Polymer compositions derived from selected alkyl (meth)acrylate ester monomers used in certain weight ratios to provide improved viscosity control and low temperature performance characteristics in phosphate ester aircraft hydraulic fluids is disclosed. Polymer compositions for use as viscosity index improving additives in aircraft hydraulic fluids contain from 40 to 100 weight percent (C.sub.1 -C.sub.10)alkyl (meth)acrylate and zero to 60 weight percent (C.sub.11 -C.sub.20)alkyl (meth)acrylate monomer units. Preferred polymer compositions based on 40 to 70 weight percent (C.sub.1 -C.sub.10)alkyl (meth)acrylate and 30 to 60 weight percent (C.sub.11 -C.sub.15)alkyl (meth)acrylate monomer units combine good solubility in the phosphate ester hydraulic fluids with good viscosity control at low and high temperatures.