Comment by verandaguy | Hacker News Reader

Comment by verandaguy | Hacker News Reader

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No, it's the opposite. Airbus does not use active side sticks; you get no haptic feedback from the other pilot. You do get an aural warning, "dual input," if split control inputs are detected, and each stick has an override button against the other side. See here[0].

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

Note that here a breakout is exactly what happened and the behaviour of the controls in case of breakout is similar to Airbus’s:

> 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.

What a horribly confusing control mechanism. It could become very hard to distinguish between (1) desynchronization (2) your copilot yielding control to you and (3) a control jam being overcome. They would all feel exactly the same. Not the sort of ambiguity you would want in an emergency situation!

I'd like to think that people designing this aren't idiots - they are very well trained intelligent engineers. And probably the reason why this method was chosen was because it has some objective advantages over any other method. I don't know what those advantages might be, but I really hope they exist.

> I don't know what those advantages might be, but I really hope they exist.

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.

So why is there not some hard-to-ignore feedback (like stick shaking) whenever this dual input occurs?

Stick shaking is already used for stalls, and only for stalls (ok, specifically, it activates right before a stall, but for all intents in this argument, that's the same thing).

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.

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