Rough Ice Is Bad Ice

in the early days of aviation, at the advent of thermal ice protection system development, aircraft designers believed that in fighting in-flight icing, the critical variables were the mass of supercooled water that an airplane would transit and the temperature. The measure of mass is liquid water content (LWC). Droplet size of supercooled water, which influences potential icing severity, is measured by the median effective diameter. Droplet size also determines how far back on the airfoil the ice collects. Temperature, mass and location of the ice on the airfoil determine the amount of heat required and the extent of ice protection needed for thermal systems to prevent phase change of water to ice. Out of the extensive airborne sampling of icing conditions starting in the late 1940s, U.S. Federal Aviation Regulations (FARs) Part 25 Appendix C was developed and defined most of the icing envelope used for certification. While well suited to anti-icing systems, Appendix C does not define the environment adequately to prevent all hazards to deicing systems. Vestiges of this concept of calculating the potential for threats from the development of icing by relying on the measure of mass alone have been slow to be revised, even in the face of icing events to the contrary. While common usage simplifies the character of the in-flight ice to two descriptors — glaze ice and rime ice — the shape, location, thickness and distribution of ice features, including roughness, are the true discriminators of the effect of ice on aircraft aerodynamics. Large ice shapes may be problematic, but research is showing that thin, rough ice can have a much greater effect on aircraft performance. These new findings call for a reconsideration of aircraft certification. This new way to consider icing and its effects began to evolve in 1967 when the University of Wyoming (UW) started operating a variety of state-of-the-art aircraft outfitted for cloud physics work. For the past 40-plus years, UW researchers participated in various weather modification projects, beginning with a search for supercooled liquid water1, without which there is no weather modification potential. Data and experience collected in this process inadvertently produced a new concept of in-flight icing: The shape and distribution of the accreted ice, and primarily the roughness, are more significant in terms of performance degradation, by an order of magnitude, than the mass of ice. Pilots often comment on how much ice they are able to handle, creating a misplaced sense of confidence about accretion of lesser thickness that may be far more adverse. Icing severity as often forecast and reported by pilots does not always equate with severity of effect. Further, the UW observations expanded awareness of the critical factors influencing in-flight icing beyond high LWC to include an understanding of atmospheric temperature and the largest droplets, particularly when considering the performance of deicing systems. There tends to be an “optimum bad” value for each of these parameters: For the flight conditions of the research airplane static air temperature, it is around minus 8 degrees C (18 degrees F). Conditions warming to temperatures well above 0 degrees C (32 degrees F) result in run-back ice — water freezing as it flows — or no ice; at colder temperatures there is mostly ice, no water. Run-back ice can also form ridges aft of ice-protected areas, which can create adverse effects. The largest “optimum bad” droplet size seems to be around 100 microns in diameter, approximately 2.5 times the thickness of a human hair. These droplets collect on the airfoil in the area of 5 percent to 15 percent of chord. They result in the formation of ice resembling small, pointed “shark’s teeth” with the teeth oriented into the local airflow. Smaller droplets collect on the leading edge of the airfoil and cause little Rough Ice A new concept in