Icing conditions
Encyclopedia
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 as carburetor icing
. Inlet icing is another engine-related danger, often occurring in jet aircraft
. These icing phenomena do not necessarily occur together. Many aircraft are not certified for flight into known icing—icing conditions certain to exist, based on pilot reports, observations
, and forecasts
.
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.
Qualitatively, pilot reports indicate icing conditions in terms of their effect upon the aircraft, and will be dependent upon the capabilities of the aircraft. Different aircraft may report the same quantitative conditions as different levels of icing as a result.
Airframe or structural ice adds to an aircraft's weight and disrupts airflow on affected surfaces. The effects include decreased lift, increased drag, increased stall speed (due to the weight increase and airflow disruption on the wing), interference with movement of control surfaces, propeller imbalance, communications interference, decrease in visibility, pitot-static blockage, jammed landing gear,or loss of control due to disruption of airflow on critical control surfaces. The crash of American Eagle Flight 4184
is an example of the latter.
In engines, carburetor ice and inlet ice can lead to reduced power or complete engine failure.
If ice (or other contaminants) are present on an aircraft prior to takeoff, this 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 engine
s are not susceptible to carburetor icing but can suffer from blocked inlets. In these engines an alternate air source is often available.
Note 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.
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...
, icing conditions are those atmospheric conditions that can lead to the formation of water ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
on the surfaces of an aircraft, or within the engine as carburetor icing
Carburetor icing
Carburetor, or carb icing, is an icing condition which can affect any carburetor under certain atmospheric conditions. Carburetor icing occurs when there is humid air, and the temperature drop in the venturi causes the water vapor to freeze. The ice will form on the surfaces of the carburetor...
. Inlet icing is another engine-related danger, often occurring in jet aircraft
Jet aircraft
A jet aircraft is an aircraft propelled by jet engines. Jet aircraft generally fly much faster than propeller-powered aircraft and at higher altitudes – as high as . At these altitudes, jet engines achieve maximum efficiency over long distances. The engines in propeller-powered aircraft...
. These icing phenomena do not necessarily occur together. Many aircraft are not certified for flight into known icing—icing conditions certain to exist, based on pilot reports, observations
METAR
METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting....
, and forecasts
Terminal Aerodrome Forecast
In meteorology and aviation, TAF is a format for reporting weather forecast information, particularly as it relates to aviation. "TAF" is an acronym of Terminal Aerodrome Forecast or, in some countries, Terminal Area Forecast....
.
Definition of icing conditions
Icing conditions exist when the air contains droplets of supercooled liquid water; icing conditions are characterized quantitatively by the average droplet size, the Liquid Water Content and the air temperature. These parameters affect the extent and speed that characterize the formation of ice on an aircraft. Federal Aviation RegulationsFederal Aviation Regulations
The Federal Aviation Regulations, or FARs, are rules prescribed by the Federal Aviation Administration governing all aviation activities in the United States. The FARs are part of Title 14 of the Code of Federal 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.
Qualitatively, pilot reports indicate icing conditions in terms of their effect upon the aircraft, and will be dependent upon the capabilities of the aircraft. Different aircraft may report the same quantitative conditions as different levels of icing as a result.
Types of structural ice
- Clear ice is often clear and smooth. Supercooled water droplets, or freezing rainFreezing rainFreezing rain is the name given to rain that falls when surface temperatures are below freezing. The raindrops become supercooled while passing through a sub-freezing layer of air, many hundred feet , just above the surface, and then freeze upon impact with any object they encounter. The resulting...
, strike a surface but do not freeze instantly. Often "horns" or protrusions are formed and project into the airflow. - Rime ice is rough and opaque, formed by supercooled drops rapidly freezing on impact. Forming mostly along an airfoilAirfoilAn airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....
's stagnation pointStagnation pointIn fluid dynamics, a stagnation point is a point in a flow field where the local velocity of the fluid is zero. Stagnation points exist at the surface of objects in the flow field, where the fluid is brought to rest by the object...
, it generally conforms to the shape of the airfoil. - Mixed ice is a combination of clear and rime ice.
- Runback ice is the result of water freezing on unprotected surfaces. Often forming behind deicing boots or heated leading edges, it was a factor in the crash of American Eagle Flight 4184American Eagle Flight 4184American Eagle Flight 4184 was an American Eagle ATR 72 that crashed after flying into unknown icing conditions on October 31, 1994. Control was lost and all aboard were killed.-History:...
.
- SLD ice refers to ice formed in Supercooled Large Droplet (SLD) conditions. It is similar to clear ice, but because droplet size is large, it often extends to unprotected parts of the aircraft and forms larger ice shapes, faster than normal icing conditions.
Effect of Icing
When flying in icing conditions, ice builds as long as those conditions exist and if left unchecked results in dangerous conditions.Airframe or structural ice adds to an aircraft's weight and disrupts airflow on affected surfaces. The effects include decreased lift, increased drag, increased stall speed (due to the weight increase and airflow disruption on the wing), interference with movement of control surfaces, propeller imbalance, communications interference, decrease in visibility, pitot-static blockage, jammed landing gear,or loss of control due to disruption of airflow on critical control surfaces. The crash of American Eagle Flight 4184
American Eagle Flight 4184
American Eagle Flight 4184 was an American Eagle ATR 72 that crashed after flying into unknown icing conditions on October 31, 1994. Control was lost and all aboard were killed.-History:...
is an example of the latter.
In engines, carburetor ice and inlet ice can lead to reduced power or complete engine failure.
Icing prevention and removal
Several methods exist to reduce the dangers of icing. 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, this must be removed from critical surfaces. Removal can take many forms:
- Mechanical means, which may be as simple as using a broom or brush to remove snow
- Application of deicing fluidDeicing fluidGround deicing of aircraft is commonly performed in both commercial and general aviation. The fluid used in this operation is called de-icing or deicing fluid...
or even hot water to remove ice, snow, etc. - Use of infrared heating to melt and remove contaminants
- Put the aircraft into a heated hangar until snow and ice have melted
- Position aircraft towards the sun to maximize heating up of snow and ice covered surfaces. In practice this method is limited to thin contamination, by the time and weather conditions.
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.
- A common approach is to route engine "bleed air" into ducting along the leading edges of wings and tailplanes. The air heats the leading edge of the surface and this melts or evaporates ice on contact. On a turbine powered aircraft air is extracted from the compressor section of the engine. If the aircraft is turbocharged piston powered, bleed air can be scavenged from the turbocharger.
- Some aircraft are equipped with pneumatic deicing bootDeicing bootA deicing boot is a type of ice protection system installed on aircraft surfaces to permit a mechanical deicing in flight. Such boots are generally installed on the leading edges of wings and control surfaces A deicing boot is a type of ice protection system installed on aircraft surfaces to permit...
s that disperse ice build-up on the surface. These systems require less engine bleed air but are usually less effective than a heated surface. - A few aircraft use a weeping wingWeeping WingWeeping Wing is a chemical aircraft ice protection system wherein a glycol-based chemical is released onto the wing surface using small orifices on the leading edge of the wing...
system, which has hundreds of small holes in the leading edges and releases anti-icing fluid on demand to prevent the buildup of ice. - Electrical heating is also used to protect aircraft and components (including propellors) against icing. The heating may be applied continuously (usually on small, critical, components, such as pitot static sensors and angle of attack vanes) or intermittently, giving an effect similar to the use of deicing bootDeicing bootA deicing boot is a type of ice protection system installed on aircraft surfaces to permit a mechanical deicing in flight. Such boots are generally installed on the leading edges of wings and control surfaces A deicing boot is a type of ice protection system installed on aircraft surfaces to permit...
s.
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
Carburetor heat
Carburetor, carburettor, carburator, carburettet heat is a system used in automobile and piston-powered light aircraft engines to prevent or clear carburetor icing. It consists of a moveable flap which draws hot air into the engine intake...
is applied to carbureted engines to prevent and clear icing. Fuel-injected engine
Fuel injection
Fuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....
s are not susceptible to carburetor icing but can suffer from blocked inlets. In these engines an alternate air source is often available.
Note 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.