If a helicopter encounters ground resonance, what should be the first action taken?

The first action to take when encountering ground resonance in a helicopter is to assess the rotor speed (RPM). If the RPM is within the normal operating range, raising to hover is a preferred action because it removes the helicopter from the ground, thus alleviating the resonance condition. If the RPM is not in the green, lowering the collective helps reduce the load and, therefore, the excitation forces can be minimized. Shutting off the engine should be done as part of the emergency procedures if the situation does not stabilize after the initial actions.

This answer emphasizes the importance of rotor RPM management and the need to minimize stress on the helicopter's structure, which can be compromised during ground resonance. Understanding that raising the collective might be inappropriate when the RPM is not in the green is crucial, as it may exacerbate the resonance situation rather than mitigate it.