Response of a research aircraft to icing and evaluation of severity indices

The relationship between four atmospheric parameters and three measures of flight degradation are investigated using data from an instrumented research aircraft. Data from flights that took place in wintertime stratus clouds over northeastern Colorado are emphasized; additional data points from encounters with large supercooled droplets over northern California and northern Arizona are included. The maximum decrease in coefficient of lift due to icing was 35%, with 68% of cases within 10% of the uniced aircraft value. Coefficient of drag increased by up to 230% as a result of icing and climb capability was reduced by up to 6.9 m/s. Greater performance loss was related to higher liquid water content, median volume diameter, and potential accumulation of ice. A combination of liquid water content > 0.2 g/m3, median volume diameter > 30 /un, and temperature > — 10°C was responsible for the largest performance decreases. Severity indices that were dependent on liquid water content, median droplet volume diameter, and temperature were tested. An index that takes into account the effects of large droplet icing provided the best relation between higher severity level and increased performance degradation.