Anti-Ice, De-Ice, Known Ice

It’s that time of year again — know what the dangers are, know your defenses…

Unless this is the first year you’ve ever flown during the cooler months, you’ve undoubtedly learned something about the hazards of flying in potential icing conditions. Ice can build rapidly when flying in visible moisture (clouds, rain, fog or wet snow) with outside air temperatures near or below the freezing mark.

Temperature: The “prime icing range” is from about 35-degrees Fahrenheit to about 20-degrees Fahrenheit below the freezing point (about 12 degrees).

How it works: Just-above-freezing air can cool to the freezing point in the venturi action of wings, fuselage and tail. Air that’s well below freezing is too cold to support significant moisture, and any water in the air at such temperatures will meet your aircraft pre-frozen and likely bounce off your airplane like a snowball.

Danger: Moist air in cumulus clouds can support significant icing at temperatures as low as minus 40F. Strong updrafts in cumulus clouds can loft large water droplets to these cold altitudes so rapidly they don’t freeze completely before hitting your airplane.

The primary hazard of ice accumulation is a change to the shape of the wing and tail, which most often results in an increase in the effective angle of attack necessary to maintain altitude.

Translation: You’re no longer flying the same airplane — it now has different wings and you are now a test pilot. Expect an increase in indicated stall speed, and wildly unpredictable stall characteristics. Among the list of other problems you might face are:

  • Power loss from ice accumulation on air inlets and filters by “impact ice” or snow, and accumulation on propeller blades, reducing thrust;
  • Pitot-Static instrument failures or inaccuracies resulting from obstructions and / or cryogenic modifications to the pitot tube and/or static port(s);
  • Fuel system failures (possibly engine failure) as fuel tank vents ice over, creating a “vapor lock” that prevents the engine from drawing fuel from the affected tank;
  • Obstructions to vision as windscreens ice over; and
  • Increased airframe weight as the heavy ice accumulates.

Important: Many airplane designs have a reputation for their ability to “handle ice” — this is usually because they have high useful loads. Unfortunately, the weight of ice accumulation is only a small part of the danger ice presents to pilot and passengers.

Almost all airplanes have at least some capability to fight back against airframe ice. The equipment they carry comes in two types:

  • Anti-ice devices, designed to prevent the accumulation of light ice; and
  • Deice equipment, capable of removing ice that has already accumulated on the airframe.

ANTI-ICE items include things like pitot heat, heated stall warning vanes, hot air windshield defoggers, and heated fuel vents. Many older, light twins have alcohol-based anti-ice devices for windshields and propeller blades.

Strategy: You have to turn anti-ice devices on before encountering ice, because they likely to not have the ability to remove a coating of ice once it’s formed.

DEICE devices are the more capable pneumatic deice “boots” on wing and tail leading edges; electric propeller deicers; and electric “hot plate” windshield deicers. Some “weeping wing,” alcohol-based wing, and even tail and windshield units also have the “oompf” to remove ice accumulations.

Strategy: With the exception of deice boots (which work best after some ice has accumulated), deicing equipment is also best turned on before encountering ice. See the deice equipment supplements to the airplane’s Pilot’s Operating Handbook for more details.

Unless an airplane is “certified for flight in icing conditions” (colloquially called “known ice“), a pilot cannot legally fly into an area where icing conditions are known to exist. A liberal and common interpretation is that ice is “known” only if there is a recent (usually, less than an hour old) Pilot Report (PIREP) indicating ice accumulation in the area and at the altitude described.

Problem: This leads some pilots to avoid making PIREPs when encountering ice, to avoid “getting themselves into trouble” — a self-perpetuating hazard as other, perhaps less-experienced pilots in less-capable airplanes, fly blindly into detected icing conditions.

There is significant case law and FAA enforcement action, however, to support the view that if the ice was forecast to possibly exist, its hazard was “known” to a pilot so briefed. This article won’t explore this thorny legal semantics issue, but we’ll say this much…

If you plan or continue flight into icing conditions, your airplane must be certified for flight in icing conditions, and all required ice certification equipment must be operable.

If your aircraft is not certified for flight into known icing conditions, you’re probably not going to be able (or want) to change that. Obtaining Federal approval for flight in icing conditions is not as simple as installing pneumatic boots and a “hot prop” on your favorite sky-buster. Ice certification entails assuring redundancy for airplane instrument and electrical systems, heated fuel vents and stall warning systems, and extensive flight testing (behind ice-generating “spray planes“) to record airplane response to various types and intensities of ice.

Inside Information: Some icing certifications require on-time overhaul or even replacement of components — regardless of their condition. The Beech 58 Baron, for instance, must have instrument air pumps replaced at intervals of 600 operating-hours, or the “ice” certification is no longer valid.

Regardless of what you’re flying, if you start to pick up ice, your first (and only) priority is to get out as fast as possible. Even your “known ice” Skymasher isn’t designed to remain in icing conditions for long. Certification for known ice gives you the flexibility to plan a flight that may include short exposures to “known” icing while en route to ice-free air. Your best options are to steer clear of it at all times and get out of it as soon as possible should the first option become irrelevant.

BOTTOM LINE: As ice season approaches, review the differences in design and operation of anti- and deice equipment on the airplane(s) you fly. Then, take a critical look at the way you make ice-related go/no-go decisions. Just remember, experience is what you get when things don’t go your way and, in this case, there’s never any guarantee you’ll survive the experience.

For more practical strategies to avoid bad icing experiences see also: