An ATR72 commuter aircraft crashed near Roselawn, Indiana, on 31 October 1994 killing all 68 people on board. Available weather data, including those from a Next Generation Radar, a radar wind profiler, a Geostationary Operational Environmental Satellite, and pilot reports of icing have been examined in combination with analysis fields from the Rapid Update Cycle model and forecast fields from the Pennsylvania State University/National Center for Atmospheric Research MM5 numerical model. Synthesis of this information provides a relatively complete and consistent picture of the ambient meteorological conditions in the region of the ATR72 holding pattern at-3.1 km above mean sea level. Of particular interest is the evidence that these conditions favored the development of supercooled drizzle drops within a strong frontal zone, as indicated by cloud-top temperatures of-10° to-15°C, weak radar reflectivity, and strong, vertical wind shear within the cloud and warm front.

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This continuing study evaluated turboprop aircraft performance response to various environmental conditions. These conditions included clear air, warm rain, ice only, mixed phase, and supercooled drops encountered during 19 separate ights. Supercooled droplets consisting of cloud, drizzle, and rain sizes were the main focus of this study. Aircraft response was quanti ed by rates of change in aircraft rateof-climb capability, lift and drag coef cients and, lift over drag ratio. The aircraft performance parameters were compared to environmental hydrometeor parameters, such as 80% volumetric diameter (80VD) * liquid water content (LWC), quantifying the environmental conditions. Results show that encounters with supercooled drizzle drops, or ZL, resulted in maximum rates of performance degradation. These high rates of degradation forced the pilot to take evasive action within 5 min of entering these hazardous conditions. The Wyoming King Air experienced substantial increases in drag and stall speed, substantial decreases in climb capability, and signi cant lift degradations while encountering ZL. Encounters with supercooled cloud and rain-sized drops resulted in minor to low rates of performance degradation, whereas encounters with supercooled drops in low ice particle concentrations resulted in only minor rates of degradation. In addition, aircraft response to high ice particle concentrations and low liquid water, following a ZL encounter, resulted in rapid performance recovery, possibly because of erosion of ice on the airframe. Aggregates of needles and hexagonal plates led to the highest rates of recovery, whereas cold, clear air resulted in the lowest rates of recovery. Furthermore, ight analyses facilitated quantifying ZL horizontal extents and encounter frequencies. For example, ZL measured by a one-dimensional cloud probe during atmospheric research ights exceeded 0.05 g/m 3 twice per 10 ight hours over a horizontal length of 29 km. The results presented herein show a strong relationship between aircraft response and environmental parameters utilizing the largest drops in the hydrometeor distribution (80VD * LWC and ZL LWC). The results suggest that the most severe icing is actually caused by drizzle-sized drops as opposed to freezing rain. In addition, the results suggest that activating the de-icing boots (with typical chordwise coverage) after a ZL encounter may have little effect on aircraft performance recovery. Nomenclature C D = aircraft drag coef cient C L = aircraft lift coef cient D = aircraft drag L = aircraft lift

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