The pilot was cooling the engine down at 2500 RPM after completing a circuit and normal landing. Suddenly, a hard, high frequency bounce developed and the helicopter bounced left skid low followed by a 45 degree yaw to the left. The pilot had tightened the control frictions after landing and he attempted to release these as he fought for control of the helicopter. He was unable to release the control frictions and the cyclic control grip was broken during his attempts to restrain the movement of this control. The engine stopped without input from the pilot and the aircraft came to rest. The period of ground resonance had been about 5 seconds. During the preflight inspection the pilot had noticed that the front left landing gear was slightly under-inflated, although he considered it to be within normal limits. However, the correct landing gear damper inflation is critical in the prevention of ground resonance in this type of helicopter and the Pilot's Flight Manual contains a caution to this effect. Incorrect or unequal landing gear damper pressure can markedly alter the resonance response of the undercarriage system of the helicopter. If a rotor head vibration is induced, for whatever reason, and it is in sympathy with the resonance response of the undercarriage system, then ground resonance will occur. The cause of the rotor vibration that initiated this occurrence of ground resonance could not be established.
