AUBURN, WA, USA
N9785C
HILLER UH-12E
DURING A SIGHTSEEING FLIGHT, THE PILOT HEARD A SERIES OF POPS AND THE WARNING LIGHTS FOR THE OIL PRESSURE AND TEMPERATURE ILLUMINATED. THE PILOT INITIATED AN AUTOROTATION TO AN OPEN FIELD. DURING THE LANDING MANEUVER, THE TAIL ROTOR WAS SEPARATED. AN ENGINE INSPECTION REVEALED THAT THE NUMBER 4 CONNECTING ROD SHANK WAS BROKEN INTO THREE PIECES. A METALLURGICAL EXAMINATION OF THE CONNECTING ROD ASSEMBLY REVEALED THAT THE CONNECTING ROD BOLT DISPLAYED A FRACTURE TYPICAL OF A TENSILE OVERSTRESS SEPARATION. THE OTHER CONNECTING ROD BOLT WAS NOT LOCATED.
On August 22, 1993, at 1700 hours Pacific daylight time, a Hiller UH-12E, N9785C, experienced a loss of engine power shortly after takeoff from the Auburn Airport, Auburn, Washington. The pilot initiated an autorotation to an open field approximately one mile south of the airport. During the landing, the tail rotor was severed. Visual meteorological conditions prevailed at the time and no flight plan was filed for the local sightseeing flight that was conducted under 14 CFR 91. The helicopter was substantially damaged and the certificated commercial pilot and his two passengers were not injured. The pilot reported that he had been giving five minute sightseeing flights in the local area for most of the day. Just after takeoff and at approximately 500 feet above ground level, the pilot heard a series of pops and the warning lights for the oil pressure and oil temperature illuminated. The pilot initiated an autorotation to an open field that was covered with six foot tall grass and shrubs. During the landing maneuver, the tail rotor was severed by the main rotor blades. During a visual inspection of the engine at the accident site, it was found that oil covered the engine and the decking below the engine. The cooling shroud was removed and the top left half of the crankcase was broken exposing the connecting rods and pistons. A push rod and pieces of the hydraulic tappets were laying loose. The engine was removed from the helicopter and inspected. It was found that the number 4 connecting rod shank was in three pieces. The cap portion was broken into two pieces. One bolt was retrieved and in two pieces. The other bolt was not located. A metallurgical examination of these parts revealed that, "the bolt fracture was typical of a tensile overstress separation." The other broken piece of the connecting rod was secondary due to excessively high stresses.
FAILURE OF THE CONNECTING ROD BOLT. THE HIGH VEGETATION WAS A FACTOR.
Source: NTSB Aviation Accident Database
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