De-ice and Anti-ice Fluids
Frozen contaminants are most often removed in commercial operations by using freezing point depressant fluids. There are a number of freezing point depressant fluids available for use on commercial aircraft and, to a lesser extent, on general aviation aircraft. De-icing and anti-icing fluids should not be used unless approved by the aircraft manufacturer.
There are four standard aircraft de-icing and anti-icing fluid types: Type I, II, III, and IV. Type I fluids are the thinnest of fluids. As such, they can be used on any aircraft, as they shear/blow off even at low speeds. They also have the shortest hold-over times (HOT) or estimated times of protection in active frost or freezing precipitation. Type II and IV fluids add thickening agents to increase viscosity. The thickeners allow fluid to remain on the aircraft longer to absorb and melt the frost or freezing precipitation. This translates to longer HOT, but it also means a higher speed is required to shear off the fluid. Type III fluids are relatively new and have properties in between Type I and Type II/IV fluids. Type III fluids also contain thickening agents and offer longer HOTs than Type I, but are formulated to shear off at lower speeds. They are designed specifically for small commuter-type aircraft, but work as well for larger aircraft.
Note: Holdover Times (HOT) are published in a range to account for variations in precipitation intensity: shorter time for heavier intensity, longer time for lighter intensity.
Type I Fluids
These fluids in the concentrated form contain a minimum of 80% glycol and are considered “unthickened” because of their relatively low viscosity. These fluids are used for de-icing or anti-icing but provide very limited anti-icing protection. Type I fluids are always applied heated and diluted. De-icing is primarily achieved through heat and hydraulic force. Anti-icing is achieved primarily by the heat imparted to the airframe.
Type II Fluids
Fluids, such as those identified as SAE Type II and ISO Type II, will last longer in conditions of precipitation. They afford greater margins of safety if they are used in accordance with aircraft manufacturers’ recommendations.
Flight tests performed by manufacturers of transport category aircraft have shown that most SAE and ISO Type II fluids flow off lifting surfaces by rotation speeds, although some large aircraft do experience performance degradation and may require weight or other takeoff compensation. Therefore, SAE and ISO Type II fluids should be used on aircraft with rotation speeds above 100 KIAS. Degradation could be significant on aeroplanes with rotation speeds below this figure.
As with any de-icing or anti-icing fluid, SAE and ISO Type II fluids should not be applied unless the aircraft manufacturer has approved their use, regardless of rotation speed. Aircraft manufacturers’ manuals may give further guidance on the acceptability of SAE and ISO Type II fluids for specific aircraft.
Some fluid residue may remain throughout the flight. The aircraft manufacturer should have determined that this residue would have little or no effect on aircraft performance or handling qualities in aerodynamically quiet areas; however, this residue should be cleaned periodically.
SAE and ISO Type II fluids contain no less than 50% glycol and have a minimum freeze point of -32°C. They are considered “thickened” because of added thickening agents that enable the fluid to be deposited in a thicker film and to remain on the aircraft surfaces until the time of takeoff. These fluids are used for de-icing (when heated) and anti-icing. Type II fluids provide greater protection (holdover time) than do Type I fluids against frost, ice or snow formation in conditions conducive to aircraft icing on the ground.
These fluids are effective anti-icers because of their high viscosity and pseudoplastic behaviour. They are designed to remain on the wings of an aircraft during ground operations or short-term storage, thereby providing some anti-icing protection and will readily flow off the wings during takeoff. When these fluids are subjected to shear stress (such as that experienced during a takeoff run), their viscosity decreases drastically, allowing the fluids to flow off the wings and causing little adverse effect on the aircraft’s aerodynamic performance.
The pseudoplastic behaviour of SAE and ISO Type II fluids can be altered by improper de-icing/anti-icing equipment or handling. Therefore, some North American airlines have updated de-icing and anti-icing equipment, fluid storage facilities, de-icing and anti-icing procedures, quality control procedures, and training programs to accommodate these distinct characteristics. Testing indicates that SAE and ISO Type II fluids, if applied with improper equipment, may lose 20% to 60% of their anti-icing performance.
SAE and ISO Type II fluids were introduced in North America in 1985, with widespread use beginning to occur in 1990. Similar fluids, but with slight differences in characteristics, have been developed, introduced, and used in Canada.
Type III Fluids
Type III is a thickened freezing point depressant fluid which has properties that lie between Types I and II. Therefore, it provides a longer holdover time than Type I, but less than Type II. Its shearing and flow-off characteristics are designed for aircraft that have a shorter time to the rotation point. This should make it acceptable for some aircraft that have a Vr of less than 100 KIAS.
Type IV Fluids
A significant advance is in de-icing fluid technology is the Type IV anti-icing fluid. These fluids meet the same fluid specifications as the Type II fluids and in addition, have a significantly longer holdover time. Type IV fluid is dyed green which aids in the application of a more consistent layer of fluid to the aircraft and reduces the likelihood that fluid will be mistaken for ice. However, as these fluids do not flow as readily as conventional Type II fluid, caution should be exercised to ensure that enough fluid is used to give uniform coverage. Research indicates that the effectiveness of a Type IV fluid can be seriously diminished if proper procedures are not followed when applying it over Type I fluid.
All fluid users are advised to ensure that these fluids are applied evenly and thoroughly and that an adequate thickness has been applied in accordance with the manufacturer’s recommendations. Particular attention should be paid to the leading edge area of the wing and horizontal stabilizer.