Servo actuator,  lateral cyclic Servo actuator,  longitudinal cyclic Servo actuator,  collective Rotor control Pressure regulator valve Pump Solenoid valve Pilot input Filter Quick disconnects RESERVOIR Vent Scupper drain Pressure Return Figure 4-25.  A typical hydraulic system for helicopters in the light to medium range.  Force Trim Active Augmentation Systems  Stability Augmentations Systems 4-17 hydraulic fluid to enter the system. When the switch/circuit breaker is put in the off position, the solenoid valve is now  de-energized and closes, which then allows the pilot to  maintain control of the helicopter with the hydraulic fluid in  the actuators. This is known as a failsafe system. If helicopter  electrical power is lost in flight, the pilot is still able to  maintain control of the hydraulic system. A pressure indicator  in the cockpit may also be installed to monitor the system. aircraft at a set attitude to accomplish other tasks or simply  stabilize the aircraft for long cross-country flights. Force trim was a passive system that simply held the cyclic  in a position that gave a control force to transitioning airplane  pilots who had become accustomed to such control forces.  The system uses a magnetic clutch and springs to hold the  cyclic control in the position where it was released. The  system does not use sensor-based data to make corrections,  but rather is used by the pilot to “hold” the cyclic in a desired  position. The most basic versions only apply to the cyclic  requiring the pilot to continue power and tail rotor inputs.  With the force trim on or in use, the pilot can override the  system by disengaging the system through the use of a force  trim release button or, with greater resistance, can physically  manipulate the controls. Some recent basic systems are  referred to as attitude retention systems. 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. So-called actual augmentation systems use electric  actuators that provide input to the hydraulic servos. These  servos receive control commands from a computer that  senses external environmental inputs, such as wind and  turbulence. SAS complexity varies by manufacturer but can  be as sophisticated as providing three-axis stability. That is,  computer-based inputs adjust attitude, power and aircraft  trim for a more stabilized flight.  Some helicopters incorporate a stability augmentation system  (SAS) to help stabilize the helicopter in flight and in a hover.  The original purpose and design allowed decreased pilot  workload and lessened fatigue. It allowed pilots to place an 