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Old 30th Nov 2010, 00:50
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SNS3Guppy
 
Join Date: Oct 2005
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Cieloitaliano,

An important difference exists between operations at cruise altitude and the central heating condensation outlet pipe at your house. You have a lot of relative humidity difference, between your home, and the air at 35,000'.

You appear to be asking specifically about condensation, though icing potential still exists in other forms.

Condensation is found on the ground, often in the form of water dripping off the wing around fuel cells (where the fuel is cold from flight), or frost under the wings in the area of the fuel tanks.

Frost sublimates fairly quickly around much of the airframe at cruise, and doesn't really have a chance to form once it's gone, due to lower pressures around the airframe. Icing can occur from various sources. Large supercooled water droplets such as may be found in convective weather can remain liquid and below freezing, and impact with the surfaces of the airplane can break the surface tension on the water droplets, leading to instant freezing. Likewise, freezing rain can cause buildups on the airplane.

Ice protection exists in two basic forms: anti-ice, and de-ice. Anti-ice prevents the buildup from ice, generally by heating the surface that's being protected. De-ice involves various methods, from chemical means, to expanding and contracting surfaces (inflatable rubber "boots"), and also various forms of heating or vibrating surfaces.

Some systems weep a fluid such as isopropyl alcohol over the nose, or wing, or propeller, to help prevent ice formation (and in some cases remove it, though anti-ice systems are never to be counted on for removing ice). Some heat surfaces such as the wing, engine nacelle inlets, windshields, etc. Some, like the boots on some inlets or leading edges, inflate to break up ice.

-60 isn't too cold for ice to attach to the airplane, but it's generally too cold for water to remain in liquid form. Water has been found in liquid form as low as -40 degrees, but much beyond that, and everything is frozen. Once frozen, it doesn't tend to adhere to the airplane.

Airplanes that operate at fast speeds tend to experience a significant change in surface temperature due to ram air effects. Ice that forms at slower speeds often won't at higher speeds. Thin surfaces and airfoils tend to ice more easily than thick ones, and tend to be more easily affected by smaller buildups of ice.

Most of all, we generally try to stay out of icing conditions, where ever possible. When we do have to fly in icing conditions, we minimize our exposure to ice, and avoid severe ice. We tend to turn on ice protection equipment early in most cases (not all; some doesn't function well until it's already iced-up). Sometimes we have operating limitations when ice protection is in use (the airplane I presently fly has minimum engine power settings when nacelle anti-ice is in use, for example, to ensure enough hot air from the engine "bleed" system is available to properly protect the engine nacelles).

Avoiding ice is always better than having to contend with it, whenever possible.
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