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I've travelled from Vancouver to London often enough to see this phenomenon as something that happens regularly enough to notice. I'm just not sure why.

So... the route from Vancouver to London is basically a series of right-hand turns. I guess it follows a set of rhumb lines or whatever. But what I noticed was that most right-turns are preceded by a smaller left turn.

ie the aircraft banks and turns left for a couple of seconds, then straightens up and goes straight into the longer right hand turn I'm expecting.

So my question is, why does it do that little turn in the opposite direction? I can't imagine it's navigational, not when it's cruising at high altitude. Is it something mechanical? Say to make sure everything is working and not iced up? I imagine the aircraft is on autopilot at that point, so it's not a pilot thing.

Does anyone know what this is? In fact, has anyone else even noticed this?! I think I've noticed it most on Vancouver-London because when you're over Iceland and the aircraft starts to turn north, you start to wonder where you are headed!

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    $\begingroup$ Can you give a specific flight and date you noticed it on. When we get questions like this it often turns out to be mild spatial disorientation and the aircraft is not really doing what it feels like from the cabin. $\endgroup$ – TomMcW Feb 7 '17 at 19:32
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    $\begingroup$ I can't, no. Do you think the flighttrackers would pick it up? Sorry. It's a question I've wondered for a while and just found this forum today. I don't think it's disorientation, or that I imagined it. I always get a window seat so I can see that we are banked to the left. $\endgroup$ – Mark Ireland Feb 7 '17 at 19:42
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    $\begingroup$ It highly depends. If you look at the flight data on, say FR24 it is usually very irregular. Sometimes you get a gps fix every 60 seconds, sometimes it's every 10 minutes. I was thinking if someone looked at a flight path they might see something specific. It could be like ymb1 says, just a momentary correction for wind that just happens to precede a turn. It could be a delayed reaction from the yaw damper, or even the yaw damper working too quickly making the rudder move before the wings. There really is no reason to bank one way before turning the other way. $\endgroup$ – TomMcW Feb 7 '17 at 20:00
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    $\begingroup$ They may be doing S-turns for spacing. $\endgroup$ – Ron Beyer Feb 7 '17 at 20:24
  • $\begingroup$ It would be helpful if you could give us more details about how much the airplane banks to the left and how much the airplane changes heading to the left (north). $\endgroup$ – J Walters Feb 8 '17 at 3:52
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In cruise, there are no opposite turns that precede every turn.

Human senses and the inner ear are very good at playing tricks, in fact pilots can't judge turns if they have no visibility (e.g., in clouds) without using their flight instruments.

Judging an aircraft's bank from a passenger seat is almost impossible at night, in clouds, over sheets of ice, over hazy oceans, etc.

On a commercial flight, the water in a cup remains level in the lateral during coordinated turns, that's because the load pulling down on the aircraft will still be pulling down perpendicular to the floor.

A shallow roll to the right or left could be just the plane correcting its course in a gust or so.

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  • $\begingroup$ Well, I'm going to maintain that I did feel/see this(!) not because I'm just stubbornly refusing to believe you that it's possible, but because I have experienced what you describe, and this just didn't feel the same (and fwiw clear day with clouds on the horizon as a reference). Anyway, it's clearly not a standard "thing" - some hidden secret of pilots or a known thing with an autopilot - and that's what I was interested to know. Thanks for your answer. $\endgroup$ – Mark Ireland Feb 10 '17 at 22:13

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