Edit: Actually, I'm completely wrong, ignore me
Boeing's control columns are mechanically linked, but given (iirc) 50 lbs of force, they will mechanically desynchronize. If memory serves, they will snap back into sync once the force is released. I don't recall there being an aural warning for this.
The issues with Airbus's approach are that (a) in high-stress environments, aural warnings are the first ones to be unconsciously ignored, and (b) you get no haptic feedback, either from the other pilot's side stick, or from the control surfaces (since modern Airbuses are FBW, though you could still do electronic feedback via an active side stick). There's some talk of Airbus moving to active side sticks in the future to mitigate this issue.
The issue with Boeing's approach is that in a high-stress, tunnel-visioned situation, you can't tell another pilot's input feedback apart from something like jammed flight controls, a runaway stabiliser, or any other trim or autopilot malfunction, to name a few. There's also some talk of Boeing moving to active side sticks in the future to mitigate this issue.
[0] https://aircrafttechnic.com/wp-content/uploads/2020/10/Ads%C4%B1z-4.png> in the event of antagonistic inputs exceeding the breakout mechanism’s activation threshold, the controls would be desynchronised and the control surface would react according to the mean control position.
The breakout threshold is 50lbs, but it’s the combined thresholds, so sudden opposite force applied by two pilots (as apparently happened here) can unexpectedly trigger it. The report confirms there is no warning of breakout situation whether visual or aural.
The advantage is obvious: it's the most similar to how it worked back when the controls were fully mechanical.
IIRC, some old planes even had each side mechanically controlling independent halves of the control surfaces, such that the end result is the average of the inputs in case one of the control surfaces (or the cabling connecting to the controls) is jammed.
Pilots are trained very thoroughly to react to stick shakers in one specific and immediate way -- execute the stall escape manoeuvre, and that's to immediately, with only very minimal instrument scanning (typically, just check altitude and airspeed), apply power and push nose down to the extent the surrounding terrain will allow you.
I do agree that dual input could be better handled, but I doubt we'll see many big leaps in that area until active side sticks become the norm. For Boeing aircraft with yokes, I can't see a good solution. Maybe have some kind of hard-to-miss, single-purpose visual indicator on the glare shield, like where Airbus puts theirs? Or an alert when the two control columns get unlinked?
Also, to be clear -- stick shakers are a Boeing-only feature. Airbus has an aural, but very distinctive "STALL, STALL" warning, plus flight envelope protection provided by the FBW system. Alpha floor protection (A.FLOOR) is the subsystem responsible for preventing stalls.
EDIT: ya know, maybe as HUDs get more widespread in civil aviation there might be better ways to surface this information. Fundamentally, we can only alert pilots in so many ways based on the five major senses, and with taste and smell being out, that leaves aural, haptic, and visual indicators.
Aural and haptic are sort-of out for the reasons outlined above, and visual indicators have to resort to being easily parseable in peripheral vision, since if a pilot's in a high-stress situation they may be looking out the windshield first. That's why it makes sense to put urgent stuff on the glare shield; it's easier to see it out of your peripheral.
With HUDs, these indicators can be more information dense. Instead of a green light or an arrow indicating dual input or stick priority, you can have DUAL INPUT flash over the centre of the HUD.
Most modern HUDs have the drawback of being monochrome green, which is a limiting factor on information density here.
My understanding is that it always averages the input, even when they're not desynchronized (but obviously, since the inputs are equal in that case, the average ends up being equal to the inputs themselves).
> The inputs cancelled each other out and triggered an audible "dual input" warning.
That is my understanding of both systems at least
> often only associated it with a jammed control
I assume in no small part because
> the description of the system in the [flight crew operating manual] was limited to an explanation about the risk of a control jamming
And it seems likely the precise behaviour of the system in case of breakout is not documented either.
Side sticks are not mechanically linked.
Is that true? I seem to recall that Airbus gave "priority" to one side or the other (with the side being switchable on demand, not fixed).
- airbus controls are not mechanically linked, save possibly on the A300 (which has a yoke not side-stick), but there’s less than 200 of these still flying
- airbus sticks have a priority / takeover button, but by default dual inputs are averaged (this is also what happens in Boeings when you trigger a breakout apparently)
- Airbus has a “dual input” aural alarm