HYDRAULICS
Most helicopters, other than smaller piston powered helicopters, incorporate the use of hydraulic actuators to overcome high control forces. [Figure 5-13] A typical hydraulic system consists of actuators, also called servos, on each flight control, a pump which is usually driven by the main rotor gearbox, and a reservoir to store the hydraulic fluid. A switch in the cockpit can turn the system off, although it is left on under normal conditions. A pressure indicator in the cockpit may also be installed to monitor the system.
 

 
When you make a control input, the servo is activated and provides an assisting force to move the respective flight control, thus lightening the force required by the pilot. These boosted flight controls ease pilot workload and fatigue. In the event of hydraulic system failure, you are still able to control the helicopter, but the control forces will be very heavy.
In those helicopters where 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 use hydraulic accumulators to store pressure, which can be used for a short period of time in an emergency if the hydraulic pump fails. This gives you enough time to land the helicopter with normal control
STABILITY AUGMENTATIONS SYSTEMS
Some helicopters incorporate stability augmentations systems (SAS) to aid in stabilizing the helicopter in flight and in a hover. The simplest of these systems is a force trim system, which uses a magnetic clutch and springs to hold the cyclic control in the position where it was released. More advanced systems use electric servos that actually move the flight controls. These servos receive control commands from a computer that senses helicopter attitude. Other inputs, such as heading, speed, altitude, and navigation information may be supplied to the computer to form a complete autopilot system. The SAS may be overridden or disconnected by the pilot at any time.