How TKS Works
The TKS Ice Protection System is based on the freezing point depressant concept. An ethylene glycol-based fluid, called TKS fluid, works as a freezing point depressant with a freezing point below -76°F (-60°C). TKS fluid disperses from the laser-drilled titanium panels mounted on the leading edges of the wing, wing strut (if applicable), and horizontal and vertical stabilizers. A traditional slinger ring provides ice protection on the propeller. The TKS fluid mixes with supercooled water in the cloud, depresses its freezing point to at least the ambient condition temperature and allows the mixture to flow off the aircraft without freezing. TKS also protects aft of the panels since fluid flows over the entire chord of the protected surfaces.
Dependent on whether the TKS Ice Protection system is either No-Hazard or FIKI certifying, the windshield is protected by either one or two on-demand pumps. A backup mode is provided for FIKI-certifying systems.
Anti-Ice and De-Ice
TKS is designed to anti-ice and is also capable of de-icing an aircraft. 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. The capability allows the system to clear the airframe of accumulated ice before transitioning to anti-ice protection.
TKS Design Philosophy
Design of a TKS Ice Protection System extends well beyond the physical properties of application to an airframe. The philosophy has been to maximize coverage, limit airframe ice accretion and maintain aircraft performace in the icing environment. In addition to placement of ice protection, optimization of fluid flow for the icing environment is key.
The fundamental concept of TKS is freezing point depression. For a given water catch during an icing encounter, a specific volume of ice protection fluid is required to mix with the water catch and depress the freezing point below the ambient temperature. The design flow rate for a system is established to provide freezing point depression over the entire FAA Continuous Maximum icing envelope, in regulation 14CFR Part 25, Appendix C. The envelope characterizes icing conditions by the specific volume of water within a cloud (Liquid Water Content, LWC) the diameter of the water droplets in the cloud (Mean Effective Drop Diameter) and the Ambient Temperature.
If the required volume of fluid is mixed with the water catch, freezing point depression will occur. When the catch starts to exceed the supplied volume, however, little change in performance will be noted. TKS transitions from freezing point depression to "natural de-icing". Natural de-icing is identified as a small scale building and shedding of ice from the protected surface. When the catch is slightly above the freezing point depression threshold, the change will hardly be noticeable. As the catch becomes more intense, the local ice accumulations will become larger prior to departure from the airframe. The upper limit of natural de-icing is reached when a continuous strip of ice accretes on a protected surface before it sheds.
For a large portion of the natural de-icing range of performance of a TKS porous panel, the end result will still appear as basically an anti-icing operation to the operator. Icing wing tunnel research and flight experience has proven that a flow rate of 50% of the required freezing point depression volume will still result in an anti-icing like build and shed performance. This performance allows the system to be designed with discrete flow rates that provide enough performance resolution to cover the entire Continuous Maximum icing envelope, yet overlap in ice protection performance to allow economical fluid consumption.
Both the active area and flow rate of the panel are designed by an analysis of the aircraft based on its flight performance and physical definition in the desired icing conditions. The analysis provides a calculation of the fluid flow required to ensure the aircraft is protected from ice. TKS provides anti-ice protection for the design condition and less severe icing encounters. The system will also provide a de-ice solution for more severe encounters that fall outside the design envelope. With this in mind, the total of fluid the system must carry (i.e.., tank size) is dependent on the desired level of protection (i.e., icing conditions anti-ice performance is expected in) and desired endurance.
Porous Panel Construction
For a TKS Ice Protection System, laser-drilled titanium panels are attached to the aircraft to form the leading edge of the protected areas. The panels are either attached over an existing leading edge as an “overshoe” or inset into the leading edge such that they form the originally defined leading edge shape. The panels are typically attached either using an adhesive or mechanical fasteners. Aerodynamic performance remains the same when panels are integrated.
The inner and outer skins of the porous panels are manufactured with titanium, with a 0.7 to 1.2 mm thickness. Titanium provides excellent strength, durability, light weight and corrosion resistance. The thickness used depends upon the leading edge radius of the panel which dictates the strength required to withstand standard internal operating pressures. The active area of the skin is perforated by laser drilling holes 0.0025 inches in diameter with 800 holes per square inch. The porous area of the titanium panels is design to provide adequate volume of fluid for a freezing point depression on a leading edge in normal operating environment.
The titanium back plate of a typical panel is formed to create a reservoir for the TKS fluid, which allows 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. Robots laser-weld the back plate to the front plate, creating an exceptionally strong joint between the two.
TKS fluid is supplied to the panels and propeller by a positive displacement, constant volume metering pump. The pump provides a flow rate for removing accumulated ice or providing complete freezing point depression performace over the entire FAA Continuous Maximum icing envelope. The fluid passes through a microfilter prior to distribution to the porous panels and propeller. 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. Systems are provided with a fluid reservoir that assures the desired ice protection endurance is obtained as required by the end user.
TKS pumps are powered with DC 14 or 28 volt pumps. Operating at full speed, a 28 volt pump will draw a continuous 1.5 to 2 Amps of power, representing 40-55 Watts of power. The motors produce a 12 Amp start current peak. Basic TKS power consumption is minimal.
CAV Ice Protection and its predecessors have designed and manufactured airborne ice protection systems for over 20 aircraft OEMs, over 30 aircraft models and even more aircraft model variants.