New ground deicing hazard associated with freezing drizzle ingestion by jet engines

A new ground deicing hazard is described, consisting of the accretion of freezing drizzle onto jet engine fan blades and cowlings, and subsequent shedding of the accreted ice during takeoff leading to damage to jet engine fan blades. Cases of damage to aircraft from hazardous surface icing conditions at Denver, Colorado and Oslo, Norway are described. The two cases at Denver cost United Airlines over $2 million in damage to 12 B737-300 engines. The hazard is identified as heavy freezing drizzle through examination of National Weather Service observations of upper level temperature and humidity, satellite, radar, and freezing rain sensor data. The official National Weather Service observation during these cases, however, was either light snow and mist or light freezing drizzle. The reason for this misreport and underestimate of intensity lies in the current reporting rules for determining freezing drizzle intensity by visibility and not by precipitation rate. Theoretical relationships are presented that show that the variation in drizzle size distribution and the difference in determining visibility from day and night is the cause of the poor correlation of drizzle rate with visibility.