend, not pick more up.
  • Low minimum en route altitudes (MEAs). Again, this gives you more usable altitude in case icing is encountered. The last thing you need is to be cruising at the MEA over mountains when you ice up and are forced to descend into hostile topography.
  • Lots of fuel. Enough to let you divert to an ice-free alternate or circumnavigate a large, ice-bearing mass of clouds. Being low on fuel is bad enough in good weather.
  • Good airports along the route. Look for airports with two or more long, wide runways served by instrument approaches (preferably ILSs), automated or human weather reporting capability, and ideally, a control tower with air traffic control radar. When you're having problems, ATC can be a great help. And knowing that you can let down with precision, quickly, to a nearby airport is a comforting feeling.
  • Day. Those who have iced up at night, far from a decent airport, and over mountainous or rough country know that demons come out at night. Fear can wreck your judgment and can lead to panic. How much better to have the reassurance that comes with the light of day. Besides, you can see any openings or escape routes in the cloud formations around you.

The airplane

What you're flying has a huge bearing on your approach to flying in the icing season. Here are some nice-to-have features:

  • Lots of power. More power means quicker climbs to on-top conditions, and faster retreats from icing situations.
  • Turbocharging. Thought you'd top that cloud deck, didn't you? Well, you're passing through 11,000 feet, still in cloud, and now your climb rate is dropping off. With turbocharging, you can carry your engine's full rated power to higher altitudes than with a normally aspirated engine. Have an engine with lots of power and turbocharging? Lucky you!
  • Oxygen. Assuming you have turbocharging and have leveled off at a high altitude in clear skies, you'll need oxygen, or pressurization, to avoid hypoxia.
  • Ice-protection equipment. Pneumatic deice boots, heated windshields, the TKS weeping-wing system, and — in many turbine-powered airplanes — leading-edge panels heated by engine bleed air all can make your life a great deal more peaceful if you find yourself stuck in icing conditions. Just remember that there's a huge difference between a full ice-protection system that's certified for flight-into-known-icing (FIKI) conditions and equipment added on via a supplemental type certificate (STC). The former has been tested and proven effective according to FAA rules. The latter has been approved merely for installation, under the sole stipulation that the equipment doesn't adversely affect the behavior of the airplane — in clear air. There's no guarantee that STCed components will cut the mustard when ice starts to build. Even FIKI-approved systems can't handle the worst ice. They just buy you some time to make your getaway, which should be a quick one, at the first sign of ice accretions.

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.


E-mail the author at tom.horne@aopa.org.


Ice Types

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