
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[1] – or the engine, resulting in carburetor icing, air inlet icing or more generically engine icing.[2] 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.[3] In order to be FIKI-certified, aircraft must be fitted with suitable ice protection systems to prevent accidents by icing.
Definition[edit]
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[4] 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.
Several methods exist to reduce the dangers of icing, using ice protection systems. The first, and simplest, is to avoid icing conditions altogether, but for many flights this is not practical.
If ice (or other contaminants) are present on an aircraft prior to takeoff, they must be removed from critical surfaces. Removal can take many forms:
All of these methods remove existing contamination, but provide no practical protection in icing conditions. If icing conditions exist, or are expected before takeoff, then anti-icing fluids are used. These are thicker than deicing fluids and resist the effects of snow and rain for some time. They are intended to shear off the aircraft during takeoff and provide no inflight protection.
To protect an aircraft against icing in-flight, various forms of anti-icing or deicing are used:
In all these cases, usually only critical aircraft surfaces and components are protected. In particular, only the leading edge of a wing is usually protected.
Carburetor heat is applied to carbureted engines to prevent and clear icing. Fuel-injected engines are not susceptible to carburetor icing, but can suffer from blocked inlets. In these engines, an alternate air source is often available.
There is a difference between deicing and anti-icing. Deicing refers to the removal of ice from the airframe; anti-icing refers to the prevention of ice accumulating on the airframe.
Unmanned aircraft are an emerging technology with a large variety of commercial and military applications. In-flight icing occurs during flight in supercooled clouds or freezing precipitation and is a potential hazard to all aircraft. In-flight icing on UAVs imposes a major limitation on the operational envelope.[5]
Unmanned aircraft are more sensitive and susceptible to icing compared to manned aircraft.[6] The main differences between UAVs and manned aircraft when it comes to icing are:
The parts of the UAV most exposed to icing are the airspeed sensor, the leading edge of aerodynamic surfaces, rotors, and propellers.
Icing on UAVs is a global phenomenon, and icing conditions at the operational altitude can occur year round around the world. However, icing risks are particularly big in the sub arctics, Arctic and Antarctic. In large parts of the Nordics, for example, icing conditions are present from 35% to more than 80% of the time from September through May.[7]
Media related to Icing in aviation at Wikimedia Commons