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Some time ago, I was on a commercial flight and while the airplane was taxiing around, there was this rhythmic bouncing. Here is a video I made of the takeoff. You can notice the bouncing at the taxi and takeoff roll of the video. What causes that rhythmic bouncing? Is it a result of uneven wear on the tires?

For the sake of anyone who wants to know in case it helps, the airplane was a Boeing 757-200.

SMS von der Tann
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    Not really obvious on the video. The nose gear running over embedded guiding lights maybe? In some places, it may be the junction between concrete slabs making up the taxiway, but this does not seem to be the case here. – jcaron May 04 '16 at 22:55
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    Good question, I also experience it sometimes. – Firee May 05 '16 at 12:14
  • It can be caused by flat spot after tire locked after hard breaking. Once I flew on F50 where you can see tires through the window. And breaking flat spot was clearly visible. – Andrius May 06 '16 at 08:29

2 Answers2

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If you are old enough to remember, the bias-ply automobile tires used in the 1960's would develop a flat spot when left sitting overnight. The flat spot would quickly go away when you started driving.

Many of today's aircraft tires still use bias-ply tires and are also subject to flat spotting.

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Source: The Boeing 737 Technical Guide

Mike Sowsun
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  • Interesting, I have not been around for that long, so I would not know. The other thing is that the airplane was sitting at the gate for some 30 minutes. It had a previous flight too. – SMS von der Tann May 05 '16 at 15:30
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    Interesting (I'd support it), but there are challenges: With the number of tires involved, and the fact that they will quickly be out of sync, due to the turns, can this effect survive a long time after the aircraft moves? Can we anticipate the frequency based on the diameter of the tires, and does it mach the video? – mins May 05 '16 at 16:06
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That is caused because the wheels are "uneven" after the plane was waiting at the stand. You have to imagine, that a plane has an enormous weight. The wheel are not fully pressurized like in a car, because they are going to expand in higher elevation.

Now, assuming an 44.5in (1.1m) tyre diameter (circumference ca 4m) and the taxying speed of approx. 17kts (ca 8m/s), we would hit the flat spot every ca 4 meters, so twice a second. That would induce oscillations. Im not a mechanical engineer, but i could imagine that this in combination with the dampers would cause the described vibrations. As the Boeing technical Guide is describing this as the cause I don't see any reason to doubt that.

SMS von der Tann
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mike
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  • So should we feel the same effect on every aircraft? – mins May 05 '16 at 16:02
  • No, that depends on the tyre type (and therefore aircraft type) the turnaround time (how long the aircraft was parked) and other things like number of passengers and weight of cargo, as well as the temperature. It also depends on the speed of the taxiing aircraft. As a sidenote it should be mentioned that boeing started to use michelin as tyre supplier, same as airbus. So this should be a thing of the past. – mike May 05 '16 at 17:37