If life were fair, no pilot would ever have to fly in conditions in which airframe icing becomes a reality. But life isn’t fair and any pilot who flies IFR in the winter or at high altitude faces the very real possibility of decorating the airplane with ice no matter how carefully she or he plans, plots and schemes.
For pilots who regularly use their airplanes to travel, having ice protection beyond a heated pitot tube often goes from the “nice to have” column to the “I really need it” column when considering upgrades to the family machine—especially if there’s recently been a pucker-inducing encounter. We did a survey of what’s available for retrofit and found, to our surprise and pleasure, that some form of retrofit de-icing system is available for just about any high-performance piston single or twin.
Before we run through what we found, we’ll note that the retrofit systems come in one of two flavors, “non-hazard” and FIKI. Non-hazard systems are designed (and certified) simply to buy time for pilots to get out of ice, not linger in it. They are not certified for flight into known icing conditions. Non-hazard icing certification only requires that the system perform its intended function—shed some ice—and that the system does not affect stall performance, controllability, stability and trim. The performance testing is done in dry air—there is no performance testing in icing conditions. Testing for ice-shedding in icing conditions is limited.
A FIKI-certified system—flight into known icing—goes through rigorous testing designed to show that the airplane can deal with most, not all, icing conditions for a period of 45 minutes. A FIKI system generally has to have two sources of power, an ice-inspection light, a heated stall-warning vane and heated static source(s). Getting FIKI certification is a big deal; nevertheless, a FIKI airplane will not handle severe icing conditions—such as freezing rain—for any length of time. FIKI certification does not make an airplane into a go anywhere, anytime machine.
TKS
Evolved from a system used by the Royal Air Force in the icy skies above Europe in World War II, CAV Ice Protection‘s TKS ice protection system exudes an ethylene glycol-based fluid from thousands of laser-drilled holes in titanium panels on the leading edges of an airplane’s wings and tail. A slinger ring dispenses the fluid onto the prop and the windshield is protected by a fluid dispenser.
TKS works as a freezing point depressant—the fluid has a freezing point of -60 degrees C and mixes with the supercooled water in the cloud through which the airplane is flying and allows the resulting mixture to flow off of the airframe without freezing.
TKS is designed to act as an anti-icing agent and, at a higher flow rate, as a de-icer. The fluid chemically breaks down the bond that has formed between the ice and the airframe. The relative wind then carries away the ice. Once the ice has departed, the flow rate often can be reduced so that the system functions in the anti-ice mode.
0.0025-inch diameter holes are laser-drilled at a rate of 800 per square inch into 0.7 to 1.2 mm thick titanium panels, which either replace the existing leading edge or are attached over an existing leading edge. Fluid is supplied to the propeller slinger ring, windshield spray bar and the wing and tail panels by a positive displacement, constant volume metering pump after passing through a microfilter that removes contaminants. A system of nylon tubing carries the fluid to proportioning units that divide the flow into volumetric requirements for each portion of the system. Depending on the aircraft, the pump is either a 14- or 28-volt unit and draws a maximum of two amps at its maximum operating speed. There are two versions of TKS available, FIKI approved and non-hazard.
Joel Jackson, sales and marketing administrator for CAV Ice Protection, the largest manufacturer of TKS systems, told us that CAV’s system is now being installed on seven different types of aircraft by six OEMs and it holds STCs for retrofit onto more than 30 different aircraft models, including Cessna, Cirrus, Beechcraft, Diamond, Mooney and Piper. The company’s website gives a full listing of the aircraft that can be retrofitted with TKS non-hazard and FIKI systems. There are additional TKS system STCs owned by Air Net II and one, for the Aero Commander 500, owned by Central Air Southwest.
Because the components of a TKS system vary based on the installed equipment on a given airplane, pricing requires that a buyer provide detail about the airplane to the provider and get a quote for the system appropriate for that airplane. We were told by CAV that its systems range in price from $23,000 to $50,000 for piston singles plus installation.
Installation is performed at one of three approved centers in the U.S.—or one in Europe—and typically takes 120 hours. System weight is 40 to 50 pounds, depending on the aircraft. Users have told us that there was a speed penalty for the system of 5-10 knots for long-wing Mooneys, 3-5 knots for the Cirrus line and none for the Cessna 206 or 210.
Early on there were reports of corrosion on some airplanes; however, the fluid is not corrosive and we have not received recent reports. Our working hypothesis is that the reported corrosion was installation-related.
CAV told us that the service life of its TKS system is infinite so long as the filter and pump brushes are serviced periodically and standard preventive maintenance is conducted.
TKS fluid weighs 9 pounds per gallon and costs $10 to $20 per gallon, although we have heard of FBOs charging upward of $50 per gallon. Cessna T210 owner Scott Dyer told us that the least expensive way to buy the fluid is in 55-gallon drums. It is hazmat, so plan on paying a surcharge for shipping. Dyer and others told us they carry a few gallons in jugs when they travel so that they don’t have to buy fluid on the road. He said that he flight plans for use of TKS fluid. He figures on two hours of endurance on the ant-icing setting and one hour on the high or de-ice setting. CAV’s sales manager, Jeff Holden, told us that one to two hours is generally accurate for endurance but it can be as long as 3.5 hours or as short as a half hour depending on the type of aircraft and the operating mode.
A TKS system has to be kept “wet”—which means using it at least once a month. Owners also told us that they run the system on the ground for about five minutes if they think they are going to encounter ice soon after takeoff. The fluid is messy and slimy—one of the reasons ice doesn’t adhere to the airframe. It means that you will have to clean your hangar floor regularly as it tends to seep out for a while after use in flight and you’ll need to warn line personnel working around your airplane that the area in front of the wings and tail may be slippery.
SLD Guard
We are watching a new product from CAV, SLD Guard. It is designed for dealing with the most dangerous type of moisture in the icing world, supercooled large droplets (freezing rain) and to meet the FAA’s evolving FIKI standards. Supercooled large droplets create a special hazard as they are so big that they remain liquid and “run back” behind leading edge de-icing systems before freezing, often creating a dam that plays havoc with airflow over the wing (particularly the ailerons) or tail.
SLD guard consists of thin titanium strips installed on the wing upper surface in front of the ailerons and on the upper and lower surfaces of the tail, behind the existing de-icing system. The strips exude TKS fluid to rid the surface of any run-back ice from supercooled liquid droplets. Wind tunnel video on CAV’s website of SLD Guard in use is impressive. We’ve seen video of ice dam buildup aft of de-icing boots on horizontal stabilizers and think SLD Guard has great potential for protection against tail stalls in ice. According to CAV’s Joel Jackson, SLD Guard can be installed on airplanes that have any type of de-icing system. He said it was not currently available for retrofit but that may change.
Boots
Pneumatic de-icing boots were first installed on an airplane in 1932 and have been in use ever since—because they work. However, the technology has improved substantially. The system consists of inflatable rubber boots attached to the leading edge of the wings, tail and sometimes wing struts. When ice begins to form on the leading edges, the tubes within the boots are inflated. (Don’t wait for ice to build—use the boots right away—the need to wait is an aviation myth that needs to be killed off.) Boots crack ice by creating fracture and shear stresses. The airstream carries the broken ice away.
De-icing boots have aerodynamically smooth polyurethane surfaces designed to enhance ice removal and resist erosion and UV damage. Kept clean and occasionally treated with commercially available products for their care, an owner who hangars a boot-equipped airplane can expect a set to last 20 years. We covered de-icing boot replacement in the December 2012 issue of Aviation Consumer.
Almost all de-icing boots are installed by the aircraft manufacturer; however, if you have one of several models of the Piper Saratoga or conventional tail Beech Bonanza lines, a full, non-hazard pneumatic de-icing boot system can be purchased from B/E Aerospace. Its full Ice Shield kit consists of wing and tail boots, prop boot, heated windshield panel and ice-viewing light. A buyer may choose to purchase just some portions of the kit.
The prices for the full kits vary slightly by the type of aircraft, but figure on $45,000 to $50,000 installed. Installation is performed by Tiffin Aire in Tiffin, Ohio, and, according to Kristian Barnett, marketing specialist for B/E Aerospace, it takes 80 hours. Depending on the type of airplane, the full kit weighs on the order of 50 pounds.
The good thing about de-icing boots is that a user doesn’t have to cart fluid around; the bad thing is that they have a lot of moving parts, although users tell us that they tend to be reliable and rarely need parts replaced. Plan on spending $12,000 to replace a set of boots.
ThermaWing
Originally developed by Kelly Aerospace in conjunction with a project to de-ice the gigantic blades of wind turbines, ThermaWing is an electric de-icing system that uses a graphite foil laminate attached to the leading edges of the wings and tail to heat up and break the bond between ice and the foil, allowing the airflow to carry the ice away. The outer layer of the foil is made up of Tedlar, an ice phobic/nonstick, polyvinal fluoride film developed by DuPont.
The system has a number of heating zones and is further broken up into leading edge and shed zones. The leading or impingement area of the wings and tail is kept warm continually so that it “runs wet”; moisture striking it remains liquid. The liquid water runs aft onto the shed zone, which is normally kept cold so that water will freeze there and not further aft.
During the de-ice cycle, the voltage to the shed zone is increased, causing it to heat up, releasing the ice bond and allowing the airflow to carry the ice away. The voltage is then reduced; the shed zone cools off and ice forms again, to be shed during the next cycle.
The pilot arms the system should ice be expected. Once the OAT drops to 41 degrees F, the de-icing cycle begins automatically. Each cycle is 60 seconds, with the entire aircraft being de-iced in 33 seconds. The system is designed to heat each zone to 40 degrees F very rapidly—as company owner Todd Bates said, “We want a pop!” The heating/cooling curve for each section is very steep.
Price for the non-hazard system is $30,000 installed. ThermaWing is in the process of transition from ownership by Kelly Aerospace to Deice Technologies Inc., run by Todd Bates. Installations are still being performed by Kelly Aerospace and take one week. The installation includes adding a 70-volt, 150-amp Hartzell alternator to the aircraft to power the system. The laminate attached to the leading edges of the wings and tail is so thin that it should not affect cruise performance. Installed weight is 40-50 pounds.
ThermaWing is a non-hazard system and is currently STC’d on the Cessna Corvalis 350 and 400 and the Columbia 300, 350 and 400. Bates told us that an STC for the Cirrus SR22 series is pending. The company is seeking FIKI certification for the Cirrus and Cessna aircraft.
Tailplane Icing and Tail Stalls
Every year structural icing claims a small but steady number of airplanes and many pilots have reported narrow escapes. Many of the accidents are on approach in conditions in which the airplane is no longer collecting ice. As the means of reconstructing icing-related accidents has gotten more sophisticated, we’ve learned that tail stalls, rather than wing stalls, may be the culprit in the descent or approach phase of flight. That matters because pilots have been taught how to recover from wing stalls (lower the nose, add power) but not from tail stalls, and the recovery from tail stalls is precisely the opposite (raise the nose, raise the flaps, reduce power).
The smaller the radius of the curve of a leading edge, the faster and wider the ice buildup is. Therefore, even with only a half inch of ice on the wing there may be an inch or more of ice on the tail.
The shape of the ice is the reason it’s a problem. When there is ice on the front of the airfoil, the airflow across the lifting surface (the top of the wing, the bottom of the tail) is no longer attached to the surface because it has had to cross an ice berm. Aft of the berm there is airflow separation creating a sort of rotor or vortex of disturbed air in the area of flow separation. (Top figure. Figures are courtesy of our sister publication IFR magazine.)
The reverse airflow means that a portion of the tail’s airfoil is stalled. If the area of disturbed airflow gets large enough, the tail stalls. This becomes important because the tail of an airplane is usually lifting downward to overcome the nose-down pitching moment of the wing.
In cruising flight icing is not as much of a concern for the tail because it is at a low angle of attack. That changes on approach: As the airplane slows and flaps are extended, the angle of attack of the tail increases, increasing the risk of a stall if ice is disturbing the airflow.
Flap extension does two things to an ice-contaminated horizontal stabilizer, both bad. It changes the airflow aft of the wings, deflecting it downward, which causes increased downwash over the tail, increasing its angle of attack, whether it is a high- or low-wing airplane. (Middle figure.)
With an increased angle of attack and an ice buildup on the leading edge, the flow separation on the underside of the tail, the lifting part, is made worse, and the rotor, the area of disturbed air, gets bigger and moves aft.
Flap deflection has the second effect of moving the center of lift of the wing aft, farther away from the center of gravity. This causes an increase in nose-down pitching force. To compensate, the tail must exert greater lift downward, thus increasing its angle of attack still more and causing it to work nearer to its performance limit. Eventually, the tail can stall. (Bottom figure.)
The common scenario for a tailplane stall event is that the airplane is picking up ice. As the pilot begins the approach, she selects approach flaps and notices that it’s difficult to trim the airplane and the elevator feels lighter than usual. The control wheel will move forward very easily but it’s difficult to pull it back. Often some mild PIO (pilot induced oscillation) begins and the pilot can’t damp it out entirely.
At some point the pilot selects full flaps and the airplane pitches down 45 degrees, the pilot tries to pull back on the yoke, but it’s immovable and the airplane crashes.
The recovery technique requires reducing the angle of attack of the horizontal stabilizer. That means raising the flaps—at least to the previous position. It also means physically pulling the elevator away from the area of flow separation by pulling back on the wheel. There are reports that on some commuter turboprops the force necessary to pull the wheel back and get the nose up to the horizon may be as high as 400 pounds. The more realistic load for smaller aircraft is as high as the 100 to 125 pound range.
If the pitch control gets “lighter,” particularly if it becomes easier to push forward on the yoke than it is to pull aft, be suspicious. It may become difficult—if not impossible—to trim the airplane and you may enter PIO. Further warning is given via buffeting in the control wheel itself, not buffeting of the airframe. If you have any amount of flap deployed and you experience shaking in the control wheel, it’s a good bet that it’s the tail that’s at risk of stalling.
The bottom line: If you have ice on the airplane, leave the flaps up on the approach and landing. If the POH has a speed for approaching with ice contamination, use it. Otherwise, fly fast and do not close the throttle until the wheels are rolling on the ground (if you reduce power in the flare you may go from being above the power-on stalling speed with ice to below the power-off stalling speed with ice—a wing stall problem).
Conclusion
We were surprised at the number of types of aircraft for which aftermarket de-icing systems were available. For the owner who regularly flies in parts of the country where the potential for icing is high, a de-icing system can increase the utility of his or her airplane and provide more peace of mind. We also think that retrofitting a de-icing system is worth considering as part of a refurb an owner may be considering.
Going forward, we are particularly interested in watching to see if more STCs are obtained for the Therma
Wing system as it looks to us as the next logical step in de-icing technology, especially at a price that appears to be well below that of retrofit boots or TKS.
