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A sidestick is spring-loaded to the centre position.

If there were servos under the stick, connected to the other side stick, so instead of being spring-loaded to centre, it was spring-loaded to the position of the opposite stick. Then each pilot would get feedback on what the opposite pilot was doing, and the application of opposite stick actions could be prevented.

Is that feasible?

EDIT: As this apparently is existing technology, then is there a reason Airbus is not using it?

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Yes. Gulfstream G500 and G600 are the first civilian transport category aircraft to have active sidestick. However, instead of mechanical linkages between the two sticks, the side sticks are coupled electrically. It's much more prevalent in military aircraft.

Source: Aviation Week

So why isn't it being adopted by aircraft manufacturers with side stick controls (e.g. Airbus, Bombardier)? It pretty much all has to do with costs.

  1. Active side sticks introduce more failure modes, such as jam and runaway, which would involve more software and mechanisms to detect and mitigate. These add complexity and introduce failure modes in their own rights, and they all add up to costs.

  2. There is very little certification mandate to make side sticks active. All the objectives, such as stick force/G, stick force/V, have been met by passive side sticks. Airbus introduced envelope protection, most likely not because it was a nice-to-have, but because certifying the combination of lack of aerodynamic feedback on the inceptors and the choice of a pure C* control laws without speed stability demands it.

  3. Certifying the first civilian active side stick is costly (but now it's been done by Gulfstream). There needs to be clear market reasons to do so. For Gulfstream, it may be (personal hypothesis) that the aircraft is being catered to the ultra-rich clientale and a novelty factor. For commercial aviation, the only reason I can think of is safety, which brings us to...

  4. Would active side stick actually increase safety by leaps and bounds? This is a topic under research so the discussion has personal bias. On the A320 and A220, if both PF and CP side stick commands are detected, an aural "Dual Input" warning would sound. Obviously, AF447 crew ignored it. The final BEA accident report didn't mention the lack of tactile feedback as a reason for the crash; rather, crew confusion on stall recovery and lack of airspeed disagreement warning were highlighted.

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  • $\begingroup$ Some good additional info about the active sticks in the brochure here: baesystems-ps.com/pdf/ais_mil_brochure.pdf $\endgroup$ – BenAdamson Aug 19 at 8:23
  • $\begingroup$ AF447 crew were not aware of the continued FO full aft stick input until too late. Indonesia Air Asia 8501 captain was not aware that the FO misunderstood his command, pulled the stick backwards, and averaged out the full forward stick input of the captain. An aural Dual Sound in a cockpit full of alarms and distractions was not enough in both cases. $\endgroup$ – Koyovis Aug 19 at 13:54
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    $\begingroup$ @Koyovis Again, no recommendation/finding on side stick from NTSB/BEA in either case. This becomes subjective discussion. $\endgroup$ – Jimmy Aug 19 at 14:34
  • $\begingroup$ Yes you may have a point there. $\endgroup$ – Koyovis Aug 19 at 14:45
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Yes! It is feasible and recommendable, because the force sensors in fingers may create the extra inputs not available when humans are not looking at their or each others hands in emergency situations.

What you describe is often applied in simulators, for instance in B737 sims where captain and F/O side column and wheel can be uncoupled:

  • Each column has its own Control Loading actuator.
  • Each actuator measures the pilot force, and applies suitable resistance forces.
  • The two actuators are normally linked in software, which can also unlink them.
  • The actuators are powered by electric motors, which are fail passive unlike the hydraulics of olden days which had to be protected from hard over failures.

enter image description here

So not only is it feasible, it is technology with a decades old experience in the aviation world. Photo above is from a sales brochure, the stick we made 25 years ago for a German car company for research in drive-by-wire. Electric motors with back-drivable gearing (not via geared wheels), the stick could still move when the motors were off. Two or more of them can be coupled in software.

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  • $\begingroup$ "which are fail passive" - until a short circuit lets the motor run away to its stops. $\endgroup$ – Sean Aug 19 at 3:59
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    $\begingroup$ @Sean Have you ever seen that huge array of circuit breakers in an airliner, that protect against short circuits? Point is, an unpowered electric motor is immediately unable to exert any force, and can move relatively freely. Unpowering a hydraulic circuit is much more complicated. $\endgroup$ – Koyovis Aug 19 at 6:15
  • $\begingroup$ Jam in the gears is a bigger worry since unlike hydraulic actuators they cannot be floated. $\endgroup$ – Jimmy Aug 19 at 10:58
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    $\begingroup$ @Jimmy yeah that's a possible failure mode, but a pretty remote one. $\endgroup$ – Koyovis Aug 19 at 11:55
  • $\begingroup$ It doesn't matter. Single failure in flight controls is single failure, regardless of probability. Unless you want to have a word with the FAA $\endgroup$ – Jimmy Aug 19 at 12:29

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