Knold, many things are hidden in words.
Manufacturers may ‘approve’ de-icing fluids, but often the small print indicates that their approval is limited to airframe materials i.e. the fluid will not harm the structure, seals, plastics, etc. Few if any manufacturers will give unequivocal aerodynamic approval for de/anti icing fluids; the fluids themselves are a contaminant, but supposedly with known characteristics.
When type 4 fluids were introduced several ‘brand’ names met the same ISO/SAE specification, but each had their own formulation. Some of the more exotic formulations caused problems, one at least was removed from the market, and others possibly re-branded as lesser types or ‘+’ versions. The ISO / SAE specification may not take account of differing glycol formulations used for environmental reasons; note fewer problems in the USA (fluid type or regulation / training?). Also things such as dynamic viscosity may not be considered, where a fluid will flow more readily with increasing airflow, but if the fluid falls into an aerodynamic quiet area (control gap) it thickens in the lower local airspeed and sticks in place. Then there are issues with thickening agents, colourants, dry out characteristics; although the latter was addressed with a specification revision.
I am not familiar with the AEA ‘approval’; what processes or to what specification do the AEA test the fluids. How many flight tests have they conducted?
A major problem for the industry is that there is no regulatory ownership of de/anti icing fluids. My understanding is that the ISO/SAE specifications are similar to industry agreed standards and guidance, and they do not regulate operational use. How can ISO/SAE govern the application or actual operating environment, have they conducted flight tests? If they have, in which aircraft and in what range of conditions, where is the supporting evidence?
I do not know of any public documents relating to the flight characteristics of fluids since the very old USA (FAA?) work based on a 737; this indicated a small, but ‘acceptable’ performance degradation with what I suspect was a type 2 fluid. Several manufacturers have in recent years conducted aircraft handling and aerodynamic tests with a range of fluids after a spate of in-service problems; I have not seen any published results. I believe that one manufacturer tested both aerodynamic and climb performance and although some performance degradation was noted, I do not know by how much or whether it warranted any change to their operating manuals.
I suspect that current problems are centred on the slower speed aircraft, turboprops, etc, and/or with manual controls; for which the type 3 fluid was designed, but has not seen wide service.