A Control Loading System (CLS, also known as Electric Control Loading), is used to provide pilots with realistic flight control forces in a flight simulator or training device. These are used in both commercial and military training applications. History The history of control loading systems starts with the history of flight simulation. The first flight simulator was the Link Trainer, also known as the Blue Box. This was developed in the 1920s and used pumps, valves and bellows to provide the flight control forces. The next development in control loading systems was the use of hydraulic actuators to provide the forces required on the flight controls. These were utilized for around 20 years in the simulator industry until the development of electric actuators. Control Loading Systems Design and Technology The main concept is to provide forces to the pilot using an actuator (hydraulic or electric). The approach used in high fidelity applications is to connect this actuator via a linkage to the pilot controls. The actuator is then controlled with a servo controller to control the torque or current of the motor. An outer-loop control then controls the torque provided to the pilot using a control loop around a force sensor. The control loading system must take in inputs from the simulator and pilot and provide outputs for the pilot and simulator. Inputs are application of force and aircraft states and outputs are flight control position and forces. An aircraft with reversible controls needs to have all of the complex components modeled within the control loading system. These include cables, rods, aero forces from the control surface, centering springs and trim actuators. As the control system gets more complicated they have to simulate effects such as bob-weights and feel units. Fly-by-wire systems are disconnected from the control surfaces and so do not need the complex features but add other functionality which is simulated. The high fidelity architecture has centralized control, individual analog signals to the control module, a brushless DC motor with low gear ratio and linkages to the pilot controls. The modular designs have localized control and digital reporting over a field bus to the central control module. The control loading systems are designed to allow situating the actuators closer to the pilot. This is necessary for mission training systems that can be easily deployed and moved around the world. Control Loading Systems are similar in design to active