What condition is most likely to cause ground resonance in helicopters?

Ground resonance in helicopters is a phenomenon that occurs when the rotor system becomes unbalanced due to vibrations or shocks that transfer through the airframe to the landing gear. This situation can lead to a self-exciting oscillation that amplifies vibrations, potentially resulting in severe damage to the helicopter or even a complete loss of control.

When a series of shocks disrupts the balance of the rotor system, it can create oscillations that resonate with the frequency of the helicopter's landing gear, causing continued vibration that can escalate into ground resonance. The mechanics of this situation are critical for pilots and maintenance crews to understand in order to prevent scenarios that might lead to such resonance.

In contrast, simply operating at low altitude, excessive lateral movements of the rotor blades, or worn skids might influence flight characteristics or stability, but they do not directly lead to the oscillatory behavior associated with ground resonance as effectively as an unbalanced rotor system does. Understanding the mechanics behind ground resonance is essential for maintaining flight safety and aircraft integrity.