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Comment by tjpnz | Hacker News Reader

Here's a video demonstrating it:

I feel like “roll” is a bad word for what’s happening here. In my brain, when I hear “roll”, I’m thinking a 360 degree rotation in either the vertical or horizontal axis (or both).

This is far from that (even in the real life footage linked in the sibling comments). This is more like a “Dutch wobble” or “tilt”.

But I’m not an aviator, so what do I know.

What you describe might be called a loop. In aviation, “roll” describes any rotation (not necessarily full 360) around the longitudinal axis, as “yaw” around the vertical and “pitch” around the lateral axis.

ETA: 360 degrees pitch is a looping, 360 degrees roll is a aileron roll or slow roll, a 360 degrees roll and pitch is a barrel roll, and 360 degrees yaw is “a 360”, ie a full circle turn.

"Roll" is the name for one of the three axes in aviation. "Vertical" and "horizontal" aren't very descriptive in three dimensions.

You could reasonably call yaw the "horizontal axis", but then assigning "vertical" to either of roll or pitch – and what do you call the other one then? They're arguably both vertical, depending on which side you look at the plane from! Additionally, at least "horizontal" implies an Earth-centered focus, which doesn't help while in, say, a barrel roll :)

Best to avoid the ambiguity entirely and use specific terms, just like how port and starboard avoid the "my left/right or yours" ambiguity nicely by always referring to the ship's frame of reference.

In any case, it sure beats a Dutch crunch.

Dutch roll was particularly pernicious with trijets. It has to do with thrust vs lift vectors having different offsets (origins) relative to the cg. Sometimes they would add stabilizers to the tail to correct for this, but eventually manufacturers gave up on the design.

I guess it’s worse than it looks? The video was kind of a dramatic let down to be honest. I preferred the mystery.

Yeah, could this be caused by turbulence? Winds?

Looking at the video, now i'm not sure I haven't been through some dutch rolls also.

The FAA reported: "AIRCRAFT EXPERIENCED A DUTCH ROLL, REGAINED CONTROL AND POST FLIGHT INSPECTION REVEALED DAMAGE TO THE STANDBY PCU, OAKLAND, CA." and stated the aircraft sustained substantial damage, the occurrence was rated an accident.

Standby PCU sounds like a control system. Was that a cause or effect of the event?

The PCU in this context is the part that moves the rudder. PCU issues were common on the 737 in the 90s[0] although that's... probably?... not relevant here.

[0] https://en.wikipedia.org/wiki/Boeing_737_rudder_issues

Exactly, rudder control issues could trigger or exacerbate Dutch roll. Loss of yaw damper control, in other words (yaw damper actuates the rudder).

Dutch Roll is a coupling of yawing and rolling dynamic modes, and is a product of the aircraft's aerodynamics. If the aircraft is disturbed off a steady-state path either by control input, changing winds, or turbulence, then it should return back to it's steady-state path with oscillations that quickly dampen. Dutch Roll is a phenomenon where these oscillations grow rather than dampen as a result of out-of-phase yaw and roll modes.

So Dutch Roll can be triggered by turbulence/wind, but the Dutch Roll itself is the result of something going wrong in reaction to that stimulus. This is different than the aircraft just being batted around by turbulence.

> This is different than the aircraft just being batted around by turbulence.

Does it actually look different or is it just a different cause for similar movement patterns?

Just being batted around by turbulence looks different if you know what you're looking for (although what to look for might be only obvious when looking at accelerometer data). Again, Dutch Roll is a very specific phenomenon as a result of coupling between the roll and yaw dynamic modes. The risk of Dutch Roll is that these oscillations can grow even without further stimulus rather than just dampen out.

For what its worth, that's very unlikely in the last decade or so. More likely to be moderate turbulence, which would be nearly indistinguishable in a non-severe event.

A real-world video looks a lot scarier.

Linked downthread: https://www.youtube.com/watch?v=2tgfkGiHhxs

Especially if close to the ground.

It looks a bit more intense in real footage.

https://www.youtube.com/watch?v=2tgfkGiHhxs

Agreed.

This should be the top comment.

Yeah, this has some elevation reference, and much more dramatic shifting.

this video posted elsewhere here seems dramatic enough

https://www.youtube.com/watch?v=2tgfkGiHhxs

Imagine you are in the cockpit at the top of the plane and your view/perspective - for a layman like me this would seem and feel like the flight is completely out of control. Even the passengers are bound to experience dramatic movements similar to severe turbulence I guess.

Idk. The idea of trying to control the aircraft which is doing this while being responsible for >100 lives seems pretty terrifying to me.

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