Aircraft Icing Caused by Large Supercooled Droplets

Abstract

The characteristic of aircraft icing environments containing large supercooled droplets are described. Substantial loss in rate of climb capability can result from less than 10 minutes duration in conditions where fewer than 0.1–1 cm⁻³ of droplets 30–400 μm in diameter are present. These conditions are found to have a greater effect than those where the liquid water was confined to smaller (generally less than about 30 μm diameter) droplets. Measurements from research aircraft flying in regions containing these large droplets, located in the Sierra Nevada in California, near Amarillo, Texas, and in northern Arizona are presented. Temperatures ranged from −5.5° to −9.4°C in 13 regions. The sizes of the droplets responsible for performance loss varied with each encounter but ranged from tens to hundreds of micrometers, and these were accompanied by few to no ice crystals. Two case studies are examined in further detail, including the weather conditions present at the time of the encounters. The meteorological situations leading to formation of these large droplets provide suitable environments for coalescence growth, or for prolonged depositional growth, and include weak atmospheric instability, warm (temperatures greater than about −15°C) cloud tops, and sufficient moisture.