November 1, 2005
By Thomas A. Horne
Pilots fortunate enough to have ice-protection systems can become blasé about icing encounters. Brother, is that a mistake! The subject of ice-protection equipment, its capabilities, and correct use is a complicated one — and one that's often neglected in conventional training materials. If you'll be flying an airplane with ice protection this winter, you'd better understand some blunt truths right up front. Then you need to know the strengths and weaknesses of each type of ice-protection system.
First blunt truth: No ice-protection system in the world will let you safely mill around indefinitely in icing conditions. Ice on the unprotected parts of the airplane — wing tips, cowlings, ventral fins, strakes, and so on — will continue to build, causing drag to increase and airspeed to decrease. For this and other good reasons, you should take no huge amount of comfort if your airplane happens to be certified for flight into known icing (FIKI). FIKI certification can buy you some time to escape icing, but it's not a foolproof antidote.
Second blunt truth: Some ice-protection systems work better than others. Pneumatically inflated boots don't shed ice nearly as well as the engine-bleed-air-driven systems used on many jets, for example. Electric windshield heat clears ice from only a small portion of the windshield. And ice can build in nasty ridges aft of boots or bleed-air panels — unless you happen to be equipped with TKS' "weeping wing" system, which oozes a glycol mixture that flows aft of its leading-edge panels.
Third blunt truth: If you fly around in so-called "light" icing conditions long enough, your ice accumulations can reach "severe" levels. That's why fleeing icing conditions is the prime rule, whether you have a full-blown FIKI setup or not. The terms trace, light, moderate, and severe are described in the Aeronautical Information Manual [Chapter 7-1-22], and relate to different rates — not amounts — of ice accumulation. The definition of light icing says that it "may create a problem if flight is prolonged in this environment (over 1 hour)" and that it "does not present a problem if the deicing/anti-icing equipment is used." But if you have minimal ice-protection equipment, believe me, "light" icing will present problems in short order.
Fourth blunt truth: There's a big, big difference between an equipment package that has been certified for FIKI and an equipment package that is installed under the basis of a supplemental type certificate (STC). The FIKI system has been tested for effectiveness in both natural and simulated icing conditions. The STCed installation need only prove that it won't adversely affect the performance of the airplane — in clear-air conditions. STCed add-ons may make you feel better about the prospect of running into unforecast ice, but there's no guarantee at all of their effectiveness in shedding it.
A fifth area of note has to do with the two operational groups of ice-protection gear. The distinctions are simple, but can be easy to forget when the heat — er, the ice — is on. One group of equipment is labeled "anti-ice," the other "deice." Anti-ice components are meant to prevent ice from accumulating, and must be turned on prior to entering icing conditions. Deice components are designed to remove ice once it has accumulated.
Today's ice-protection systems come under five additional main categories, based on how they are powered. Here's a brief rundown of each, along with some operational tips.
Electrothermal. This includes the familiar heated pitot tube (and static ports, too, on some aircraft), heated stall-warning and angle-of-attack probes, as well as heated windshields, propeller heating elements, engine air intakes, and the Northcoast Technologies "Thermawing" leading-edge heating panels. With the exception of the Thermawing system, these components are designed as anti-ice systems. The advantage of eletrothermal systems is their relative simplicity of design and operation. The disadvantage is a potentially high electrical draw, and that failure of a component may not be detectable in flight — until it's too late. The lowly pitot-tube heating element comes to mind. Loss of airspeed information can be deadly in an icing encounter, so be sure to check your pitot heat by briefly touching the tube as part of a preflight test of the system. If your flight involves the chance of an encounter with ice-bearing clouds (i.e., those with temperatures roughly between zero and minus 20 degrees Celsius/32 and minus 4 degrees Fahrenheit) or other visible moisture, then don't forget to turn your pitot heat on several minutes before flying into these conditions. Many experts advocate turning on the pitot and stall-warning heating elements as part of the pretakeoff checklist on any flight.
Pneumatic. This type of system uses inflatable rubber boots to break up ice accumulations. As a deicing system, boots are activated once ice appears on the wing leading edges. Traditional lore says that you should let up to one-quarter of an inch of ice build on the wings before activating the boots. The rationale was that if you cycled the boots too often, ice would build above the inflating tubes and "bridge" over them, rendering them useless. But researchers have learned that ice bridging is a myth. Now the consensus is that boots should be inflated at the first sign of ice accretions, and kept in operation for the duration of an icing encounter. It's thought that the myth originated back in the 1930s, when the first boot designs operated under low pressures and could be slow to inflate and deflate.
Pneumatic systems can be complicated. They use dedicated air pumps or — in the case of turbine airplanes — engine bleed air to make high-pressure air sources. Timers and sequencing valves then send this high-pressure air via a network of tubes to the wing, tail, and other leading-edge surfaces (the Cessna Caravan has pneumatic boots installed on the wing and landing-gear struts). This air inflates the boots' channels and also exerts suction to draw the boots flush with the leading-edge contours when the inflation cycle ends.
Most systems have two or more boot cycling modes. A one-time inflation cycle may be provided by one switch position, or other switch positions may be available to select for inflation sequence cycles separated by one or three minutes — one minute for heavier icing rates, three minutes for less rapid accumulations.
Maintenance can be a problem for pneumatic systems. Pumps can fail, as can sequencing valves, timers, and pneumatic plumbing. The rubber boots can deteriorate over time, and impact damage can put holes in the inflation channels. Holes can be patched, but eventually boots must be replaced — an expensive proposition. Still, boots can be very effective in helping to shed ice.
Bleed air. Turbine aircraft tap into the hot air produced by their engines' compressor sections for ice protection. This hot air is routed to the airfoil leading edges or windshield, where it vaporizes any supercooled water droplets instantly. This is an anti-ice system, although some forms of bleed-air protection can be used for deicing. Some late-model Beechcraft King Airs can be ordered with bleed-air fittings that can melt any slush that accumulates on wheels and brakes during cold and wet winter takeoffs.
Glycol based. These systems use a mixture of glycol and water to rid propellers, windshields, and leading edges of ice. Depending on the installation, glycol systems can have either anti- or deice functions, or sometimes both. Like pneumatic systems, there can be multiple-position activation switches (for low and high flow rates) and multiple distribution lines, plus fluid reservoirs and a pump to energize the fluid flow. Propellers using glycol will have slinger rings that send the fluid flowing over the propeller blades. Windshields have spraybars that also let the glycol flow out a series of tiny nozzles and over the windshield. The TKS system is unique in that its titanium leading-edge panels have thousands and thousands of tiny, laser-drilled holes perforating them. The glycol oozes from the holes — hence the "weeping wing" analogy — then spreads aft in the slipstream, covering all the leading-edge surfaces as well as the entire unprotected airfoil sections aft of them. Glycol systems are known for their reliability and simplicity — no boots to replace, no complicated electronics — but the fluid is heavy (9 pounds per gallon) and they carry hefty price tags.
EIDI. This abbreviation stands for "electro impulse deicing" — an old Russian-invented system that until recently was not certified in the United States. Raytheon's Beechcraft Premier I and IA now use EIDI — the company calls it an EMED, for "Electro-Magnetic Expulsive Deicing" — to deice the horizontal stabilizer. EIDI uses a series of capacitors embedded behind aluminum leading-edge panels. These build and store electrical charges, which are then released to mechanical actuators in a series of rapid discharges. This shocks the adjoining leading edges, and causes them to experience small-amplitude deflections. This blasts ice accretions free. During preflight checks of the system, you can hear a loud, regular rapping sound coming from the discharges, but in the cabin you hear nothing. EMED eliminates the need for boots and bleed air, but requires an electrically heated anti-ice parting strip at the stabilizer's leading edge to augment the EMED's operation. EIDI systems have been successfully tested on Cessna piston singles, but the Premier I and IA are the only aircraft certified with this system.
Other ice-protection methods remain in the experimental stage. They include the use of ice phobics, or low-friction materials or treatments that don't allow icing droplets to cling to leading edges, and memory metals, special alloys that, when briefly heated, distort to shed ice — then return to their original shape.
While most of us may never fly with EIDI or memory metals, it's nice to see that icing research continues to promise future safety benefits. In the meantime, we still have fairly reliable systems to help us fly safely in the icing season — assuming we use and maintain them as intended.
E-mail the author at firstname.lastname@example.org.
Links to additional information about aircraft icing systems may be found on AOPA Online ( www.aopa.org/pilot/links.shtml).
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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