Abstract: This application discloses a high performance hydraulic fluid suitable for use as an aircraft hydraulic fluid comprising a major amount of tri(n-butyl)phosphate, a minor amount of a triaryl phosphate and optionally a minor amount of tri(isobutyl)phosphate. The compositions of this invention provide superior performance with respect to flash and fire points as well as extended useful fluid life in service. Such properties are highly valuable as hydraulic fluids modem jet aircrafts.

Abstract: There is disclosed herein novel functional fluids useful as aircraft hydraulic fluids comprising at least about 50% trialkyl phosphate ester and at least one aryl compound whereby the fluid exhibits in combination the fire resistant properties as indicated by a fire point in excess of 177° C., a flash point of at least 160° C. and an autoignition temperature of at least 400° C. The trialkyl phosphate esters contain between 6 and 18 carbon atoms and preferably between 12 and 15 carbon atoms. The aryl compounds are most preferably polyphenyls, fused ring aryl compound containing from 5 to 24 carbon atoms in the aryl portion such as biphenyl or terphenyl or alkyl substituted polyphenyls wherein the alkyl groups contain from 1 to 5 carbon atom. Other aryl compounds includes fused ring aryl compounds such as naphthalene, anthracene, diphenyl oxide and the like.