Common snowfall conditions associated with aircraft takeoff accidents

Snowfall and surface meteorological data from five takeoff accidents related to inadequate deicing or antiicing are examined. Despite common values of liquid-equivalent snowfall rate, temperature, and windspeed, the visibility varied widely. The common values of liquid-equivalent snowfall rates are consistent with recent studies showing that the water content of the snow is the primary factor responsible for the failure of deicing fluids to protect an aircraft from reicing. Liquid-equivalent rates, however, are not available to pilots in real time, and so they instead rely on their own vision or a National Weather Service snowfall intensity estimate based on visibility to estimate snowfall rate. It is shown that snow intensity estimates based on visibility alone can often mislead pilots into thinking that conditions are not as bad as they actually are. We define the hazard as high-visibility-high-snowfall-rate conditions. Nighttime conditions lead to a factor of two increase in visibility during snowfall as compared to daytime, also contributing to the high-snowfall-rate-high-visibility condition. Wind is shown to result in an enhanced accumulation of snow on a wing when an aircraft is facing downwind and stationary due to the approximate 10 deg angle of the wing to the horizontal. Nearly all of the accidents also occurred during the peak snowfall period of a storm in association with snowbands.