Refer to figure.
Ground resonance can be defined as a forced or self induced large amplitude vibration of a helicopter in contact with the ground. It is recognized from a rocking motion or oscillation of the fuselage. If early corrective action is not taken, the amplitude can quickly increase to a point where it will be uncontrollable and the helicopter will roll over.

For ground resonance to occur 4 conditions must be present:

• Rotor system self-excited vibrations
• Fuselage vibrations
• Rotor and fuselage vibrations must be of similar frequency to reinforce each other
• Helicopter must be in contact with ground

Undercarriage “padding” can sometimes cause ground resonances, this is felt by lateral oscillation with increasing frequency. Lateral oscillations occur on skidded helicopters in particular, which isn’t a problem unless frequency or severity increases to the level of ground resonance.

During ground resonance the main rotor blades may be out of phase in relation to each other, causing the fully articulated disc to become unbalanced. Main rotor Centre of Gravity (CG) displaced from axis of rotation (rotor mast) and/or destructive oscillations are present.

During engagement of fully articulated rotor systems the main rotor blades might be out of sync leading to the risk of ground resonance, preventing this can be done by correct tyre pressure or proper gas pressure in the oleo dampers.

The causes of ground resonance are:

• Bad lead-lag dampers
• Incorrect tire pressure (wheeled helicopters)
• Bad oleo dampers
• Hard ground contact

To correct for ground resonance the Pilot must eliminate the vibrations at an early recognition stage. The Pilot must shut down the helicopter immediately and apply rotor brake if at ground idle or take off if at flight idle to bring the helicopter airborne.