Common snowfall conditions associated with aircraft takeoff accidents

Snowfall and surface meteorological data from five takeoff accidents relate d to inadequate deicing or antiicing are examined. Despite common values of liquid-equivalent snow fall rate, temperature, and windspeed, the visibili ty varied widely. The common values of liquid-equivalent snowfall rates are consistent with recent studies showing that the mater content of the snow is the primary factor responsible for the failure of deicing fluids to prot ect an aircraft from reicing, Liquid-equivalent rates, however, are not ava ilable to pilots in real time, and so they instead rely on their own vision or a National Weather Service snowfall intensity estimate based on visibil ity to estimate snowfall rate. It is shown that snow intensity estimates ba sed on visibility alone can often mislead pilots into thinking that conditi ons are not as bad as they actually are. We define the hazard as high-visib ilily-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 s hown to result in an enhanced accumulation of snow on a wing when an aircra ft 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 duri ng the peak snowfall period of a storm in association with snowbands.