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Aircraft icing on the ground
De-icing is the process of removing frozen contaminant, snow, ice, slush, from a surface
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Anti-icing is the process of protecting against the formation of frozen contaminant, snow, ice, slush on a surface.
De-icing can be accomplished by mechanical methods (scraping, pushing); through the application of heat; by use of dry or liquid chemicals designed to lower the freezing point of water (various salts or brines, alcohols, glycols); or by a combination of these different techniques. De-icing fluids are always applied heated and diluted.
Anti-icing is accomplished by applying a protective layer, using a viscous fluid called anti-ice fluid, over a surface to absorb the contaminate. All anti-ice fluids offer only limited protection, dependent upon frozen contaminant type and precipitation rate. A fluid has failed when it no longer can absorb the contaminant and it essentially becomes a contaminant itself. If it fails it must be washed from the surface using a de-icing fluid.
When there are freezing conditions and precipitation, de-icing an aircraft is crucial. Frozen contaminants cause critical control surfaces to be rough and uneven disrupting smooth air flow and greatly degrading the ability of the wing to generate lift and increasing drag. This situation can dramaticaly change the shape of the control surfaces, increase weight, and badly affect performance.
De-icing techniques are also employed to ensure that engine inlets and various sensors on the outside of the aircraft are clear of ice or snow.
When Does Ice Form?
Generally the heaviest ice formation occurs when the water content in the air is highest, which is close to 0˚C. This is the real danger zone as it some may consider it ‘not sufficiently cold to worry’, when in fact it is exactly the time to be concerned – as the temperature falls further the air actually becomes increasingly drier so produces less ice.
Commercial aircraft fly at high enough altitudes above the cloud base to not be troubled by ice in-flight, but during take-off and landing, exposed surfaces are at risk of ice and frost accumulation, and it is when the aircraft is actually on the ground that it is at most danger from winter conditions. What many people fail to realise is that even a small amount of build-up can substantially degrade an aircraft’s performance.
What Other Kinds Of Ice Are There?
Hoar frost is certainly much easier to spot but is all too often also underestimated. It commonly accumulates on aircraft overnight, particularly when the sky is clear, and forms because the aircraft’s body becomes cooler than the outside air. This process is known as radiant cooling, and while only a few degrees difference is needed, it’s not uncommon for the aircraft skin to actually be as much as 5˚C or, in very severe conditions, 10˚C cooler than the outside air temperature. This is why frost will readily form on an aircraft even when the outside air temperature is as much as 4˚C above zero.
What About Safety Checks?
So how should you check an aircraft for ice deposits? Initial safety checks are imperative, and you should give particular attention to critical surfaces like the flying wing – especially its leading edge as this is where the majority of the lift is produced. Visually check for ice deposits, but if you can, run a finger over to feel for any change to the smoothness of the wing edge – even the Civil Aviation Authority advises that your eyes and your hands are the best tools for checking for ice and frost. Most airports and major airlines will have their own operating guidelines, and where these differ we would always advise to err on the side of caution and if in doubt, treat the aircraft exposed surfaces.
Why Is Ice So Dangerous?
If the leading edge becomes contaminated with ice, this has been shown to substantially reduce the effectiveness of lift. Contrary to some thought, it is not the weight of the ice which causes the problems, but the surface roughness – even something that’s only as rough as the finest emery paper is enough to cause a distortion of the aircraft’s aerodynamic properties.
After an accident in Colorado during the winter of 2004 / 2005, the National Transportation Safety Board in Washington studied the effect of ice build-up and found that particles of frost or ice as fine as a grain of table salt and distributed as sparsely as one per cm2 over an aircraft wing’s upper surface can destroy enough lift to actually prevent it taking off. And according to their wind tunnel data, this level of frost and ice accumulation can cause lift losses of between 22% and 33% both on the ground and in the air. Combine this loss of lift with other factors such as an aircraft’s load and the length of a runway, and it becomes much clearer why we see so many avoidable accidents every year.
How Do We De-Ice An Aircraft?
To de-ice an aircraft we need to remove the rough surface and leave the wings – particularly the leading edges – as clean as possible. There are various methods of de-icing but it’s most common for an aircraft to be sprayed, and a number of fluids are available. De-icing fluids typically consist of glycol and water mixtures that are heated in advance and applied hot. The heat melts the ice while the residual glycol prevents re-freezing. The degree of protection against re-freezing depends on the type of liquid that is applied.
What Type of De and Anti-Icing Fluids are Available and How Do They Differ?
There are three core families of fluids: Type I, Type II and Type IV and they differ according to their function and the holdover (ongoing anti-icing protection) they provide.
Type I fluids are primarily used for de-icing and do not offer any significant holdover protection, so are often used as part of a two step de-icing / anti-icing procedure. A range of this type of fluid is available. The fluids are all coloured orange for ease of recognition, offer good sprayability with minimal foaming, and have a pH around 8.5 to 9.5. Type I fluids are always applied diluted and heated.
Type II fluids can be used for de-icing purposes, but unlike Type I fluids, they also offer extended anti-icing holdover protection and can be used unheated and diluted or undiluted for anti-icing, heated and undiluted for de and anti-icing as a one step process, diluted with water and heated for de and anti-icing as a one step process, or diluted with water and heated as the de-icing stage in a two-step process when used with unheated and undiluted fluid. These fluids are a light straw colour for ease of recognition and have a pH around 7.0.
Type IV fluids offer maximum anti-icing holdover protection but can also be used for de-icing purposes – similarly to Type II fluids, Type IV fluids can be used in a variety of ways. Type IV fluid, offers maximum holdover against freezing precipitation – especially snow – and can be used cold to prevent the build-up of frost, ice and snow or applied hot as a one-step de and anti-icer. Very useful at airports which experience severe weather conditions and / or long taxi times, Type IV fluids offer extended holdover times, excellent aerodynamic flow off properties and good dry out characteristics, as well good shear and storage stability. For ease of recognition these fluids are coloured green.
All of the fluid types are designed with a low immediate viscosity, and when applied to frozen deposits they melt and dissolve the ice which then drains away. With Type II and Type IV fluids, a small amount does remain on the wings, however, as the aircraft takes off this shears completely away to leave the wing clean and smooth.
In some cases several types of fluid are applied, first the heated glycol/water mixture to remove contaminants, followed by the unheated thickened fluid to keep ice from reforming before the aircraft takes off. This is called "a two-step procedure".
Taken from various sources including Kilfrost who are the global market leader in the supply of de-icing and anti-icing products to the civil aviation and transportation industries as well as ice protection fluids to general aviation.
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Posted : Thursday 7th Jan 2010, 9:06pm
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