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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.