Supercooled large droplets (SLD) are one category icing weather conditions with liquid water diameter more than 100μm which extensively exist worldwide and out of the coverage of Appendix C of 14 CFR Part 25 icing conditions. Moreover, SLD icing conditions are much more rigorous and hazardous for flight than Appendix C. After several years study, Federal Aviation Administration (FAA) and European Aviation Safety Agency (EASA) have issued final rules on SLD icing conditions and corresponding airworthiness requirements in 2015. A few regulations are newly established including §25.1420 and Appendix O which present the SLD icing conditions as well as the general airworthiness requirements of operating in SLD icing conditions for transport category aircraft. This work firstly focuses on the background analysis of the SLD icing conditions (Appendix O) and the general SLD icing requirements (§25.1420), which can help us to understand these requirements. Secondly, the technical definition and explanation of these requirements are presented. Finally, a fully discussion is given on how to comply with these SLD icing protection airworthiness requirements, including how to consider these requirements during aircraft icing protection system (IPS) design, how to verify the compliance of the design with these requirements and which Means of Compliance (MOC) to use. We hope this work can provide some useful information for aircraft designers during their aircraft design practice, as well as for aircraft inspectors from aviation authorities during their certification practice.
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