Icing - a risk factor in aviation. Case study: The plane crash in the Apuseni Mountains (Romania) on 20.01.2014. Icing is a potentially harmful weather phenomenon for flight safety. Icing, irrespective of its forms, has a negative impact on all aviation activities since it severely impedes the aerodynamic properties of an aircraft, sometimes to such an extent that flying and landing may become impossible. Icing is a serious weather threat to aviation and may ultimately lead to deadly events. One such unfortunate event took place in the Western (Apuseni) Mountains on the 20.01.2014, when an aircraft which was transporting a medical team on a humanitarian mission, simply crashed down in the area of the Western (Apuseni) Mountains, at around 4:00 pm local time. The present study actually makes an inventory of the extremely hazardous flying conditions, by thoroughly analyzing the weather reports and data, as well as visual and synoptic messages or official elements and information from that very day. All these materials show that the flight route and the airdrome of destination were under the influence of a front which accounted for very poor meteorological conditions. On such bad weather, the flight was practically doomed to failure since low snow and sleet-bearing clouds were hanging down over the mountaintops. Flying straight through these clouds made any landmarks impossible to be seen and, furthermore, lack of orientation created false perceptions which led to fatal misjudgements and errors.