Use of a functional fluid composition as an aviation hydraulic fluid wherein the functional fluid composition comprises: (a) from 70% to 99.99%, by weight of the functional fluid composition, of a base oil composition comprising: (i) from 50% to 95%, by weight of the base oil composition, of a naphthenic base oil, having a kinematic viscosity at 20°C in the range of from 4.75 to 5.10 mm 2 /s, a kinematic viscosity at 40°C in the range of from 2.90 to 3.20 mm 2 /s, a minimum flashpoint of 101°C according to ASTM D92, and a minimum pour point of -66°C, according to ASTM D5950, wherein the naphthenic content of the naphthenic base oil is in the range of from 50% to 90% by weight of the naphthenic base oil as determined by liquid chromatographic fractionation by HPLC carried out according to a modified method based on IP 368/95 wherein the modification is that pentane rather than hexane is used as the solvent, Field Ionisation Mass Spectrometry (FIMS) to determine the concentration of hydrocarbon types in terms of their carbon number and hydrogen deficiency, and proton NMR in order to determine the %weight olefins; and (ii) from 5% to 50%, by weight of the base oil composition, of a Fischer-Tropsch derived base oil having a kinematic viscosity at 100°C in the range of from 0.5 to 5 mm 2 /s, and wherein the functional fluid composition has a pour point of at or below -30°C, and a kinematic viscosity of at least 3 mm 2 /s at 100°C. (a) from 70% to 99.99%, by weight of the functional fluid composition, of a base oil composition comprising: (i) from 50% to 95%, by weight of the base oil composition, of a naphthenic base oil, having a kinematic viscosity at 20°C in the range of from 4.75 to 5.10 mm 2 /s, a kinematic viscosity at 40°C in the range of from 2.90 to 3.20 mm 2 /s, a minimum flashpoint of 101°C according to ASTM D92, and a minimum pour point of -66°C, according to ASTM D5950, wherein the naphthenic content of the naphthenic base oil is in the range of from 50% to 90% by weight of the naphthenic base oil as determined by liquid chromatographic fractionation by HPLC carried out according to a modified method based on IP 368/95 wherein the modification is that pentane rather than hexane is used as the solvent, Field Ionisation Mass Spectrometry (FIMS) to determine the concentration of hydrocarbon types in terms of their carbon number and hydrogen deficiency, and proton NMR in order to determine the %weight olefins; and (ii) from 5% to 50%, by weight of the base oil composition, of a Fischer-Tropsch derived base oil having a kinematic viscosity at 100°C in the range of from 0.5 to 5 mm 2 /s, and wherein the functional fluid composition has a pour point of at or below -30°C, and a kinematic viscosity of at least 3 mm 2 /s at 100°C. (i) from 50% to 95%, by weight of the base oil composition, of a naphthenic base oil, having a kinematic viscosity at 20°C in the range of from 4.75 to 5.10 mm 2 /s, a kinematic viscosity at 40°C in the range of from 2.90 to 3.20 mm 2 /s, a minimum flashpoint of 101°C according to ASTM D92, and a minimum pour point of -66°C, according to ASTM D5950, wherein the naphthenic content of the naphthenic base oil is in the range of from 50% to 90% by weight of the naphthenic base oil as determined by liquid chromatographic fractionation by HPLC carried out according to a modified method based on IP 368/95 wherein the modification is that pentane rather than hexane is used as the solvent, Field Ionisation Mass Spectrometry (FIMS) to determine the concentration of hydrocarbon types in terms of their carbon number and hydrogen deficiency, and proton NMR in order to determine the %weight olefins; and (ii) from 5% to 50%, by weight of the base oil composition, of a Fischer-Tropsch derived base oil having a kinematic viscosity at 100°C in the range of from 0.5 to 5 mm 2 /s,