A functional fluid that is particularly useful as an aircraft hydraulic fluid is provided. The fluid contains a major amount of an organo/phosphate ester base stock; based on the total weight of the fluid of from about 0.5 to 10 wt % of certain alkoxylated polyether amines and from about 4 to about 20 wt % of auxiliary additives selected from the group consisting essentially of antioxidants, VI improvers, rust inhibitors, erosion inhibitors and defoamers.