Servos & Simulation's Blog

How Are Feedback Control Loaders Used in Simulation?

Posted by Rachel Baker on Jan 28, 2017 3:44:17 PM

How are feedback control loaders used in simulation? Why are they used in full-flight simulators? How did they come into use? Let's explore...

Let's start with the basics... what is a feedback control loader? or more simply, a control loader? Well, it isn't a large piece of machinery for farming...

A control loader 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 (wikipedia).  Ok... so what? So let's explore a little history here...

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 (see our other blog article on the history here). 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 (wikipedia). Here's where we depart from the wikipedia article... Electric actuators have been around since the late 1980's... over 30 years! I was there at the beginning. Servos & Simulation was on the forefront as our first models were all electric!

So from a little history, we learned that pumps, bellow and valves were used to provide flight control forces from the aircraft to the pilot. Over time, the force the pilot puts on the flight controls and the force that the aircraft dynamics communicates to the flight are interconnected. This reading of the pilot force and the writing of the aircraft's response to that force via its flight dynamics math model creates what is known as a feedback loop.

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The best example of a feedback control is power steering in a car... if you have ever driven a car without power steering, you know that the wheel is very heavy and the car is hard to turn. With power steering, the wheel is light and easy to move and the car is easier to turn. You know that if you move the wheel very fast (force), the car will respond, in kind, to that force and move as quickly to the direction that you turned.

Seems simple... but, in aircraft feedback control loading, it is a bit more complicated. How complicated the control loading is actually determined by 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. They are posted on the FAA web site.

Basically, the FAA level of the flight trainer indicates how nice (fidelity) the flight trainer will be. For example, a low end flight trainer (level 1 through 7) may or may not have any control loading as referenced by the FAA. The lower level trainers are usually used for procedural trainers... how to read the gauges, turn things on and off, read the navigation and auto-pilot and the like. FAA Level 6 usually has a nice feedback control system, but it isn't as good as the highest, Level D, which is like the difference between driving a inexpensive car and the nicest car on the planet.

If you want to learn more about the FAA regulations for flight trainers, check out their web site. There is a wealth of information on what is required by the FAA for each level of the trainer and what they are expecting.


So to come back to the point, why are control loaders used and why is it important in flight simulation? Its rather like asking why was it important to take driver's education in high school and then have your parent (or someone else that was willing and brave) to teach you how to drive... one, so that you could drive the vehicle and understand what the car was telling you and second, so that you didn't injure yourself or others, but most imporantly, no one is killed. Same premise... flight simulation is training to learn to drive some of the largest vehicles on earth - or above it as the case may be. Some react very fast, some react very slow and every single one of them are different. They behave different. Therefore, the FAA says pilots have to have training for each and every type of aircraft that they want to fly and that training has to be continuous and ongoing, year after year. This would be extremely difficult to do using actual aricraft!! They are expensive to operate and the amount of flight training that is ongoing as compared to actual flights ongoing is more than likely 4 to 5:1. Could you imagine 5x more aircraft in the air just to train a single pilot? This is why control loaders are important... so that pilots can continue their training without actually flying the real aircraft. By the time they have completed their training so that they can fly, they have logged hundreds of hours of trainer time. Emergency procedures and knowing how to react in an emergency is key to training too.

Therefore, control loaders must accomplish a few things:

  1. accurately reproduce the aircraft dynamics model at the pilot's flight control stick while both on the ground and in the air under all conditions whether it is clear skys or the worst of storms and turbulance
  2. they must have an excellent frequency response between the pilot and the loader (approximately 4KHz banwidthor greater at times).
  3. they must run forever basically... downtime on a trainer has a severe impact on the training company's business. Trainer downtime can cause economic hardship for the company, scheduling backup for pilots that need to be recertified, and possible loss of business.
  4. must be easy to maintain... the less maintenance, the more the trainer can be used.
  5. no system induced noise. The control loader must be dead quiet. The pilot should feel nothing but the actual flight control.
  6. time delay of the system must be kept to a minimum.

Some of the nice things that a control loader might need to accomplish:

  1. be energy efficient for the customer. Less cost to operate is a consideration
  2. easy to adapt for an array of aircraft. To be able to change the type of aircraft so that the trainer is reconfigerable might be a consideration
  3. easy to install. The ability to install the system into an existing system is key. Either for a new system or when upgrading an existing system due to age or failure.
  4. easy to integrate to the Host and other equipment such as visuals and a motion base platform.
  5. technical support and integration for the system

For more information on our entire control loader product line, please check out our web site at

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Topics: control loader, feedback control loading, flight simulator, full-flight training