Structural Ice
Structural ice forms when the airplane is flown in visible moisture and the temperature of the wings, tail, and other collecting surfaces are below freezing. Icing not only weighs down the airplane, but also disrupts the smooth flow of air across the wings, tail, and control surfaces, adversely affecting everything about the way the airplane flies. Icing chokes off the engine, gathering around air intakes and on air filters, and it gathers on the wind screen, limiting the pilots forward visibility.
Probably the most dangerous icing results from flight through freezing rain. Freezing rain has a very high accumulation rate and is extremely hazardous.
However, for a VFR pilot flying a light airplane that isn't certified for flight in icing conditions, any icing encounter is extremely hazardous.
The key factor in avoiding icing is avoiding flight through visible moisture.
Pitot Heat
The airplane's pitot tube senses ram air pressure, which drives the airspeed indicator. Should the pitot tube become iced over, the airspeed indicator could provide the pilot erroneous indications. To prevent this from occurring, pitot tubes are fitted with internal electrical heaters. Pitot heat should only be required when flying through some type of visible moisture.
Carburetor Heat
In addition to carburetor ice, which forms inside the carburetor, structural ice may also form on the engine air intake and air filter, restricting airflow into the engine. Activation of carburetor heat also switches the engine to an alternate intake air sources, provided unfiltered air from a someplace where structural ice will not form. If the engine suddenly recovers full power with the application of carburetor heat, this is a good indication that airflow through the primary air intake and its air filter are restricted.
De-ice versus Anti-ice
De-ice refers to the removal of ice which has already accumulated, while anti-ice refers to the prevention of ice accumulation.
De-ice Boots
De-ice are rubber tubes attached to the leading edges of wings and tail surfaces. When activated, they are inflated by an air pump, breaking off any accumulated ice. They are intended to be activated after the accumulation of about 1/4 to 1/2 inch of ice, depending on the system and the airplane, and are not as effective at the prevention of ice accumulation.
Weeping Wings
These systems pump de-ice fluid through thousands of tiny holes drilled into the leading edges of the wing and tail surfaces. This fluid can prevent the accumulation of ice and rid the airplane of accumulated ice.
The pilot is usually able to select the rate of fluid flow, depending on the severity of the icing conditions, in order to minimize the use of the deicing fluid.
Hot Wings
Thermal systems pass hot air through tubes behind the leading edges of the wing and tail surfaces. This hot air heats these surfaces to high temperatures, preventing the accumulation of ice.
These systems are considered anti-ice systems, since they are not very effective at removing accumulated ice. They should be activated prior to entering icing conditions.
Due to high air temperatures required to make hot wings effective, they are primarily found on turbine powered airplanes and utilize engine bleed air as their heat source. However, piston airplanes could be fitted with a special combustion heater to provide hot air for this purpose.
Propeller Ice Protection
To prevent ice accumulation on the propellers, they may be fitted with electric heating elements or a system that allows anti-icing fluid to be slung onto the propellers from the their hubs.