NTSB Identification: LAX04LA197.
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Accident occurred Tuesday, April 27, 2004 in Van Nuys, CA
Probable Cause Approval Date: 10/27/2005
Aircraft: Schweizer 269C-1, registration: N61415
Injuries: 2 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The helicopter encountered ground resonance during the initial run-up prior to takeoff. During the run-up, 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 and the engine stopped. 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 1 month 61 hours prior to the accident.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

the failure of maintenance personnel to ensure the proper charge of the aft landing gear dampers in accordance with the manufacturer's recommendations.

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