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. This is essentially only a relevant for such a salt based non-glycol fluid.
Next, came a sorbitol, or sugar, based fluid in the early 2000s. As with the formate and acetate salts, it passed all the required tests of AMS1424 including the additional corrosion test. But then in field tests, where the fluid was heated as per usual use, there were problems with foam, sticky and slippery residues. All standard specification laboratory tests are conducted on cold fluids, since this is the worst case for glycol-based fluids, where they are most viscous. However, other FPDs may have the fluid increase in viscosity with heating and evaporation. Following the failed field tests, tests were conducted in the laboratory which showed that when the fluid was heated to high levels of evaporation, the aerodynamic acceptance test was not met and with further evaporation, the fluid solidified. This does not occur with glycol-based fluids since glycol is a liquid. Furthermore, in the lab, mold developed on some exposed fluid left out in a Petri dish.
The FAA has since removed this fluid from their list of qualified fluids in the official FAA Holdover Time Tables [1]. More recently, there have been newer fluids that are non-glycol-based, or have another FPD along with the glycol (low-glycol). These fluids all are qualified at least for aerodynamic performance and anti-icing endurance and two are on the current FAA list of qualified fluids. However, the specification has no tests to address stickiness, solidification or tendency for mold to form. For the foam, a test was added to the specification since this issue was arising equally with glycol-based fluids.
As part of a grant from the FAA, AMIL is developing test protocols to be added to the test specifications to address the new potential issues that may be required of non-glycol fluids before their use on aircraft.
Beyond the corrosion and foam issues for which tests currently exist in the specification, test for aerodynamic acceptance on evaporated heated and sheared fluids, tendency to from mold and slipperiness are proposed.