Deicing fluid
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Ground deicing of aircraft is commonly performed in both commercial and general aviation.
Deicing fluids come in a variety of types, and are typically composed of ethylene glycol or propylene glycol, along with other ingredients such as thickening agents, wettening agents, corrosion inhibitors, and colored, UV-sensitive dye. Propylene Glycol is more common due to the fact it is less toxic than ethylene glycol.
The Society of Automotive Engineers publishes standards (SAE AMS 1428 & 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) at high pressure to remove snow, ice, and frost. Often 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. Typically the fluid film will remain in place until the aircraft attains 100 knots or so, at which point the viscosity breaks down due to sheer 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 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 rarely used.
- Type IV fluids, commonly referred to as anti-icing fluids because an aircraft must first be deiced prior to a Type IV fluid application, meet the same viscosity specifications 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.
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.
Deicing fluids are typically sold in concentrated form and diluted with water according to the ambient weather conditions. The freezing point of undiluted fluid is typically about -30 °C. Adding water reduces the freezing point, with a minimum of -55 °C being reached at a dilution of 75% fluid and 25% water. Further dilution raises the freezing point again. The dilution of deicing fluid is carefully done in order to minimize costs while maintaining safety.
Deicing fluid performance is measured by holdover time, 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 has to take off right away or else wait to be deiced again. Type IV fluids generally provide a holdover time between 30 and 80 minutes.
The toxicity of deicing fluids is an environmental concern, and research is underway to find less toxic alternatives. For example, the US Air Force Research Laboratory (AFRL) is currently studying a chemical known as METSS ADF-2. Other strategies can be used to minimize the environmental impact such as collecting used fluid and using the maximum dilution consistent with safety.
