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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)1 environment and analyze its impact on aircraft operation and performance. Currently no database exists on freezing rain and freezing drizzle to describe the operating environment in which the aircraft may be exposed. There is a present need to have a better understanding of freezing rain and freezing drizzle conditions. 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.

Two cases of unusual inflight icing encountered by turbine engine-powered airplanes are discussed. The atmospheric conditions in which the apparent icing occurred are unusual in that in both cases the temperature was colder than that typically associated with icing (at the altitudes at which the events occurred). The pilots' accounts of the events suggest that large supercooled droplets were encountered. These conditions are rare at any temperature, but especially at the temperatures at which these events occurred.

Extreme rain and hail conditions have caused engine rundowns during commercial airplane flights. In some cases, rain and hail conditions have caused all of the engines on the airplane to rundown simultaneously. This paper presents an analysis that quantifies the rain and hail threat in terms of the probability of encountering a storm of a given intensity for a given period of time. This analysis was used to establish recommended rain and hail concentration levels for design and certification of turbine engines.

Curves are derived that show probability versus rain water content and hail water content. These curves apply at the “worst case” single point. The rain curve indicates rain water contents of 15.3, 18.9, and 22.4 g m-3 at probability levels of 10-7, 10-8, and 10-9, respectively. The hail curve indicates hail water contents of 8.5, 12.5, and 16.5 g m-3 at probability levels of 10-7, 10-8, and 10-9, respectively. The rain and hail water content values listed above apply at 19,700 feet above mean sea level (rain), 15,000 feet above mean sea level (hail). Complete atmospheric vertical profiles for rain and hail are shown.