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.[1][2] Environmental concerns include increased salinity of groundwater where de-icing fluids are discharged into soil, and toxicity to humans and other mammals.[3][4]
Deicing a Boeing 737 with Type I fluidA layer of Type IV anti-icing 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, 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 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 100 knots (190km/h) 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.
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.[5] 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. [6]:2–29[2]
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, are used as corrosion inhibitor.[7]
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 (1,900L) and 1,000 US gallons (3,800L) 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).
The total annual usage of deicing fluids in the U.S. is estimated to be approximately 25million US gallons (95,000,000L), broken down as follows (figures from 2008, adjusted to show totals for undiluted fluid):[8]:43
Fluid type
Annual amount
Fraction
Type I Propylene Glycol
19,305,000USgal (73,080,000L)
77.1%
Type IV Propylene Glycol
2,856,000USgal (10,810,000L)
11.4%
Type I Ethylene Glycol
2,575,000USgal (9,750,000L)
10.3%
Type IV Ethylene Glycol
306,000USgal (1,160,000L)
1.2%
Note that type II and type III fluids are rarely used in the United States.
Measurement of performance
Deicing fluid performance is primarily measured by Holdover Time (HOT), and Lowest Operational Use Temperature (LOUT).
Holdover Time (HOT) is the length of time an aircraft can wait after being treated prior to takeoff. Holdover time is influenced by the fluid dilution, ambient temperature, wind, precipitation, humidity, aircraft skin material, aircraft skin temperature, and other factors. If the Holdover Time is exceeded the aircraft must be re-treated before takeoff.
Lowest Operational Use Temperature (LOUT) is the lowest temperature at which a de/anti-icing fluid will adequately flow off aircraft critical surfaces and maintain the required anti-icing freezing point buffer for type-II, III and IV fluid which is 7°C (13°F), and 10°C (18°F) for type-I fluid below outside air temperature (OAT).
In the United States, the Federal Aviation Administration (FAA) publishes official Holdover Time and Lowest Operational Use Temperature tables for all approved deicing fluids, and revises them annually.[9]
For Type I fluids, the Holdover Time listed in the FAA tables ranges from 1 to 22 minutes, depending on the above-mentioned situational factors. For Type IV fluids the Holdover Time ranges from 9 minutes to 160 minutes.
Dilution
Deicing fluids work best when they are diluted with water. For example, undiluted Dow UCAR Deicing Fluid[10] (Type I ethylene glycol), has a freezing point of −28°C. Water freezes at 0°C. However, a mixture of 70 percent deicing fluid and 30 percent 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.
Layer thickness
Thickened fluids (types II, III, and IV) are intended to remain on surfaces after application in order to provide anti-icing protection. They are also designed to slough off during the takeoff run so that they do not negatively affect flight performance. In order to obtain these objectives they must be applied at the correct thickness.
For a typical type IV fluid, a layer thickness of between 1 mm and 3 mm is required, however each manufacturer will document their own requirements.
Standards compliance
Manufacturers of aviation deicing fluids must certify that their products conform to the AMS 1424 and 1428 standards using the defined High Speed Ramp Test, Low Speed Ramp Test, and Water Spray Endurance Test.[11]
The objective of these standards is to ensure acceptable aerodynamic characteristics of the deicing/anti-icing fluids as they flow off aircraft lifting and control surfaces during the takeoff ground acceleration and climb.
With the development of non-glycol deicing fluids these standards are evolving to address additional factors such as corrosion, foaming, thickening, residue formation, slipperiness, and mold formation.[12]
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.[13]
Environmental impacts
Many de-icing fluids, including glycol-based fluids, are toxic to humans and other mammals, and damage the ecosystems where the fluids are discharged, such as the areas around airports. The use of such fluids can cause changes to nearby acquatic habitats that harm fish and other wildlife.[6]:2–23[14][15]
Ethylene glycol and propylene glycol exert high levels of biochemical oxygen demand (BOD) during degradation in surface waters. Large quantities of dissolved oxygen (DO) in the water column are consumed when microbial populations decompose propylene glycol.[6]:2–23 This process can adversely affect fish and other aquatic life by consuming oxygen needed for their survival.
Thickened fluids typically use alkylphenol ethoxylate (APE) surfactants, the biodegradation products of which have been shown to be endocrine disruptors, and as such these are banned in Europe and are under EPA scrutiny in the U.S.[16] A number of fluids also use benzotriazole or tolyltriazole corrosion inhibitors, which are toxic and non-biodegradable and thus persist in the environment.[17] Research is ongoing to find less problematic alternatives.[18] This is proving to be challenging due to the many performance and safety factors that need to be considered.[12]
One U.S. FAA-approved deicing fluid (Kilfrost DF Sustain) is 1,3-propanediol, a fermentation product of corn, as a freezing point depressant instead of ethylene glycol or propylene glycol.[19]
Benzotriazole (and tolyltriazoles), although not highly toxic, is not readily degradable and has a limited sorption tendency. Hence, it is only partly removed in wastewater treatment plants and a substantial fraction reaches surface water such as rivers and lakes.[7]
Petrochemicals are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sources such as maize, palm fruit or sugar cane.
Brine is a high-concentration solution of salt in water. In diverse contexts, brine may refer to the salt solutions ranging from about 3.5% up to about 26%. Brine forms naturally due to evaporation of ground saline water but it is also generated in the mining of sodium chloride. Brine is used for food processing and cooking, for de-icing of roads and other structures, and in a number of technological processes. It is also a by-product of many industrial processes, such as desalination, so it requires wastewater treatment for proper disposal or further utilization.
Ethylene glycol is an organic compound with the formula (CH2OH)2. It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odorless, colorless, flammable, viscous liquid. It has a sweet taste, but is toxic in high concentrations. This molecule has been observed in outer space.
Glycerol, also called glycerine or glycerin, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known as glycerides. Because it has antimicrobial and antiviral properties, it is widely used in wound and burn treatments approved by the U.S. Food and Drug Administration. Conversely, it is also used as a bacterial culture medium. Its presence in blood can be used as an effective marker to measure liver disease. It is also widely used as a sweetener in the food industry and as a humectant in pharmaceutical formulations. Because of its three hydroxyl groups, glycerol is miscible with water and is hygroscopic in nature.
Propylene glycol (IUPAC name: propane-1,2-diol) is a viscous, colorless liquid, which is nearly odorless but possesses a faintly sweet taste. Its chemical formula is CH3CH(OH)CH2OH. As it contains two alcohol groups, it is classed as a diol. It is miscible with a broad range of solvents, including water, acetone, and chloroform. In general, glycols are non-irritating and have very low volatility.
A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids are excavators and backhoes, hydraulic brakes, power steering systems, automatic transmissions, garbage trucks, aircraft flight control systems, lifts, and industrial machinery.
Synthetic oil is a lubricant consisting of chemical compounds that are artificially modified or synthesised. Synthetic lubricants can be manufactured using chemically modified petroleum components rather than whole crude oil, but can also be synthesized from other raw materials. The base material, however, is still overwhelmingly crude oil that is distilled and then modified physically and chemically. The actual synthesis process and composition of additives is generally a commercial trade secret and will vary among producers.
An antifreeze is an additive which lowers the freezing point of a water-based liquid. An antifreeze mixture is used to achieve freezing-point depression for cold environments. Common antifreezes also increase the boiling point of the liquid, allowing higher coolant temperature. However, all common antifreeze additives also have lower heat capacities than water, and do reduce water's ability to act as a coolant when added to it.
Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure, and to amplify braking force. It works because liquids are not appreciably compressible.
Deicing is the process of removing snow, ice or frost from a surface. Anti-icing is the application of chemicals that not only deice but also remain on a surface and continue to delay the reformation of ice for a certain period of time, or prevent adhesion of ice to make mechanical removal easier.
A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosion of the cooling system. Some applications also require the coolant to be an electrical insulator.
USAir Flight 405 was a regularly scheduled domestic passenger flight between LaGuardia Airport in Queens, New York City, New York, and Cleveland, Ohio. On March 22, 1992, a USAir Fokker F28, registration N485US, flying the route, crashed in poor weather in a partially inverted position in Flushing Bay, shortly after liftoff from LaGuardia. The undercarriage lifted off from the runway, but the airplane failed to gain lift, flying only several meters above the ground. The aircraft then veered off the runway and hit several obstructions before coming to rest in Flushing Bay, just beyond the end of the runway. Of the 51 people on board, 27 were killed, including the captain and a member of the cabin crew.
Triethylene glycol, TEG, or triglycol is a colorless odorless viscous liquid with molecular formula HOCH2CH2OCH2CH2OCH2CH2OH. It is used as a plasticizer for vinyl polymers. It is also used in air sanitizer products, such as "Oust" or "Clean and Pure". When aerosolized it acts as a disinfectant. Glycols are also used as liquid desiccants for natural gas and in air conditioning systems. It is an additive for hydraulic fluids and brake fluids and is used as a base for "smoke machine" fluid in the entertainment industry.
Fuel system icing inhibitor (FSII) is an additive to aviation fuels that prevents the formation of ice in fuel lines. FSII is sometimes referred to by the registered, genericized trademark Prist. Jet fuel can contain a small amount of dissolved water that does not appear in droplet form. As an aircraft gains altitude, the temperature drops and the fuel's capacity to hold water is diminished. Dissolved water can separate out and could become a serious problem if it freezes in fuel lines or filters, blocking the flow of fuel and shutting down an engine.
In aviation, icing conditions are atmospheric conditions that can lead to the formation of water ice on an aircraft. Ice accretion and accumulation can affect the external surfaces of an aircraft – in which case it is referred to as airframe icing – or the engine, resulting in carburetor icing, air inlet icing or more generically engine icing. These phenomena may possibly but do not necessarily occur together. Both airframe and engine icing have resulted in numerous fatal accidents in aviation history.
In aeronautics, ice protection systems keep atmospheric moisture from accumulating on aircraft surfaces, such as wings, propellers, rotor blades, engine intakes, and environmental control intakes. Ice buildup can change the shape of airfoils and flight control surfaces, degrading control and handling characteristics as well as performance. An anti-icing, de-icing, or ice protection system either prevents formation of ice, or enables the aircraft to shed the ice before it becomes dangerous.
Propylene carbonate (often abbreviated PC) is an organic compound with the formula C4H6O3. It is a cyclic carbonate ester derived from propylene glycol. This colorless and odorless liquid is useful as a polar, aprotic solvent. Propylene carbonate is chiral, but is used as the racemic mixture in most contexts.
Aircraft engines produce gases, noise, and particulates from fossil fuel combustion, raising environmental concerns over their global effects and their effects on local air quality. Jet airliners contribute to climate change by emitting carbon dioxide, the best understood greenhouse gas, and, with less scientific understanding, nitrogen oxides, contrails and particulates. Their radiative forcing is estimated at 1.3–1.4 that of CO2 alone, excluding induced cirrus cloud with a very low level of scientific understanding. In 2018, global commercial operations generated 2.4% of all CO2 emissions.
Ethylene glycol poisoning is poisoning caused by drinking ethylene glycol. Early symptoms include intoxication, vomiting and abdominal pain. Later symptoms may include a decreased level of consciousness, headache, and seizures. Long term outcomes may include kidney failure and brain damage. Toxicity and death may occur after drinking even in a small amount as ethylene glycol is more toxic than other diols.
In aviation, ground deicing of aircraft is the process of removing surface frost, ice or frozen contaminants on aircraft surfaces before an aircraft takes off. This prevents even a small amount of surface frost or ice on aircraft surfaces from severely impacting flight performance. Frozen contaminants on surfaces can also break off in flight, damaging engines or control surfaces.
↑ US 20090283713,""Environmentally benign anti-icing or deicing fluids employing industrial streams comprising hydroxycarboxylic acid salts and/or other effective deicing/anti-icing agents""
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