Recent aircraft icing regulations, enacted in January 2015, introduce a new icing certification rule, §25.1420, and an FAA engineering standard, Appendix O, that defines supercooled large drop (SLD) environments (freezing drizzle and freezing rain) for certification of affected aircraft. These SLD regulatory changes necessitate improvements to icing weather information available, in the terminal area and en-route, and the identification of and detailed discrimination between SLD, its subcategories, and other icing and non-icing environments.
As part of the development and testing of icing weather tools and capabilities, and to facilitate their improvement, it is critical to compile a new, high-quality set of in-situ measurements covering a broad spectrum of icing conditions (freezing drizzle, freezing rain, “small drop” icing, high liquid water contents (above the limits of Appendix C – “exceedance conditions”), and mixed phase) as well as non-icing conditions (glaciated environments and clear air).
To this end, the FAA is planning ICICLE (the In-Cloud ICing and Large-drop Experiment), a field program where the FAA and other agencies will work with the National Research Council of Canada (NRC) to fly their Convair-580 research aircraft into the conditions described above in a 5-6 week window between January and March of 2019. To characterize the environment, the NRC Convair-580 will carry a wide array of in-situ and remote sensing systems (W and X-band radar, LIDAR and radiometer) that will provide atmospheric and aircraft state parameters, aerosol and cloud microphysical properties, and cloud structure. It is intended for the program to be focused over the western Great Lakes and nearby Plains States, with the aircraft tentatively planned to be based out of Rockford, Illinois (KRFD). Climatology studies and past flight programs have shown that the atmospheric icing conditions described above are relatively common in the proposed sampling domain, while rich operational datasets are also available there to allow the icing tools to be thoroughly assessed. The terrain is relatively flat, enhancing safety and allowing samples to be made down to low altitudes. Profiles will be flown to capture the entire vertical structure, from near the surface to above cloud top, and missed approaches will sometimes be flown to capture sub-cloud and near surface conditions. Although heavy air traffic can limit access to conditions favorable for icing in close proximity to cities like Chicago, most areas in the domain have reasonably light traffic, allowing for thorough sampling 24 hours a day.
The planned ICICLE field program will allow scientists to deepen their understanding of the mechanisms associated with SLD environments, including initiation, persistence and cessation, as well as operationally-critical transitions between SLD, small-drop and non-icing environments. It will also provide critical, high-quality data for real-time testing and post-analysis validation/verification of current operational icing products and candidate icing tools that are in use or under development for TAIWIN and IFI. Such icing weather information is derived from operational icing products (e.g. Current Icing Product (CIP) and Forecast Icing Product (FIP)) output and numerical models, as well as ensembles and enhancements of those models. Icing relevant information is also derived from satellite (e.g. GOES-16), NEXRAD (e.g. MRMS mosaics and products), lightning networks, pilot reports and surface weather observations (e.g. ASOS and post-processing enhancements to the algorithms applied to ASOS data). The planned ICICLE field program will be described in the context of its value for the FAA’s icing programs.
This research is in response to requirements and funding by the Federal Aviation Administration (FAA). The views expressed are those of the authors and do not necessarily represent the official policy or position of the FAA.