Improvements in or relating to functional fluids

A fire-resistant aviation hydraulic fluid comprises a mixture of (a) a brominated aryl ether, or a mixture of such ethers, having the formula Brm-Ar-O-Ar1-Brn, where Ar and Ar1 are benzene or naphthalene nuclei which may contain alkyl substituents, m is 1, 2 or 3 and n is 0, 1 or 2, the ether or ethers containing at least 40% by weight of bromine, (b) an alkyl di- or polysiloxane and (c) at least one hindered ester, the proportion of the ingredients being such that the composition has a viscosity of not more than 5000 centistokes at - 40 DEG F. and a bromine content of at least 20% by weight. Constituent (c) is preferably an ester of an a ,a ,a -trimethylol substituted alkane, wherein the alkane has 2 to 6 carbon atoms, with an aliphatic carboxylic acid or acids having 4 to 9 carbon atoms, e.g. trimethylol propane tricaprylate. Alternatively, the ester may be fully esterified pentaerythritol, e.g. pentaerythritol tetracaproate. The composition may also contain thickening agents such as polyisobutylene or alkyl polymethacrylate, or copolymers of C12-24 alkylmethacrylates with N-vinyl pyrrolidone, or copolymers of fatty fumarates with N-vinyl pyrrolidone or vinyl acetate, and antioxidants such as phenyl-a -naphthylamine and hindered alkyl phenols, e.g. 4-methyl-2,6-ditertiarybutyl phenol. The composition may futher contain metal deactivators, e.g. benzotriazole, load-carrying additives, e.g. di-isopropyl phosphite and, to improve low temperature properties mixed liquid brominated alkyl benzenes.