A functional fluid composition containing a fluid base stock (e.g., a phosphate ester fluid base stock) as a major component, and an erosion inhibitor mixture as a minor component. The erosion inhibitor mixture contains at least a first erosion inhibitor (e.g., a perhalometallate or perhalometalloidate salt) and a second erosion inhibitor (e.g., an alkali metal salt of perfluoroalkyl sulfonic acid). Erosion inhibition of the functional fluid composition is improved as compared to erosion inhibition achieved using singly the first erosion inhibitor or the second erosion inhibitor, as measured by one or more of ASTM D2624, ASTM D974, ASTM D130 and ASTM D6304. A method for operating and lubricating a hydraulic system by utilizing as a hydraulic fluid, the functional fluid composition. A method for identifying erosion inhibition effectiveness of the functional fluid. The functional fluids are useful as aircraft hydraulic fluids.