The basic function of the deicing fluids is to lower the freezing point of freezing precipitation as it collects on the airplane and thus delaying the accumulation of ice, snow, slush or snow on critical surfaces.
According to the US Environmental Protection Agency (EPA), propylene glycol-based ADFs are used more frequently than ethylene glycol-based ADFs. This conclusion is based on information provided in survey responses for the 2002-2003, 2003-2004, and 2004-2005 deicing seasons (Table 1).
Table 1 - US Commercial Airports – National Estimate of Aircraft Deicing Fluid Use/Purchase
Four types of aircraft deicing fluids are currently used worldwide, according to SAE standards AMS 1428 & AMS 1424.
Type I – A Newtonian fluid with a high concentration of ethylene glycol or propylene glycol ( more than 80%). It is the most commonly used fluid. It has no thickening agents and has the shortest holdover time of any type of ADF. Concentrated Type I fluids are diluted with water to achieve a freezing point compliant with the application procedure. When used as a deicing fluid, it must be heated to ensure a minimum temperature of 60 °C (140 °F) at the nozzle.
For standardization it is colored amber.
Type II – A non-Newtonian (long chain polymers) fluid containing minimum 50% ethylene glycol or propylene glycol and thickening agents. It has a HOT between Type I and Type IV. Its viscosity is a function of the temperature and shearing force so when the airplane is on the ground it will stay on the treated surfaces and offer protection. During takeoff, the shearing force of the airflow will act on the fluid layer decreasing the viscosity and the fluid will flow off the treated surfaces. As a certain takeoff speed must be achieved, it is forbidden to use Type II fluids without the airplane manufacturer`s approval.
Type II fluids are used in 100% concentration and diluted with water. The dilution will affect the fluids performance (by increasing the freezing point and decreasing the holdover time).
Destructive Type II/IV fluid has the properties of thinned Type I fluid.
For standardization it is colored light yellow (or light green).
Type III – A non-Newtonian (long chain polymers) fluid with the same composition of Type II but containing less thickening agents; therefore it has a lower viscosity level and a shorter holdover time (it has a HOT between Type I and Type II). It was developed to answer the necessity of a low rotation speed airplane (which could not use Type II). It can be a diluted Type II of Type IV that meets the aerodynamic test for turbo-propeller airplane.
For standardization it is colored pale straw.
Type IV – It has almost the same composition as Type II, but the use of an advanced thickener allows a longer holdover time than that of Type II used in the concentrated form.
In addition, it is forbidden to use Type IV fluids without the airplane manufacturer`s approval. In order not to destroy the viscosity and thus the fluid, Type II/IV fluids at 100% concentration may not be heated (above 60°C).
For standardization it is colored emerald green.
Picture 1 - Types of Aircraft Deicing Fluids
All deicing fluids must meet an aerodynamic acceptability standard to ensure proper aircraft performance.
Picture 2 - During taxi (at speeds below 30 knots), the fluid viscosity is high because air stream shearing force is not sufficient to disturb the fluid film. At higher speeds, the increased shearing force of the air stream causes fluids to flow off (the viscosity is lower).
Picture 2 - Deicing fluid viscosity (Image courtesy of NAA)