Icing conditions

In aviation, icing conditions are atmospheric conditions that can lead to the formation of water ice on an aircraft. Ice accretion and accumulation can affect the external surfaces of an aircraft – in which case it is referred to as airframe icing – or the engine, resulting in carburetor icing, air inlet icing or more generically engine icing. These phenomena may possibly but do not necessarily occur together. Both airframe and engine icing have resulted in numerous fatal accidents in aviation history. Not all aircraft, especially general aviation aircraft, are certified for flight into known icing (FIKI) – that is flying into areas with icing conditions certain or likely to exist, based on pilot reports, observations, and forecasts. In order to be FIKI-certified, aircraft must be fitted with suitable ice protection systems to prevent accidents by icing. Icing conditions exist when the air contains droplets of supercooled liquid water. They freeze on contact with a potential nucleation site, which in this case is the parts of the aircraft, causing icing. Icing conditions are characterized quantitatively by the average droplet size, the liquid water content and the air temperature. These parameters affect the extent, type and speed that characterize the formation of ice on an aircraft. Federal Aviation Regulations contain a definition of icing conditions that some aircraft are certified to fly into. So-called SLD, or supercooled large droplet, conditions are those that exceed that specification and represent a particular hazard to aircraft, which all aircraft must try to avoid. Qualitatively, pilot reports indicate icing conditions in terms of their effect upon the aircraft, and will be dependent upon the preexisting capabilities of the aircraft. Different aircraft may report the same quantitative conditions as different levels of icing as a result. Ice detectors are often used to indicate the presence of icing conditions. Clear ice is often clear and smooth. Supercooled water droplets, or freezing rain, strike a surface but do not freeze instantly.