Control loading system

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. There are differing types of systems to cover the variety of training devices. The most common being the traditional control loading system where actuators are connected to linkages. These are most commonly used in full flight simulators. In the last ten years, smaller and more modular systems have been developed for use in mission trainers, especially significant for the military training market.

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 1920’s 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.

One of the first names in control loading was that of McFadden, known for their hydraulic actuators used from the early 60’s. Fokker Control Systems, formerly part of the aircraft manufacturer Fokker from the Netherlands, have been involved in control loading systems for more than 40 years. FCS have recently been bought by Moog, Inc to consolidate on simulation sales. Wittenstein aerospace & simulation moved into the control loading systems with the development of compact actuators based on their industrial components.

Control Loading Systems

Design and Technology

The main concept is to provide forces to the pilot using an actuator (hydraulic or electric). The traditional approach 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. A traditional 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 thy 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 complex features. Traditional architectures have centralized control, individual analog signals to the control module, large motor with low gear ratio and linkages to the pilot controls. The modular designs have reduced the cabling with localized control and digital reporting over a field bus to the central control module. The smaller actuator sizes have also allowed situating the actuators closer to the pilot which allows for greater design freedom for the simulator manufacturer and has led to the development of mission training systems that can be easily deployed and moved around the world. One modular device that has been developed is the side-stick which is used in aircraft such as the F-35 and F-16 as well as Airbus aircraft. These systems have also led to the development of active sidesticks which can provide active cues to the pilot during flight. The first production example of this was in the T-50 Golden Eagle jet trainer developed by KAI in partnership with LMCO.

Standards and Regulations

The regulations governing control loading systems for civil simulators are the Federal Aviation Administration regulations in North America and EASA (formerly JAA) in Europe. The FAA documents are AC 120-40B for airplane simulator qualification, Advisory circular 120-45A for Airplane Flight Training Device Qualification and AC 120-63 for helicopter Simulator Qualification. The EASA regulations are similar to the FAR’s. Between 2006 and 2008 the International Working Group of the RAeS’s Flight Simulation Group met on several occasions to redefine the standards applicable to flight simulation. This resulted in the release of a draft standards document to ICAO. This will be released by ICAO in 2009 and at this time the FAA and EASA should incorporate this into the regulations. The changes behind the standards will define different levels of simulator training devices which define what training requirements can be trained on with particular levels of simulators.