What happened
During the initial run-up prior to takeoff, a Schweizer 269 encountered ground resonance. While performing magneto checks, the engine began running roughly. At 2,000 rpm, the certified flight instructor attempted to correct this roughness by leaning the mixture. Following this adjustment, the helicopter began shaking from side-to-side, signaling the onset of ground resonance.
Because the engine was at a low rpm, the instructor elected to close the throttle instead of attempting to lift the aircraft from the ground. The intensity of the ground resonance increased to the point where neither the student pilot nor the instructor could maintain control of the flight controls. To terminate the event, the instructor pulled the mixture control, resulting in an engine stop.
The investigation
Investigators examined and tested the skid damper assemblies for their charge levels. While the forward dampers were found to be within specified limits, the rear dampers possessed a lower charge than required by the aircraft maintenance manual. According to the maintenance instructions for the Schweizer 269, these dampers must be checked and correctly pressurized during every 100-hour inspection. The most recent 100-hour inspection had been completed 1 month and 61 hours before the incident.