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I have noticed on all Airbus of the A320 series that I have flown as a passenger (which includes the 319, the 320 and the 321), that whilst in level flight, the aircraft has always a longitudinal axis rocking motion. It is very low but if you focus on the horizon from the window you can actually notice the aircraft rocking left and right. Almost not perceivable but it is actually there! This does not happen on the A330's or A340's

Is there any particular reason for this type of behaviour?

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    $\begingroup$ Probably some degree of Dutch roll (the same oscillations should be seen on the yaw axis), due to yaw damper less effective on shorter aircraft. The A319 seems to be known for that. $\endgroup$ – mins Dec 10 '15 at 10:32
  • $\begingroup$ Interesting. The fact is that on aircraft of comparable role and size such as the 737 this effect is not noticeable. $\endgroup$ – Fabrizio Mazzoni Dec 10 '15 at 13:57
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As nobody has answered this in 4 days I'll add this "layman's" input.

I am not a pilot or aeronautically trained, but I understand that essentially all airliners have a dynamic yaw instability aka Dutch Roll and have stabilisers designed to address this. ANY feedback error correction system requires SOME error signal to derive the feedback from and how much error and how it manifests changes with system.

@mins I'm no expert in this area BUT all aircraft are different and you'd be surprised (or should be) if they all behaved the same. The overall combination of a wide range of dynamic forces leads to the end result.

As I recall (probably from Peter R March's very interesting 'The 747 story' Dutch roll was not a major factor in large aircraft until the larger jets and they lost a prototype with an experienced pilot inexplicably. Subsequent investigations and close calls showed the cause was a dynamic yaw-roll coupling. AFAIR this is caused by the high yaw-induced rolling moment of swept wings and as these are standard fare as speeds rise all large/fast modern airliners have this issue and have a yaw damper to make it unnecessary for the pilot to deal with it.
Without a yaw damper, the vertical tail would need to be much larger to provide enough damping. This costs weight and drag and creates problems of its own due to the location of the tail upwards of the line of symmetry. When the aircraft yaws, the wing creates a rolling moment. This rolling motion causes a drag difference which, together with the directional stability of the tail, swings the aircraft's tail around to the other side, and the process repeats in the opposite direction.

It may be, and I'm 'mentating' this with no proof at all, that they are up against a regulatory barrier and that correcting the problem may lead to them having to do something else expensive. eg early British cars were long stroke not for any technical reason but due to taxation advantages as taxing was based on bore AREA. For a given cc size bore area was reduced by increasing stroke. This limited achievable RPM. Japanese engines had no such limits and I remember our amazement in the early days of Japanese motorcycles at the under-square high revving Japanese motorcycles - which gave them a power per cc rating advantage.

Similarly some America's cup yachts at one stage had their ratings improved by tacking light metal sheeting on their decks. This made NO difference whatsoever.

As an example of stabiliser interaction in another field - many modern cameras have inbuilt motion stabilisers. These need to be turned off when the camera is used on a tripod - or else the compensation system causes the camera to "hunt" about the essentially perfectly stable tripod. I've read of people who had major blur issues through failing to realise this.

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    $\begingroup$ Thanks for answering! The stabiliser is actually called a roll damper, and the first one was installed in the Me-262. A small yaw oscillation was even present in early propeller aircraft, because the vertical tail is too small in order to minimize drag. It was so bad in the Convair 880 that the aircraft had to be withdrawn from passenger service. $\endgroup$ – Peter Kämpf Dec 14 '15 at 7:56
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I've noticed this on Boeing widebodies, e.g. 767, moreso than on any Airbus types. However it isn't a design "thing" with the aircraft as such, whether Airbus or Boeing, but is an interaction with the autopilot and the current weather. Widebodies have more inertia to overcome, so the cyclic corrections come further apart unless the crosswinds are particularly strong.

If the aircraft is tracking through a fairly strong crosswind at altitude, it makes more frequent and more enthusiastic course corrections. In Europe that would be on north-south sectors more often than west-east sectors, since the winds at altitude are predominantly westerlies. How aircraft deal with this is down to autopilot function - the point at which it "decides" to make a correction, its "hysteresis", to borrow a term from elsewhere. If these corrections are continuous then it manifests itself as "rocking" - it's just the autopilot working hard at maintaining a heading.

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  • $\begingroup$ It seems odd that a crosswind would cause such a thing, but this is certainly a very interesting answer $\endgroup$ – quiet flyer May 12 at 2:12
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What you are most likely observing is the behavior of the AFCS tracking a given course and making corrections as the aircraft drifts to the left or to the right of the course. It could also be due to turbulence, wind shear or as described above a dutch roll flight characteristic.

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I am real world pilot and flying on the A320 Family.

These bank variations are caused by calibration between the yaw roll and the flight computers. It`s in case of stabilization and recalibration. It's important to hold the aircraft right on track and to maintain a stable information exchange between the board computers and the rudder section sensors.

In normal flight mode (good weather, calm winds, stable horizontal flight without any changes), the aircraft banks a maximum of 4 degrees +- alone to calibrate (more if needed in case of turbulence etc.) and in order between flightdeck and computers a maximum bank of ca. 30 degrees +- (input by A/P or Pilot).

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  • $\begingroup$ Welcome to aviation.stackexchange.com! I don' t fully understand your last sentence, can you clarify? $\endgroup$ – DeltaLima May 10 at 12:38
  • $\begingroup$ Does the flight manual explain this calibration process? $\endgroup$ – ymb1 May 10 at 14:14

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