Most people would assume adding motion to flight simulation would automatically make it better, however that's simply not the case. Motion is very hard to get right in a simulator because the simulator doesn't really fly, so we have to fake it. Humans detect motion by detecting acceleration not velocity1. Think about it, if you're in an airline and you close your eyes you don't feel like you're traveling 444 knots2. Of course you feel some motion but almost all of that perception is due to vibrations. The other motion you feel is subtle accelerations along the heave and sway3 axes. Lastly it should be pointed out that motion certainly does have its place such as initial training in a B777. To do it right requires proper implementation of the motion system. There is no doubt that a level D FFS that allows for a pilot to go directly from the simulator to the cockpit is a boon to airlines and proper. However it comes at a tremendous increase in cost.
Negative Skills Transfer
Negative skills transfer occurs in a simulator, when a person learns something that is improper because the simulator misrepresents or incorrectly emulates the task being taught. The classic example of this, can be illustrated when you drive a manual transmission car for a while, and then switch to an automatic, you initially try and press a clutch pedal that doesn't exist. This of course can have negative consequences in a car, but the consequences in an aircraft can be even more serious.
If a student is used to a simulator which provides a certain sensation of motion and they then go to fly an aircraft which is otherwise very close to the simulator they used when the motions don't match up they'll make incorrect judgements about their current situation. Had the simulator not had any motion at all there would be no expectation of what the motion should be.
Things that move can make some people sick. Who and how varies a lot. Some people are nauseated just riding in a car, or a boat, and that's real motion. When you have to simulate motion, nausea becomes a much much bigger problem. Subtle differences between what the simulation creates and what the brain expects can quickly lead to problems. That can be incorrect angles, mismatches in timing between the visual and the motion system and just about any other imperfection. If you ever used one of those "fly a fighter jet" rides that were popular in the late 1990s end early 2000s you may have experienced the problem. If you used a Virtual Reality headset for any length of time prior to the 2010s you've almost certainly experienced some degree of "cyber sickness".
It's simply not possible to build a flight simulator which won't make anyone sick, after all airplanes make some people sick. But getting it right is very hard, and very expensive.
Many people also find the motion very distracting, particularly new students who are just learning procedures. Now eventually if they're going to fly they're going to have to learn to deal with a moving aircraft. However learning proper procedures and concepts in the simulator doesn't require motion and motion can slow down the learning of students. Slower learning means more frustration and less time and money for aircraft time. Therefor it's important to use simulator time efficiently so as to produce the best pilots possible. Sometimes that means the right thing is a simulator that doesn't simulate everything, including motion.
Should We Do Away With Motion?
Absolutely not. Motion is required for some regulatory classes of simulators. There's good reasons why Level D Full Flight Simulators must have highly accurate motion systems. However those simulators have specific use cases and cost millions of dollars. Most operators of such simulators also have non-moving simulators in their training facilities as well. However, as technology has progressed, more simulators include motion. The global community of manufacturers and simulator operators we must be careful that motion is being used effectively and is not a badly implemented a gimmick. It's interesting to note that as of November 2014 although the FAA was encouraging[d] manufacturers of AATDs to provide motion in their AATDs, it would seems only two major manufacturer have any kind of motion offering in their AATDs. Also interesting to note is that during the evaluation of an AATD, the evaluation criteria states “motion is acceptable as long as it does not negatively impact the training” There is no mention of cuing accuracy.
That's because velocity can only be measured in reference to something else. Can you feel the earth spinning? Can you feel the galaxy whirling? For more on this we recommend this Scientific American article "How fast is the earth moving?" Prof. Rhett Herman, Radford University October 26, 1998 ↩
That's 511 mph or 823km/h, roughly the cruising speed of the popular Boeing 737-800 ↩
Heave is the axis of up down motion, sway is the side to side motions. Surge is the axis of forward and backward motion, however in a large transport jet fluctuations in velocity on surge axis at any given moment are so small as to not be detectable most of the time. ↩