It seems the usefulness of six-axis flight simulator motion may have been misconceived ever since it was introduced. Training in fixed-base high-fidelity simulators - provided they have first-class "external" synthetic visual systems - may be equally effective.

That is what research appears to be revealing. If it were to be established as unadulterated truth, it would have major implications. Without the need for six-axis motion systems, the cost of acquiring a full-flight simulator (FFS), maintaining it, housing it, and training in it would reduce considerably, and the value of FFS assets and the training centres that house them would take a severe hit.

Recent research carried out by the US Department of Transportation's Volpe Center has concluded that aircraft handling skills learnt theoretically by pilots in simulators that have sophisticated motion systems have never been proven to transfer to flying the aircraft itself. This negates the value of the training.

But, if the research is to be believed, the simulator time may have failed to generate or refresh flying skills on the type. So regulatory approval of the simulator training as an alternative to time in the real aeroplane may have been based on false assumptions about the effectiveness of FFS motion systems on pilot handling performance.

During type rating training, for example, a pilot will unquestionably learn about aircraft systems handling and procedures from practising them in a simulator. However, the new research suggests that pilots go to the line for the first time with only their inherent flying skills, plus what they have learned from the visual system about attitudes and perspectives.

Simulator training develops certain types of transferable skills, the research reveals. But, according to Volpe, there is no evidence that adding a motion system has ever made a positive difference to the manual flying skills the pilots have attempted to hone, particularly since high-fidelity visual systems have been used with full-flight simulators.

This is because pilots have been taught to rely on their eyes to judge aircraft performance, and whereas in a modern FFS the visual systems faithfully represent a real world perspective - as do the aircraft instruments - the motion systems cannot represent real-world sustained accelerations and the sensations associated with sustained attitude change.

Motion systems can only represent the onset of a physical cue, such as vertical or lateral acceleration, pitching or rolling, but then they have to bleed it off, which provides human physiology with a feeling of acceleration in the opposite direction. The latter sensation tends to lead to pilot induced oscillation, and if, during long simulator training sessions, a pilot's handling performance improves, it tends to be because he/she is becoming an expert at "flying" the simulator, which is not the same as flying the aircraft. This is a description of a phenomenon that rings true in most professional pilots' experience.

The only way in which the sense of sustained acceleration can be generated in a simulator is to vary the gravity vector relative to the flightdeck, and that is achieved by rolling or pitching the flightdeck to an angle so that the normally vertical gravity vector is felt as having a longitudinal or lateral component.

Sometimes when these attitude changes are introduced to simulate sustained acceleration resulting from a power change, for example, the aircraft attitude does not alter, so although the visual display and flight instruments appear the same, the pilots' vestibular (balance) system feels an actual difference in attitude. This is confusing.

Recent Volpe tests showed that motion cues in simulators with existing motion systems do not provide pilots with handling skills that transfer to the aeroplane. Even tests run in a simulator with an enhanced, NASA-developed motion system showed that practising a take-off decision speed (V1) single engine power failure did not result in faster pilot reaction speed or improved handling accuracy when the same scenario was enacted in the real aeroplane. But following training in the same simulator with the motion system switched off, the pilots developed "steadier and more precise control that [did] transfer [to the aircraft]".

"Decades of research have failed to show an effect of motion on transfer," says Volpe scientist Dr Judith Burki-Cohen.


Meanwhile, fitting simulators with pilot seats that can reproduce sensations of airframe vibration, and provide motion representing the onset of "heave", might be all that is required to induce feelings of realism for the pilots, especially backed up by engine noise, systems operation, slipstream and surface roll that pilots already hear in full-flight simulators. Apart from low cost and simplicity, the other advantage of such a system would be that it does not cause undesirable secondary effects produced by conventional motion systems when they attempt to provide the sensation of sustained accelerations. As for attitudes, and pitch and roll rates, the visual system and instruments provide all the cues pilots need, says Volpe.

Simulator manufacturers know all this is going on, and they already offer simulators containing new "tricks", such as seat-cueing in high-fidelity, fixed-base machines.

But long-held assumptions will take a long time to change - if indeed they are required to change. Metaphorically speaking, turning around the regulatory supertanker would take a huge amount of time, because it would entail an admission that it has been operating for nearly two decades on a costly assumption that may now have been proven false.

Meanwhile, simulator manufacturers and training organisations have invested heavily in training centres equipped with six-axis motion full-flight simulators. Therefore, they have a vested interest in seeing change delayed while any doubt remains about whether the now-conventional motion systems might yet show some kind of benefit.

Simulator manufacturers can cope with changes and will be prepared if this one comes. Thales has launched its RealitySeven simulator series, which marketing director Mark Dransfield says "is based on a modular design, with a common core element independent of the aircraft type. Different aircraft modules can effectively be 'swapped' out without the entire simulator being replaced. It suits those who need [either] a fixed-base training device, with or without visual display, or a full-flight simulator."

 Mechtronix FFT X simulator
  © Mechtronix

Meanwhile, Mechtronix chief Xavier Herve, heading a company that has made its name through supplying second- and third-tier airlines with lower-cost simulation solutions so they can bring their training in-house, is more adventurous in his predictions. He says this potential upset in assumptions about simulator motion systems, and the issue of what might be an alternative, is in its early days.

Now it would be rash to predict where the graph will go, he says, but he concedes that it may become exponential. The big unknown is how long the process will take. He uses the example of his company's FFT-X simulator, which is a level B or above FFS. French regulator DGAC allows recurrent training to be carried out on a fixed-base version, requiring only the check rides in the motion-driven FFS.

Another option for making motion systems feel more real is providing less travel on the rams. Training organisations have already been allowed to reduce ram travel from 60in (1.52m) to 35in, says Herve. Meanwhile, the cost of motion systems has been reduced by powering them - almost universally in machines coming off the production line - with electric motors rather than hydraulics.

Finally, Sabena Flight Academy's Development section has produced a bolt-on motion system modifier called LM2, that takes the lurching sensation out of any of the existing motion systems, making simulated flying feel much more like the real thing, according to Flight International's experience of "flying" a Boeing 737-200 simulator fitted with it.

So the manufacturers are getting ready with alternatives to the motion systems accepted today. The open question is whether they will have to be modified or abandoned.

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Source: Flight International