NTSB Identification: CEN12LA343
14 CFR Part 91: General Aviation
Accident occurred Saturday, June 02, 2012 in Denver, CO
Probable Cause Approval Date: 02/03/2014
Aircraft: SCHWEIZER 269C, registration: N2196F
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 pilot reported that, during an introductory flight, he was conducting a 2-foot hover in the helicopter. An airplane started to taxi in front of the helicopter, so the pilot set the helicopter back down on the ground, left skid first and then the right. The helicopter drifted right as the skid made contact, and the pilot added left cyclic input to remedy the drift as he also lowered the collective. As the skids started to settle apart, the pilot noticed a low frequency vibration that immediately got worse. He determined that the helicopter was entering ground resonance, but the engine and collective parameters were too low to lift the helicopter off of the ground, as indicated in the helicopter's operating procedures. The pilot reduced the throttle to idle and "within seconds, the helicopter had shaken itself apart."
Postaccident testing revealed that all four landing gear dampers failed to meet the manufacturer's serviceable testing specifications, and two dampers were overcharged. Maintenance records indicated that the front dampers had been replaced with overhauled units about 10 months and about 534 flight hours before the accident. The aft dampers had been replaced with overhauled units about 7 months and about 280 flight hours before the accident. A review of the manufacturer's overhaul procedure for the landing gear dampers revealed that the procedure used by the overhaul facility did not include functional testing of the dampers following overhaul, which would have identified the out-of-specification condition before installation. Maintenance procedures required periodic inspection of the landing gear dampers, and the operator stated that the 100-hour inspection procedure was accomplished about 1 month before the accident. The mechanic who accomplished this inspection reported that he used the manufacturer's procedures to complete the inspection. However, the out-of-specification damper conditions were not detected.
As a result of this and a similar accident (CEN12LA353), the helicopter manufacturer has committed to revising the overhaul manual to include testing procedures and criteria for overhauled landing gear dampers.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The inadequately overhauled landing gear dampers that allowed initiation of a ground resonance event from which the pilot was unable to recover. Contributing to the accident was the overhaul facility's failure to identify the overcharged dampers and a lack of any published acceptance testing criteria for the dampers in the manufacturer's overhaul procedure manual, resulting in the out-of-specification dampers being provided to the operator. Also contributing was the ineffective damper inspection conducted by the operator's maintenance personnel. Full narrative available
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