In aviation, icing conditions are those atmospheric conditions that can lead to the formation of water ice on the surfaces of an aircraft, or within the engine. Many aircraft, especially general aviation aircraft, are not certified for flight into known icing—icing conditions certain or likely to exist, based on pilot reports, observations and forecasts. Icing conditions exist when the air contains droplets of supercooled liquid water. I c i n g negatively effects aerodynamics of the aircraft, decreasing lift and gaining weight. In any case ice must be removed from the aircraft before the flight by treating its skin with anti-icing fluid. Also almost every commercial airliner is equipped with antiicing system, removing ice from leading edges of the aircraft by heating or mechanically.
Air France Airbus A-330 performed a scheduled flight from Rio de Janeiro to Paris on the night of 01.06.2009 over the Atlantic Ocean, when the crew faced a severe storm in Cumulo nimbus-clouds. The aircraft crashed after temporary inconsistencies between the airspeed measurements—likely due to the aircraft\’s pipot-tubes being obstructed by ice crystals—caused the autopilot to disconnect, after which the crew reacted incorrectly and ultimately led the aircraft to an aerodynamic stall from which they did not recover.
UTAir ATR-72 crashed just few moments after getting airborn from Tyumen airport on 02.04.2012. The investigation revealed that the aircraft hadn’t undergo pre-flight anti-icing treatment (which sadly was common practice in the company in order to save money), though the crew were aware snow and ice had accumulated, and, as a result,stalled on climb-out.