In ground deicing of aircraft, aircraft deicing fluid (ADF), aircraft deicer and anti-icer fluid (ADAF) or aircraft anti-icing fluid (AAF) are commonly used for both commercial and general aviation. Environmental concerns include increased salinity of groundwater where de-icing fluids are discharged into soil, and toxicity to humans and other mammals.
Fluids used
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, colors, and UV-sensitive dye. Propylene glycol-based fluid is more common because it is less toxic than ethylene glycol.
SAE International (formerly known as the Society of Automotive Engineers) publishes standards (SAE AMS 1428 and AMS 1424) for four different types of aviation deicing fluids: 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. Type II fluids are pseudoplastic, which means they contain a polymeric thickening agent to prevent their immediate flow off aircraft surfaces. Type II prevents snow, ice or frost contamination from adhering to the aircraft from the apron to takeoff. Typically the fluid film will remain in place until the aircraft attains or so, 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 clear in color. 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 bright yellow in color. Type IV has the same purpose and meets 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 11075 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.
TKS fluid is similar to Type I fluid and is used by in-flight TKS ice protection systems. It can also be used for ground-based deicing. It conforms to different standards than Type I fluid, namely DTD 406B, AL-5, and NATO S-745.
Chemical composition
The main component of deicing fluid is a freezing point depressant (FPD), 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.
Ethylene glycol (EG) fluids are still in use for aircraft deicing in some parts of the world because it has a lower operational use temperature (LOUT) than propylene glycol (PG). However, PG is more common because it is less toxic than ethylene glycol.
In non-aviation contexts, deicing chemicals typically contain chloride salts, such as calcium chloride (CaCl2). These are prohibited in aircraft deicing fluids due to their corrosive properties.
Based on chemical analysis, the U.S. Environmental Protection Agency has identified five main classes of additives widely used among manufacturers: Benzotriazole and methyl-substituted benzotriazole, used as corrosion inhibitor. Alkylphenol and alkylphenol ethoxylates, nonionic surfactants used to reduce surface tension. Triethanolamine, used as a pH buffer. High molecular weight, nonlinear polymers, used to increase viscoelasticity. Dyes used to aid in identification.
TKS fluid (used by TKS in-flight deicing systems) contains 85% ethylene glycol, 5% isopropyl alcohol, and 10% water.
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 and 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$2.10 to US$3.20 per liter).