Hydraulic fluids, also called hydraulic liquids, are medium by which power is transferred in an hydraulic machinery. Common hydraulic fluids are mineral oil based. Examples of equipment that might use hydraulic fluids include excavators and backhoes, brakes, power steering systems, transmissions, garbage trucks, aircraft flight control systems, elevators, and industrial machinery. Hydraulic systems like the ones mentioned above will work most efficiently if the hydraulic fluid used has low compressibility.