Most helicopters, other than smaller piston-powered helicopters, incorporate the use of hydraulic actuators to overcome high control forces. A typical hydraulic system consists of actuators, also called servos, on each flight control, a pump which is usually driven by the main rotor transmission and a reservoir to store the hydraulic fluid. Some helicopters have accumulators located on the pressure side of the hydraulic system. This allows for a continuous fluid pressure into the system. A switch in the cockpit can turn the system off, although it is left on under normal conditions.
When making a control input, the servo is activated and provides an assisting force to move the respective flight control, thus reducing the force the pilot must provide. These boosted flight controls ease pilot workload and fatigue. In the event of hydraulic system failure, a pilot is still able to control the helicopter, but the control forces are very heavy.
In those helicopters in which the control forces are so high that they cannot be moved without hydraulic assistance, two or more independent hydraulic systems may be installed. Some helicopters are designed to use their hydraulic accumulators to store hydraulic pressure for an emergency, allowing for uninterrupted use of the controls for a short period of time following a hydraulic pump failure. This gives you enough time to land the helicopter with normal control.