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Deicing and anti-icing fluids are used to remove and prevent ice formation on aircraft before takeoff. Holdover times (HOT) published by the FAA are used by pilots as guidelines indicating the amount of effective time of a fluid under certain freezing precipitation types. However, the times on these tables are based on endurance time tests involving a visual estimate of failure on a flat plate [1]: when 30% of the fluid is covered with white snow under snow precipitation, although the times have been correlated to aircraft wing tests [2] they do not address the mechanism of fluid failure. To measure and understand the fluid mechanisms conducting to failure, the Anti-icing Materials International Laboratory (AMIL) developed a simplified test with a generic deicing propylene glycol-based fluid. The test consisted of pouring 400 mL of the generic deicing fluid on a 5 dm by 3 dm level flat plate where the plate edges were rimmed with insolated walls to make a waterproof open box.

Deicing fluids are used to remove and prevent ice formation on aircraft before takeoff. These fluids are essentially composed of water, a freeze point depressant (FPD) usually glycol, a surfactant or wetting agent and a corrosion inhibitor. All commercial fluids are qualified to SAE (Society of Automotive Engineers) specifications, which test for aerodynamic acceptance, anti-icing endurance, corrosion inhibition, material compatibility, fluid stability and environment. However, these tests have been built around a fluid with a glycol FPD. More recently, with environmental pressure, fluids with other FPDs have been developed and qualified. The other FPDs include: acetates and formate salts, sorbitol, and other undisclosed FPDs. The acetates and formates, which came out in the early 1990s led to suspected corrosion problems. This led to the additional requirement for corrosion tests for non-glycol deicing fluids in paragraph 3.1.1 of AMS1424.

Gel residues occur as the result of repeated anti-icing fluid application that leaves a powdery film upon dryout that, when rehydrated, can swell up to over 600 times its weight. When these gels collect on aircraft flight control surfaces in aerodynamically quiet areas and freeze, they give rise to reduced performance, increased stick force, slowed rotation and have caused jammed flight controls. Laboratory tests have been developed to simulate the gel formation by drying out fluids and rehydrating them. However, by their complex nature, much variation is seen between test results from different laboratories and the results are not yet considered by fluid users. Testing carried out at AMIL on different fluids with different test methods has led to a more reproducible results and a potential classification of fluids based on their gel formation potential (GFP).

Simulating freezing and frozen precipitation in an indoor laboratory setting can permit year round evaluation of de/anti-icing systems and fluids. At AMIL, freezing rain, freezing drizzle, icing fog and in-cloud icing as well as frost, snow, ice pellets and icing clouds can be simulated in a variety of cold chambers of different heights and with different wind conditions using specialized spraying systems and temperature set-ups. Freezing rain is simulated using a 9 m high vertical chamber capable of supercooling water droplets from 100 to 1000 μm, so they freeze not long after impact. The freezing drizzle is simulated in a 4 m high chamber where supercooled droplets from 50 to 250 μm freeze on impact. Icing fog and in-cloud icing are simulated with the help of a pneumatic spray nozzle system which allows for a finer water spray, in the 20 μm diameter range. The frost is simulated by saturating a cold room with humidity generated from a heated, temperature controlled water bath.

The certification process for aircraft ground anti-icing fluids involves flat plate wind tunnel aerodynamic flow-off tests. This test method was developed in 1990 from flight and wind tunnel tests results on full scale and model airfoils, and flat plates; the resulting lift losses were then correlated to the Boundary Layer Displacement Thickness (BLDT) on a flat plate. This correlation was made for Type II fluids existing at the time. Since the introduction of Type IV fluids in 1994, with significantly longer anti-icing endurance times, the same test procedure was applied. However, Type IV fluids are generally more viscous than Type II fluids of the same concentration. At the FAA's request, a study was undertaken to see if aerodynamic certification testing should be different for Type IV fluids as opposed to Type II.

Three performance certification tests are required for the assessment of aircraft de/anti-icing fluids. The two first, measuring anti-icing endurance, consists of the Water Spray (WSET) and the High Humidity Endurance Tests (HHET). The third, for aerodynamic performance, consists of the Flat Plate Elimination Test (FPET). The three performance tests, used for both deicing and anti-icing fluids, are described in the annexes of AMS1424 and AMS1428. Since February, 2003 they are covered by aerospace standards AS5900 for aerodynamic, and AS5901 for anti-icing endurance performance. The WSET anti-icing endurance test measures the time that a fluid protects a plate, inclined at a 10° angle, subjected to freezing precipitation, from a specified amount of icing. This WSET time then defines the fluid type: it must exceed 3 minutes for a Type I, 30 minutes for a Type II and 80 minutes for a Type IV fluid.

The objective of this communication is to present the new capability at AMIL in ice accretion simulation on 2D Airfoils at low speed. AMIL, in a joint project with CIRA (Italian Aerospace Research Center), has developed a numerical model called CIRAMIL. This model is able to predict ice shapes in wet and dry regimes. The thermodynamic model used is similar to existing ones. The major difference is in the approach of calculating the surface roughness and the residual, runback and shedding liquid water masses on an airfoil surface. The numerical ice shapes are compared to rime and glaze shapes obtained experimentally in wind tunnel for a NACA0012 wing profile. The new roughness computation method generates the complex ice shapes observed experimentally in wet and dry regimes and the results agree well with icing profiles obtained in wind tunnel experiments and in many cases are better than those predicted by the models available.