Types of Flight Simulators:
Full Motion Simulators

From a regulatory point of view (at least in the US and EU) Full Flight Simulators are the only things called1 "flight simulators".

These are at the very top end of the flight simulator food chain, they're the most realistic, the most capable, and the most expensive2. Although there are very high end stationary simulators with advanced capabilities that exceed the price for entry level for Full Flight Simulators it certainly isn't the norm.

Entry level Full Flight Simulators are themselves not the norm. FFSs are ranked Levels A3 through D in the US and EU; however there are probably less than two dozen Level A or B FFS in the entire US4, and there are hundreds and hundreds of Level C and D units operating across the US. This is because a Level 5 or higher Flight Training Device generally provides a better experience and can be used (from a regulatory point of view) for almost everything a Level B FFS can, without spending as much.

Full Flight Simulators virtually all move using something called a stewart platform5. It looks something like a tripod but which uses six variable length actuators (usually hydraulic rams) to move a platform. The platform can be moved backwards and forwards (the surge axis), side to side (the sway axis), up and down (the heave axis) as well as be rotated about the pitch roll and yaw axis.

They simulate motion through something called Acceleration Onset Cuing. Since humans sense motion by detecting acceleration they can provide a little bit of acceleration to signal to you that your velocity has changed. Then they can return to another position slowly so that you don't notice.

For example say you're sitting blind folded in a chair strapped to a Stewart platform and you want to simulate the chair falling down a mine shaft. The imaginary shaft itself might be five hundred feet deep, but in order to give you the falling sensation the platform only needs to accelerate downward briefly and then slowly come to a stop so that you don't feel the stop as strongly. It can then gently lift you back up to normal operating height over a few seconds so that you don't feel that either. Then when it wants to simulate hitting the bottom it will accelerate upwards quickly. Now of course if you drop down a five hundred foot shaft you'll accelerate the whole six seconds it would take you to reach the bottom and you'd feel it all the way. This is why flight simulators are only simulators, not replicators. The truth is the only way to replicate all of the acceleration forces of flying, is by actually flying.

Full Flight Simulators must simulate all systems available from the cockpit, All the breakers, switches, dials and knobs must be present and must work exactly as they do on the aircraft. If the aircraft has a chrome clip board in a specific place, so must the simulator. The controls must precisely replicate any loading forces found in the aircraft and do so identically to the aircraft for all situations simulated.

The visual systems must also provide incredible wrap around cross-cockpit collimated6 displays which use expensive hemispherical mirrors7 and multiple projectors.

Interestingly enough for these high end Level C and D simulators, accuracy of the airport runway environment is a must, but not the surrounding area (unless it has unusual consequence such as a lake right at the approach end). The collimated displays assure that both the pilot and co-pilot see the visual exactly the same. The detail required by regulation is quite specific and generally focuses on the runway environment, lighting, and ground texturing coupled to accurate vibration such as taxiing across a rough surface.

Unlike the lower end FTDs and AATDs the specific scenery requirement is limited to those factors. Many lower level devices include vast databases of graphical information. Some examples: Taxiways, airport signage and lighting, structures such as specific gates to pull up too, FBOs, terrain geometry etc. The list can go on with anything one imagine to be represented in the virtual environment. It's not unusual for a simulator like the ones we build to ship with 60 - 250 GB of data to generate the virtual environment.

Having said all of that, today’s Level C and D FFS can and do (usually as an option) provide much of what has been provided in the new technology AATDs and BATDs for years. The price difference is still astronomical. However, for transport category airplanes it is the best way to train. To emphasis that point one might ask just how good are Level D simulators? The answer: They're so good that the FAA allows pilots to receive Zero Flight Time (ZFT) type ratings with them. That means a pilot at an airline can switch from flying one kind of aircraft to another without spending any time in the new aircraft before their first flight. If that disturbs you, it shouldn't they're that good.

  1. For a discussion of the proper use of the term "flight simulator" you can find more information on this page

  2. At the low end around $2 million, at the high end $50 million. 

  3. New Level A certifications are no longer issued. 

  4. AOPA "ABCs of Simulators" By Alton K. Marsh May 1, 2011 

  5. There are certainly other means of producing six degree of freedom motion systems, however the stewart platform is by far the most popular. 

  6. A collimated display uses a "trick" of optics where by using a concave hemispherical mirror reflecting a convex projection screen observers of the mirror will see the projected image on the screen. What is reflected appears distant and in focus and all observers will see the same apparent position for projected images even if there is several feet between the observers. This is the same "trick" used in reflex optics and certain heads up displays to provide infinite focus information and accurate reference over the real world. 

  7. The mirrors are often made of metallized BoPET film drawn into a hemispherical shape when in use by a vacuum pump.