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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 which are certain to exist based on pilot reports, observations, and forecasts.

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 with which ice will form on an aircraft. Aircraft which are certified for flight into known icing condition use the definitions of to define icing conditions. So-called SLD, or Supercooled Large Droplet, conditions are those which 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

• Glaze ice is often clear and smooth. Supercooled water droplets, or freezing rain, 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 airfoil's stagnation point, 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 4184.
• SLD ice refers to ice formed in SLD conditions. It is similar to glaze ice, but due to the larger droplet size often extends to unprotected parts of the aircraft and forms larger ice shapos, faster than normal icing conditions.

Effect of Icing

When flying in icing conditions, ice gradually builds as long 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 increased stall speed (due to the weight increase and airflow disruption on the wing), 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.

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 fluid or even hot water to remove ice, snow, etc.
• Use of infrared heating to melt and remove contaminants

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:

• One 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 and propellers are equipped with pneumatic deicing boots which 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 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 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 boots.

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.

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.

See also

• PIREPS

References

1. FAR 25 Appendix C

External links

• Aircraft Icing - NASA Page
• 14 CFR 25 Appendix C : Atmospheric Icing Conditions for Aircraft Certification

• Articles lacking sources from December 2007
• Aviation terminology