Ground deicing of aircraft is commonly performed in both commercial and general aviation. The fluid used in this operation is called de-icing or deicing fluid. The abbreviation ADF (Aircraft Deicing Fluid) is often used.
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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 & AMS 1424) for four different types of aviation deicing fluids:
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.
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:[1]
The use of 1,3-propanediol (a fermentation product of corn) as a base for deicing fluid was patented in 2009.[2] At least one manufacturer of deicing fluids (Kilfrost) is now using that as a base for their product.
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.
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): [3]
| Fluid type | Annual amount | Fraction |
|---|---|---|
| Type I Propylene Glycol | 19,305,000 US gal (73,080,000 L) | 77.1% |
| Type IV Propylene Glycol | 2,856,000 US gal (10,810,000 L) | 11.4% |
| Type I Ethylene Glycol | 2,575,000 US gal (9,750,000 L) | 10.3% |
| Type IV Ethylene Glycol | 306,000 US gal (1,160,000 L) | 1.2% |
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.[4]
Deicing fluids work best when they are diluted with water. For example, undiluted Dow UCAR Deicing Fluid[5](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.
The toxicity of deicing fluids is an environmental concern, and research is underway to find less toxic (i.e. non-glycol-based) alternatives. Other strategies can be used to minimize the environmental impact such as collecting used fluid and using the maximum dilution consistent with safety.