Comment by throwaway110823 | Hacker News Reader

Comment by throwaway110823 | Hacker News Reader

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throwaway110823

Someone in my extended family works at Airbus as an engineer (hence the throwaway account)

When I asked him about this accident, I brought up this specific point - the mechanical non-linkage.

Sure, the Airbus is fly by wire (there are no "mechanics"), but you can still program one joystick to mimic the other joystick, right? As far as I know, the Airbus plane actually averages the inputs..?? [0]

Anyway, he sorta-angrily gave me the same explanation as I just saw posted here as well: it was a crew management issue. (which of course may have played a huge role).

I am not a pilot so I am probably missing something. But he (the Airbus family member) did seem quite defensive about this. What portion was internalized corporate-comm "we are not at fault" reasoning? What portion was engineering hubris of "fly by wire is unquestionably superior to mechanical linkage"?

I don't know. But I do find it strange... and indefensible. When does the average of inputs make sense? I'm open to an explanation. Is there a good one?

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[0] See https://news.ycombinator.com/item?id=4224707 from 2012.

"This input was averaged (read: canceled) with the other pilot's nose up input."

(and further down in the same sub-thread)

"The AF447 inputs were averaged."

Averaged inputs on the sticks was the worst thing I read about back when this happened. I can't imagine a circumstance where this is desirable, and it can only lead to confusion.

> When I asked him about this accident, I brought up this specific point - the mechanical non-linkage. Sure, the Airbus is fly by wire (there are no "mechanics"), but you can still program one joystick to mimic the other joystick, right?

That's come up at least twice in NTSB ship accident reports. Some ships have more than one control station. This is usually to allow driving from a control station out on a bridge wing, where the pier can be seen clearly. A high speed ferry plowed into a dock in New York City in 1993 because of confusion over which station had control. "The NTSB concludes that the propulsion control system on the Seastreak Wall Street used poorly designed visual and audible cues to communicate critical information about mode and control transfer status."[1] Big throttle levers at each station, but only the ones at one station at a time did anything. The levers did not move together.

The U.S. Navy went all the way to touch-screen throttles. After a collision involving confusion over which of three control stations was driving, they're going back to big handles.[2]

The Airbus system has been criticized, but it's two people sitting side by side. The ship systems have control stations much further apart.

[1] https://www.ntsb.gov/investigations/AccidentReports/Reports/...

[2] https://www.theverge.com/2019/8/11/20800111/us-navy-uss-john...

The 777 and 787 are fly-by-wire aircraft with mechanically-linked yokes. In extreme situations (one yoke is actually stuck and won't move at all), there is a breakaway on that link, and if you exert enough force upon it (like, really hulk at the thing), you can snap it. I don't know whether this has been necessary in flight, but the facility is there. The control surfaces are split between the yokes, so for example the left yoke pitch control commands the left-side elevator, making the plane still controllable with only one working yoke.

> I don't know whether this has been necessary in flight, but the facility is there.

It happened in one accident, where pilots were literally tugging the yokes in opposite directions. They were literally struggling against each other to fix the attitude.

What would you have the system do instead of averaging the inputs? If you have 2 inputs when you should have 1, you warn with a light and sound, and what input do you use in the meantime?

There are plenty of situations where averaging two safe-but-opposite inputs would cause a crash. For instance, if you're holding hands/handcuffed with someone and walking toward a pole, you must both either go left, or both go right. This is a very common scenario in life and we have many solutions for the problem.

In the Air France situation, there's no way for the plane to "know" which pilot's input is correct, so I don't have a good answer, but "just average two drastically different inputs" seems crazy.

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