According to the NTSB a certificated flight instructor and student pilot were conducting a hover taxi in a Schweizer 269C helicopter from the hangar area to a fuel pump. The student was initially at the controls. The flight instructor took the controls from the student upon reaching the fuel pump, after the student stated he was uncomfortable landing on a raised platform in the confined area. The flight instructor landed the helicopter on the platform, where it then entered into ground resonance. The flight instructor rolled off the throttle immediately, but the ground resonance intensified, resulting in substantial damage to the helicopter.
Ground resonance happens in helicopters with fully-articulated rotor systems (rotor systems with three or more blades), or more specifically rotor systems with lead-lag hinges. These hinges permit the blades to independently move slightly forward and aft in the plane-of-rotation allowing them to speed up and slow down at different points as they spin around the mast. Known as a drag hinge they are necessary to relieve the stress that might otherwise damage the blades from the acceleration and deceleration of the rotor system. To prevent this back-and-forth movement from creating a serious vibration, hydraulic dampers are used to slow down the movement. Ground resonance cannot occur in a two bladed semi-rigid rotor system because the blades do not lead and lag.
Ground resonance typically occurs during a hard landing when the pilot sets the helicopter down on one corner of a skid or on one tire of a wheel equipped helicopter. The jolt transmits a shock through the fuselage to the main rotor system causing the blades to move out-of-phase with each other. In this condition the weight of the rotor system becomes concentrated on one side of the rotor disk causing the rotor system to become unbalanced. As long as the helicopter stays in contact with the ground the out-of-balance condition in the rotor system rapidly increases in frequency until the helicopter shakes itself apart.
If ground resonance starts, the best option is to lift the helicopter into the air allowing the blades to realign. If flight is not achievable then some improvement might be possible by reducing blade pitch and shutting down the engine. However, since the out-of-phase condition can cause major damage in a matter of seconds this approach is only sometimes successful. Helicopters with fully-articulated rotor systems can have shock-absorbing landing gear that will absorb the energy that feeds ground resonance. When ground resonance happens in these helicopters, it is usually because dampeners or shock absorbers have been improperly serviced.