Incident Details
During the initial run-up prior to takeoff, the helicopter encountered ground resonance. The magneto checks produced a rough running engine. At 2,000 rpm, the certified flight instructor (CFI) attempted to correct the roughness by leaning the mixture. The helicopter then began shaking from side-to-side, indicating ground resonance. Due to the low rpm, the CFI elected to close the throttle rather than lift the helicopter from the ground. The ground resonance increased, and neither the pilot-undergoing-instruction nor the CFI could hold onto the controls. The CFI then pulled the mixture control, causing the engine to stop.
Post-Incident Examination
Following the incident, the skid damper assemblies were examined and tested for charge. The rear dampers had a lower charge than that specified in the maintenance manual for the helicopter. The forward dampers were within limits. The Schweizer 269 maintenance instructions call for the dampers to be checked and correctly pressurized at each 100-hour inspection. The last 100-hour inspection was completed 1 month and 61 hours prior to the incident.