The design and certification icing envelopes for ice protection systems do not consider potentially severe icing conditions associated with supercooled large droplets (SLD). Criteria for the SLD icing conditions have not been established.
Since aircraft operate in freezing rain and freezing drizzle during the winter months, it is important that we recognize the supercooled large droplet (SLD)
Drop-size distributions (DSD) in icing clouds are analyzed. These data were obtained during the Canadian Freezing Drizzle Experiments (CFDE) I and III over the St. John’s Newfoundland regions and the Great Lakes, respectively. The DSD data were collected using several probes giving full coverage in the drop-size range from 2 to 6400 μm drop diameters. The data are averaged over one-minute intervals yielding 1755 individual DSDs in 38 instrumented flights. The distributions are grouped into three types, namely, 1) (Appendix C) (ZC), 2) Freezing Drizzle (ZL), and 3) Freezing Rain (ZR). The types are grouped based on the range of drop size. ZL and ZR conditions are considered as subsets of SLD conditions.
Preliminary work is presented in exploring a possible SLD engineering design standard. The maximum limiting value of liquid water content (LWC) for ZL and ZR with median volume diameter > 50 μm has been estimated based on the exceedance probability of 0.01. Also, the liquid mass associated with droplet diameter greater than 50 μm has been identified. To simplify the drop-size distribution for engineering analysis, the mass is distributed among several droplet-size bins.
An engineering analysis of the distributions indicates that the measured type ZC data is in general agreement with the current icing envelopes in Federal Aviation Regulations (FAR) Part 25, (Appendix C). No changes are necessary in the current (Appendix C) envelopes used for the ice protection system design and certification. The ZL and ZR conditions show interesting characteristics. Many conditions have a bi-modal drop-size distribution. The distributions with high LWC have median volume diameter less than 50 μm, but with many drops exceeding 135 μm in diameter. Those distributions in which the MVD exceeds 50 μm and the maximum droplet size is up to 3000 μm tend to have LWC values below 0.45 g m-3.