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A resonance that manifests itself as a violent rocking of the helicopter when it is on the ground. This phenomenon is most evident when the helicopter is on hard surfaces that do not absorb vibration. It is caused by the resonance between the skid and the rotor system. It is most likely if the helicopter has a fully articulated rotor system that experiences a hard impact, such as the helicopter hitting the ground with a single skid while landing. The resonance can either build up slowly, with the oscillations gradually increasing in amplitude, or extremely rapidly. If uncorrected, the helicopter can roll over and break up. To prevent this critical situation, either the engine should be switched off if the rotor RPM is low or the helicopter should takeoff by applying more power.
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