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It is assumed the Operator in question did not appreciate that de-/anti-icing fluid applied to the aerodynamic surfaces of the aircraft is in itself a contaminant that will degrade aerodynamic performance. In this case the level of performance degradation is known and allowed for in the performance calculations, however, the calculations are based on both wings receiving the same treatment and the standard adopted within the industry currently reflects this requirement.
Over the past few years Japan has been well represented at industry standardisation meetings, such as the SAE G-12 Aircraft Ground De-icing Committee, so it’s quite possible that Operator concerned has since brought his standard in line with that of the industry.