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While I was reading a small article on "Reasons for RTO", I came across a situation where the pilots aborted takeoff because of "landing gear shimmy". What does that mean?

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the nose gear support strut contains a bearing which allows the nose wheel to swivel like a caster so the plane can turn on the ground. the nose wheel assembly itself is turned either via a hydraulic cylinder or a mechanical linkage connected to pads on the upper tips of the rudder pedals, or in some cases it freely turns on its own during the application of asymmetrical braking to the main gear.

But whatever the details of the nosewheel design are, "shimmy" occurs when the wheel assembly wobbles rapidly back and forth (left-right-left-right) as the plane is rolling down the runway at speed. this is called castering instability and occurs when there is excessive play or looseness in the caster bearing or the support strut or the wheel bearings, or slop in the steering control circuit, or improper inflation of the nose wheel.

The shimmy action sets up strong vibrations in the airframe that the pilot can feel and if strong enough can hear as well. Those vibratory stresses are acting to wiggle loose any bolted connections in the entire nosewheel assembly and can also seek out and activate any fatigue stress raisers present there- in addition to making the airplane difficult to accurately steer.

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    $\begingroup$ Like it occasionally happens with worn out shopping carts, right? $\endgroup$ – PerlDuck Nov 15 at 8:48
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    $\begingroup$ @Johnson You're basically putting 42k of horsepower behind a 300,000 kg triangular / 3-wheeled shopping trolley or pram, and trying to keep it going in a straight line. $\endgroup$ – Chronocidal Nov 15 at 14:34
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    $\begingroup$ @Johnson When you're going very fast in close proximity to the ground, you do not want something applying uncontrolled and unpredictable sideways shoves to the nose of your aircraft. $\endgroup$ – anaximander Nov 15 at 14:36
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    $\begingroup$ @Johnson also, after takeoff, you have to land, and unlike takeoff, completing a landing is not negotiable - even if you abort and go around, you still have to try again. It's the old aviation saying: "It's better to be down here wishing you were up there, than to be up there wishing you were down here". $\endgroup$ – alephzero Nov 15 at 15:36
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    $\begingroup$ @Johnson "How do pilots know?" They can probably feel the vibrations in the controls (at least of a non-complete FBW system) or through the seat-of-their-pants, just as you would in your car. Either way, that's excellent fodder for a whole new question so you can get a complete answer. $\endgroup$ – FreeMan Nov 15 at 17:59

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