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In the past (before the pandemic, of course), when riding commercial jets, I noticed that sometimes, the pilots would bank to port, then to starboard, and repeat that multiple times, while climbing after takeoff. I was wondering if there was a reason they did that. Maybe they were just turning? Then why bank back to the other side? Perhaps they did it because there's nothing illegal with banking while you're climbing? But that might generate lift in a direction not quite perpendicular to the horizontal plane. I'd think that they would do it for a reason. Perhaps no one is sure of what I'm talking about. Any help?

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  • $\begingroup$ How much banking are we talking about? And for how long did the plane remain banked? $\endgroup$
    – Bianfable
    Dec 15 '20 at 16:34
  • $\begingroup$ Well...I'm pretty sure the plane continued travelling in a straight direction. The banking was most likely 45 degrees or less. And the plane remained banked for around 10 (?) seconds or a bit less. $\endgroup$ Dec 15 '20 at 16:37
  • $\begingroup$ Yes, pilots turn the aircraft, and make heading corrections, both of which require some banking. Looking out the side window at a long wing, even minor course corrections may seem larger than they really are. $\endgroup$ Dec 15 '20 at 17:07
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    $\begingroup$ @Sovereign Inquiry: 45 degrees is a pretty major bank angle. I'd suspect your commercial flights bank much less. As for why they bank, unless the runway is pointing directly at the destination, they pretty well have to turn. And then there's noise abatement, traffic avoidance, and so on. As for banking back and forth, I suspect much of that is basically a perceptual illusion. The first bank starts a turn, then when you reach the desired heading, you "un-bank" to straighten out. But if you don't have a good view of the horizon, or instruments, it feels like a bank the other way. $\endgroup$
    – jamesqf
    Dec 15 '20 at 17:42
  • $\begingroup$ Ahh. And you're probably right about the bank angle. You could make your comment an answer... $\endgroup$ Dec 15 '20 at 18:13
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When a commercial airliner takes off, they are vectored (directed) by ATC along or to a certain course. Or, they are given a specific published, codified procedure to follow. This provides the aircraft proper obstacle and traffic avoidance and clearance. It might also provide for avoiding sensitive areas like restricted airspace, environmentally and noise sensitive areas. You can view published procedures for your airport on Sky Vector

When ATC or the published procedure requires the aircraft to change direction, the aircraft has to bank to turn. When an airliner turns, they typically bank no more than 30° unless there is an emergency. The 45° bank you mentioned would be avoided due to discomfort for most passengers. Bank angle exponentially increases G-load. A 45° bank would increase G-loads by almost 50%. And some passengers would get the sensation that the wings were pointing perpendicular to the ground.

S-turns (one turn immediately followed by another turn in the opposite direction), while not illegal, are not normal in an airliner. And, it would not make sense to do them on takeoff nor climb-out.

I would suggest that you were suffering from a vestibular illusion called Somatogyral Illusions. A mild form of this is usually termed “The Leans”. A more severe form of this is termed “Graveyard Spins” and “Graveyard Spirals”. You can combat and overcome this illusion by sitting with your head straight up and as still as possible. Abrupt head movements will greatly aggravate this condition. Being able to see the horizon out of the front windowsheild also helps. Unfortunately, you do not have this view.

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    $\begingroup$ So in other words, commercial jets basically never bank for no reason. And I think the Somatogyral Illusions aren't serious. jamesqf described it perfectly: "As for banking back and forth, I suspect much of that is basically a perceptual illusion. The first bank starts a turn, then when you reach the desired heading, you "un-bank" to straighten out." $\endgroup$ Dec 15 '20 at 19:57
  • $\begingroup$ @SovereignInquiry - If they are banking, there is probably a reason. $\endgroup$
    – Dean F.
    Dec 15 '20 at 20:33
  • $\begingroup$ As far as the illusion, you would be surprised. It is very easy (for me) to tell when an aircraft is in level flight. It is very easy to tell when an airplane is banking. It is also very easy to tell when it is unbanking. When you do not have a view of the horizon or flight instruments as a reference, it is harder to tell the degree or magnitude of a bank or unbank. When an aircraft banks slow enough, remains banked long enough, or unbanks too rapidly, you could have the sensation of a bank in the opposite direction when the aircraft returns to straight and level. $\endgroup$
    – Dean F.
    Dec 15 '20 at 20:33
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    $\begingroup$ Maybe remaining banked for an extended period of time is the explanation. $\endgroup$ Dec 15 '20 at 20:37
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    $\begingroup$ @SovereignInquiry They will usually be doing turns at standard rate, which means a 180° turn would take one minute. Without any outside reference or instruments, that’s easily long enough for your body to trick itself into thinking the plane is level again, so you feel a “turn” the other way when they straighten back out. $\endgroup$
    – StephenS
    Dec 16 '20 at 14:13

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