Deicing fluid

Ground deicing of aircraft is commonly performed in both commercial and general aviation. The fluids used in this operation are called deicing or anti-icing fluids. The abbreviations ADF (Aircraft Deicing Fluid) or AAF (Aircraft Anti-icing Fluid) are commonly used.^[1]

Types

Deicing an aircraft with type IV fluid

Deicing fluids come in a variety of types, and are typically composed of ethylene glycol (EG) or propylene glycol (PG), along with other ingredients such as thickening agents, surfactants (wetting agents), corrosion inhibitors, and colored, UV-sensitive dye. Propylene Glycol-based fluid is more common due to the fact that it is less toxic than ethylene glycol.

The Society of Automotive Engineers publishes standards (SAE AMS 1428 and AMS 1424) for four different types of aviation deicing fluids:

1. Type I fluids have a low viscosity, and are considered "unthickened". They provide only short term protection because they quickly flow off surfaces after use. They are typically sprayed on hot (130–180°F, 55-80°C) at high pressure to remove snow, ice, and frost. Usually they are dyed orange to aid in identification and application.
2. Type II fluids are pseudoplastic, which means they contain a polymeric thickening agent to prevent their immediate flow off aircraft surfaces. Typically the fluid film will remain in place until the aircraft attains 100 knots or so (almost 200 km/h), at which point the viscosity breaks down due to shear stress. The high speeds required for viscosity breakdown means that this type of fluid is useful only for larger aircraft. The use of type II fluids is diminishing in favour of type IV. Type II fluids are generally light yellow in color.
3. Type III fluids can be thought of as a compromise between type I and type II fluids. They are intended for use on slower aircraft, with a rotation speed of less than 100 knots. Type III fluids are generally light yellow in color.
4. Type IV fluids meet the same AMS standards as type II fluids, but they provide a longer holdover time. They are typically dyed green to aid in the application of a consistent layer of fluid.

The International Organization for Standardization publishes equivalent standards (ISO 11074 and ISO 11078), defining the same four types.

Deicing fluids containing thickeners (types II, III, and IV) are also known as anti-icing fluids, because they are used primarily to prevent icing from re-occurring after an initial deicing with a type I fluid.

Chemical Composition

The main component of deicing fluid is usually propylene glycol or ethylene glycol. Other ingredients vary depending on the manufacturer, but the exact composition of a particular brand of fluid is generally held as confidential proprietary information.

Based on chemical analysis, the U.S. Environmental Protection Agency has identified five main classes of additives widely used among manufacturers:^[2]

1. Benzotriazole and methyl-substituted benzotriazole, used as corrosion inhibitor/flame retardants to reduce flammability resulting from the corrosion of metal components carrying a direct current.
2. Alkylphenol and alkylphenol ethoxylates, nonionic surfactants used to reduce surface tension.
3. Triethanolamine, used as a pH buffer.
4. High molecular weight, nonlinear polymers, used to increase viscoelasticity.
5. Colored dyes, such as azo, xanthene, triphenyl methane, and anthroquinone, used to aid in identification.

The use of 1,3-propanediol (a fermentation product of corn) as a base for deicing fluid was patented in 2009.^[3] At least one manufacturer of deicing fluids (Kilfrost) is now using that as a base for their product.

Usage statistics

The amount of fluid necessary to deice an aircraft depends on a wide variety of factors. Deicing a large commercial aircraft typically consumes between 500 US gallons (1,900 L) and 1,000 US gallons (3,800 L) of diluted fluid.

The cost of fluid varies widely due to market conditions. The amount deicing service companies charge end users is generally in the range of US$8 to US$12 per diluted gallon (US$2,1 to US$3,2 per litre).

The total annual usage of deicing fluids in the U.S. is estimated to be approximately 25 million US gallons (95,000,000 L), broken down as follows (figures from 2008, adjusted to show totals for undiluted fluid):^[4]

Measurement of performance

Deicing fluid performance is measured by holdover time (HOT), which is the length of time an aircraft can wait after being treated prior to takeoff. Holdover time is influenced by the ambient temperature, wind, precipitation, humidity, aircraft skin temperature, and other factors. For Type I fluids, the holdover time is only about five to 15 minutes, so the aircraft must take off immediately or else wait to be deiced again. Type IV fluids generally provide a holdover time between 30 and 80 minutes.

The FAA publishes official Holdover Time Tables for all approved deicing fluids, and revises them annually.^[5]

Deicing fluids work best when they are diluted with water. For example, undiluted Dow UCAR Deicing Fluid ^[6] (type I ethylene glycol), has a freezing point of −28 °C. Water, of course, freezes at 0 °C. However, a mixture of 70% deicing fluid and 30% water freezes below −55 °C. This is known as the eutectic concentration, where the freezing point of the mixture is at its lowest point, and lower than either of the component substances.

Depending on the manufacturer, deicing fluids may be sold in concentrated or pre-diluted formulations. Dilution, where necessary, must be done according to ambient weather condition and the manufacturer's instructions in order to minimize costs while maintaining safety.

The dilution of a particular sample of fluid (and hence its freezing point) can be easily confirmed by measuring its refractive index with a refractometer, and looking up the result in the deicing fluid manufacturer's tables.

Manufacturers of aviation deicing fluids must certify that their products conform to the AMS 1424 and 1428 standards by using a standard Aerodynamic Acceptance Test.

Cautions

The repeated application of Type II, Type III or Type IV anti-icing fluid may cause residues to collect in aerodynamic quiet areas, cavities and gaps. These residues may rehydrate and freeze under certain temperature changes, in high humidity and/or rain conditions. In addition, they may block or impede critical flight control systems.

An appropriate inspection and cleaning program should be established when using these types of fluids. ^[7]

Toxicity

The toxicity of deicing fluids is an environmental concern, and research is underway to find less toxic (i.e. non-glycol-based) alternatives.^[8][9] Other strategies can be used to minimize the environmental impact such as collecting used fluid and using the maximum dilution consistent with safety.

References