De-ice Technology Develops
New systems you can bet your boots on
By Greg Napert
There are three basic types of ice prevention systems that have been in common use over the years: The rubber bladder type — typically used on corporate and general aviation aircraft airfoil leading edges, bleed air heat — used on commercial transport and business jets, and electric heating elements — used primarily on propellers and windshields. Ice detection, on the other hand has been virtually non-existent — the only system being one that consisted of a small probe which vibrated at a given frequency which changed as ice formed over it.
Researchers, over time, have developed new systems for detection and protection, but for the most part, manufacturers haven't incorporated the new technology into new aircraft. That still holds true.
However, an aircraft accident which happened in Oct. 31, 1994 — where an American Eagle ATR-72 in Roselawn, Indiana (Flight 4184) crashed due to icing problems — led industry icing experts, investigators, and manufacturers to study new ways to prevent and eliminate such incidents.
Although the de-ice manufacturers say they have been researching alternative approaches to ice detection and prevention for years, Supplemental Type Certificates (STCs) for new detection and prevention systems since 1994 seem to be on the rise.
The following pages give you a look at of some of the new detection and prevention systems that you will see appearing in the marketplace over the upcoming years as well as some new technology that is currently available.
According to Bryn Young, customer service for BFGoodrich Aerospace Ice Protection Systems Division, the company's newest integrated ice detection/protection system called SMARTboot™, incorporates the ice detector into the de-ice boot. "Up until SMARTboot came along, the kind of ice detection systems that were on aircraft were probe-like, single point detectors. And so, they detected ice near the de-ice boot and not on the de-ice boot. Its disadvantages were twofold: its location, which wasn't in the critical area of the de-ice boot, and, it was a one-point sensor as opposed to SMARTboot, which is a wide area sensor," she says.
Dave Sweet, manager of research and development for BFGoodrich says the company received an STC on the SMARTboot on the Piper Malibu/Mirage in 1997. The product was in development for around three years.
Sweet continues, "BFG began working with a small company out of Ithaca, NY, which had adopted a system developed by NASA/Langly research. This system initially did not work, but it had merit and led us in the direction that we were able to solve the problems it had. The original capacitance technique that was used worked well if the ice was completely frozen. But, ice is often very wet in reality, especially when flying through clouds. So, in order to resolve the issue, we instead had to measure the impedance of the ice/moisture. Using impedance, we are able to sense the onset of ice at about .020-inch thickness, and the system tells us the thickness of the ice as it increases."
Sweet says the system is designed primarily for the tail of the aircraft — an area that is not visible to the pilots. "Tail icing was what drove the development of the SMARTboot," he says. "He can see the wing, so that's not really a concern when it comes to de-icing." He continues, "The horizontal stabilizer is also a smaller airfoil, so it is a more efficient collector of ice. The system is designed to alert the pilot when there is ice, and it also tells them when there is enough ice to activate the system. Once you've detected the ice, it works as a conventional boot does."
According to Sweet, the sensor consists of conductive strips built into the surface ply of the boot. "They're actually strands of graphite, embedded into conductive rubber. What happens is when ice forms on the surface, one of the electrodes (called the driver or positive electrode) sends a signal to the receiving electrode, and through this, we can measure the impedance, which gives us the thickness of the ice. Polyurethane has very good erosion resistance. We are finding, through testing, that the boots are equivalent in erosion resistance to our standard boots. However, the boots are activated less because you only turn it on when you have an indication, so life expectancy is expected to increase. One of the things we're looking at in the future is installing a closed loop system (automatic activation). But for now, the de-ice has to be activated manually."
Maintenance of the SMARTboot
Sweet says that SMARTboots are maintained as other esthane/urethane boots are.
"In terms of maintenance, the SMARTboot is an esthane/urethane surface material, so you don't use the AgeMaster™ product, but you can use the Icex™ and the ShineMaster™ with it," he says.
The electrical sensing system does require testing periodically by checking the continuity of the Ice Thickness Sensors, to confirm there are no intermittent shorts within the air connection and manifold area. To check continuity of the SMARTboot de-icers, BFGoodrich provides a procedure requiring the use of a digital ohmmeter and an air pressure source capable of 6 psi.
Pneumatic Impulse Ice Protection
Young says a product which BFGoodrich has in development is called Pneumatic Impulse Ice Protection (PIIP). The key improvements in this system over the standard pneumatic systems is that it could remove smaller accretions of ice, called "rime ice." With standard pneumatic systems, you need to let the ice build to .040 inches before activating the system. PIIP is able to break ice off at .025 inches. The other advantage is that the leading edge is made of Titanium, which could conceivably last forever.
Sweet says BFGoodrich began development of the product in the early 80's. "It is considered to be the next generation system designed to upgrade the Pneumatic de-icers. Installation of the system basically involves removing the aluminum leading edge strip on the aircraft and installing the new PIIP leading edge in its place. This new leading edge has the ice protection system built right into it.
"The PIIP edge is basically a composite panel with a titanium surface. It contains a tube or tubes which are opened to the atmosphere at the end," he says. The way it works is we build a high pressure charge (600 psi) with a valve. This charge is released and creates a shock wave in the tube. As that shock wave travels down the length of the tube, high surface acceleration is generated on the titanium and this expels the ice from the surface. The acceleration is actually quite dramatic. In fact, if you would place your hand on the surface when the air is expelled, it feels like when you fall down and slap your hands on the concrete. In wind tunnel tests, with a high speed camera, you can actually see the ice come off and go forward in the airstream, before it's carried away. So, it's launched with quite a bit of acceleration," he explains.
Sweet says PIIP is activated upon detection or when icing is suspected. "The number of cycles you can get out of the system is about four times what you would get out of a pneumatic de-icer, then the tubes have to be replaced," he says.
"We are currently working with a business jet manufacturer who is testing it onboard an aircraft, but I can't give you a date when it will be certified. It will originally be targeted for business aircraft. We feel it will offer quite a few advantages over the conventional systems that are out there," says Sweet.
Two other systems in development at BFGoodrich are Electro-Impulse De-icing system (EIDI), and Eddy Current De-Icing Systems (ECDS).
EIDI uses a spiral wound coil placed in proximity to the inner surface of the protected airfoil. When electrical current is discharged into the coil, opposing magnetic fields are generated between the coil and skin. The result is a rapid outward motion of the airfoil surface, and shedding of accreted ice.
ECDS systems contain a flat ribbon coil distributed over the airfoil surface. An electrical discharge into the coil produces eddy currents, causing the surface of the de-icer to move rapidly away from the airfoil. This method is better suited for stiff, complex shapes that may not "flex" as easily as two-dimensional airfoils.
A New Approach to Ice Protection
Pneumatic de-icing systems have been the standard for general aviation aircraft for many years. Their utility and reliability have been taken for granted by pilots and maintenance personnel alike. Yet, the boots require a level of maintenance that is sometimes undesirable.
A company cal
led Aerospace Systems and Technologies, Inc. (ASandT) began introducing an alternative to general aviation aircraft in 1987, and as the company gains a foothold in the industry, you (the technician) may be called on to repair, service, or troubleshoot it — so take note!
The system, which it calls TKS, achieves ice protection by mounting laser-drilled titanium panels to the leading edges of the wings, horizontal and vertical stabilizers. Secondary fairings or structures such as wing lift struts can be protected in a similar manner. Propellers are protected with fluid slinger rings, and windshields are provided with spraybars.
The basic system is not really new. According to TKS, fluid ice protection started in the 1930's as companies experimented with methods of introducing de-icing fluid at the leading edges of wings. The TKS system was first developed during World War II as a method of providing ice protection for armored leading edges. The concept evolved through the 50's and into the early 60's when TKS ice protection was applied to the HS-125 business jet. Since then, every 125 produced has been equipped with TKS ice protection. Then, in the early 80's, laser-drilled panels were developed and first applied to the Cessna Citation SII as standard equipment.
What is new is the introduction of this systems to the single engine, general aviation market. TKS received its first STC for the Beech Bonanza in 1987. Since 1987, a handful of additional STC have become available to customers .
According to ASandT, TKS ice protection offers a level of ice protection unsurpassed by any other method. It has the major advantage of providing anti-ice capability, as opposed to de-ice capability. The end result is an ice protection system that keeps ice off of the aircraft, maintaining aircraft performance in the icing environment. This level of protection, coupled with the ease of use of the system, provides effective, simple ice protection.
The TKS ice protection method is based upon the freezing point depressant concept. An antifreeze solution is pumped from panels mounted on the leading edges of the wings, horizontal and vertical stabilizers. The solution mixes with the super-cooled water in the cloud, depresses its freezing point, and allows the mixture to flow off of the aircraft without freezing.
The system is designed to anti-ice, but it is also capable of de-icing an aircraft as well. When ice has accumulated on the leading edges, the antifreeze solution will chemically break down the bond between the ice and airframe, allowing the aerodynamic forces on the ice to carry it away. This capability allows the system to clear the airframe of accumulated ice before transitioning to anti-ice protection.
A valuable side-effect of TKS ice protection is the reduction of runback icing on the wings and tail. Once fluid departs the panel on the leading edge of the surface, it flows aft, over the upper and lower surfaces, and departs the aircraft at the trailing edge. This runback effect keeps ice accumulation in check aft of the panels, from runback or from impact of larger water droplets. This side-effect is a positive benefit in today's environment of concern for ice protection during large droplet encounters.
According to ASandT, the outer skin of the TKS ice protection panels are manufactured with titanium, typically 0.7 to 0.9mm thick. Titanium provides excellent strength, durability, light weight, and corrosion resistance. The panel skin is perforated by laser drilling holes, 0.0025 inches in diameter, 800 per square inch. The porous area of the titanium panels is designed to cover the stagnation point travel on the appropriate leading edge over a normal operating environment. This range is typically from best rate of climb at the low end to VNO, maximum structural cruising speed. Conservative margins are added to this range.
The back plate of a typical panel is manufactured with stainless steel or titanium. It is formed to create a reservoir for the ice protection fluid, allowing fluid supply to the entire porous area. A porous membrane between the outer skin and the reservoir assure even flow and distribution through the entire porous area of the panel.
The porous panels can be bonded or attached as a cuff over a leading edge (typical in STC installations), or built in as the leading edge. Most high performance general aviation singles and twins utilize the cuff method. Panels are bonded to the airframe with a two-part, flexible adhesive.
Fluid is supplied to the panels and propeller by a positive displacement, constant volume metering pump. The two-speed pump provides two flow rates to the panels and propeller. The low speed supplies fluid for the design point of anti-icing during a maximum continuous icing condition. The high speed doubles the flow rate for removing accumulated ice or providing ice protection for more severe conditions. Flow rates are designed for this level of performance, regardless of the certification basis for the system.
For systems that are not certified for flight into known icing, one metering pump is provided. For systems certified for flight into known icing, two pumps are installed for redundancy. The pumps are individually selectable.
Fluid for the windshield spraybar system is provided by an on-demand gear pump. The spraybar may be activated as needed to clear forward vision through the windshield. Similar to the metering pump, one or two pumps are provided depending on the certification basis. The fluid passes through a microfilter prior to distribution to the porous panels and propeller(s). The filter assures all contaminants are removed from the fluid and prevents panel blockage. A system of nylon tubing carries the fluid to proportioning units typically located in the wings and tail of the aircraft. The proportioning units divide the flow into the volumetric requirements of each panel or device supplied through the unit. Each panel and device is fed again with nylon tubing. Each system is provided with a fluid reservoir that ensures a minimum ice protection endurance when filled. All systems are designed for a minimum anti-ice endurance of 2.5 hours. The endurance can be increased dependent upon available volume for the reservoir and weight constraints on the amount of carried fluid. For the high performance single, the design fluid quantity typically falls in a range of 6 to 7.8 gallons.
The system is operated and monitored through a control panel in the cockpit. All modes of operation and selection for the metering and windshield pumps are controlled through the panel. Coupled to a float sensor in the reservoir, the remaining quantity of fluid is displayed. The operational state of the system may also be monitored with the panel.
One very important aspect of TKS ice protection is its inherent low power consumption, says the company. Little demand is placed on the aircraft electrical system by the TKS system. A 28-volt TKS system will typically draw 1.5 amps of current during normal operation. Complete airframe ice protection is provided for a fraction of the power consumption of a resistance heating device on the airframe.
The weight of a TKS ice protection system for a high performance, single engine aircraft will typically fall between 35 to 40 pounds without fluid. The final value is dependent upon the size of the aircraft, the coverage and size of the leading edge panels, and the configuration of the mechanical components. Systems with components individually mounted, tend to be lighter that pallet-based systems, because of the added weight of the pallet framework. The tradeoff is weight versus ease of installation of these components.
Fluid weight depends entirely on the quantity carried. At a weight of 9.2 pounds per gallon, fluid weight falls in the range of 55 to 70 pounds. This weight can, however, be eliminated in non-icing seasons by draining the fluid tank or not replenishing it.
Handling, Servicing, and Maintenance
Prolonged out of Service Care During Flyable Storage Ensure that the de-icing fluid tank contains at least the minimum take off quantity of fluid (1 gallon), and that all system components are filled with fluid. If necessary, operate pump(s) until all air is dispelled from components and pipelines (see Priming, below). Recheck tank contents.
1. De-icing Fluid Tank (See Limitations for specified de-icing fluids). The filler cap is located on the right fuselage aft of the baggage compartment. To preclude the possibility of contaminated fluid, always clean the top of fluid containers before dispensing, and if required, maintain a clean, measuring vessel solely for de-icing fluid. Secure the filler cap immediately after filling. The tank is vented through the filler cap and an additional vent line is provided.
2. Ice Protection Fluid Strainer Ordinarily, the de-icing fluid strainer in the fluid tank outlet should not require cleaning, unless there is a definite indication of foreign matter in the tank.
3. Ice Protection Fluid Filter
Illumination of the HIGH PRESSURE warning in flight (or during ground testing) indicates the need for filter element renewal. Warnings arising from system operation in the MAXIMUM flow mode and/or at abnormally low temperatures (below -30ûC, -22ûF) may be ignored. 4. Pump Priming
The airframe/propeller pumps may not be self priming, and are primed, when required, by operation of the corresponding windshield pump. Windshield pump 1 primes main pump 1, and windshield pump 2 will prime main pump 2.
Porous panels contain a plastic membrane which may be damaged by certain solvents, particularly Methyl Ethyl Ketone, lacquer thinner, and other types of thinners. Mask panels when painting aircraft or when using solvents for other purposes in the proximity of the porous panels.
Only the solvents listed in the Limitations are permitted for use on porous panels. The porous panels may be washed with mild soap and water using a brush or cloth.
Ice Protection Fluid
The active element of any TKS system is the antifreeze solution. In most instances, three fluids have been approved for use in TKS systems certified in the United States, but only one is typically used. This fluid is commonly known as AL-5, manufactured to the British specification DTD 406B.
DTD 406B is a relatively simple mixture consisting of 3 parts: 85 percent ethylene glycol, 5 percent isopropyl alcohol, and 10 percent de-ionized water by volume. The fluid weighs 9.2 pounds per gallon. Simple as it is, the fluid is well filtered before distribution to remove even the smallest contaminants that could adversely affect the operation of a TKS system. The fluid is available from a number of manufacturers in the U.S. and is readily available at a number of fixed-base operators throughout the country.