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.