A hydraulic drive system is a quasi-hydrostatic drive or transmission system that uses pressurized hydraulic fluid to power hydraulic machinery. The term hydrostatic refers to the transfer of energy from pressure differences, not from the kinetic energy of the flow. A hydraulic drive system consists of three parts: The Pump, driven by an electric motor or a combustion engine or a windmill; valves, filters, piping etc. and the actuator.An aircraft hydraulic system just like all other hydraulic system is used to control some mechanical movements in the system. The hydraulic system if used for controlling the movement of the control surfaces of the aircraft. These are the most crucial of the mechanical movements in an aircraft as they involve direction control of the aircraft itself. Along with these other movements that are supported by the aircraft hydraulic system are extension and retraction of the landing gear, direction control of the swivelling nozzle, movement of the air intake hatch cover and mechanical movement of air brakes (if any).In operation of all these components, most crucial of the components as mentioned above are pump, valves, pipelines and the actuators.