DUMONT, TX, USA
N796
HUGHES 269C
DURING WILD LIFE CONTROL ACTIVITIES THE HELICOPTER EXPERIENCED A CATASTROPHIC ENGINE FAILURE AT 40 FEET ABOVE GROUND LEVEL. THE PILOT ATTEMPTED TO REGAIN POWER BY 'MILKING THE COLLECTIVE.' THE HELICOPTER IMPACTED THE GROUND IN A LEVEL ATTITUDE WITHOUT FLARING. FOLLOWING THE INITIAL IMPACT THE HELICOPTER BOUNCED 13 FEET FORWARD AND ROLLED RIGHT. EXAMINATION OF THE ENGINE REVEALED THAT THE #3 CYLINDER CONNECTING ROD CAP BOLTS HAD FRACTURED RESULTING IN A CATASTROPHIC FAILURE OF THE BEARING CAP AND PISTON. THE SAFETY BOARD'S METALLURGICAL EXAMINATION REVEALED THAT THE FRACTURE FEATURES AT THE CENTER OF EACH BOLT WERE THE RESULT OF OVERSTRESS SEPARATIONS. HOWEVER, THE FRACTURE FEATURES ADJACENT TO THE OUTSIDE DIAMETER OF EACH BOLT CONTAINED MECHANICAL DAMAGE THAT OBLITERATED THE ORIGINAL FRACTURE FEATURES.
On January 12, 1995, approximately 1730 central standard time, a Hughes 269C, N796, was destroyed during a forced landing near Dumont, Texas. The helicopter, owned and operated by the U.S. Department of Agriculture, was on a local public use flight. There was no flight plan filed and visual meteorological conditions prevailed. Neither the pilot, nor the gunner were injured. The pilot reported in a written statement that he was at an altitude of "about 40 feet above ground level," maneuvering south at "about 30 knots" herding some "hogs." The pilot further stated that, he experienced severe vibrations, to a level that interfered with his vision. The pilot then began "milking the collective" in an attempt to regain power, in an attempt to land. The pilot further stated that he determined that he "did not have enough airspeed to flare." According to local law enforcement officials and the pilot the aircraft struck the ground in a "straight and level attitude;" however, "the helicopter landed hard," bounced 13 feet, and rolled over onto its right side. An examination of the engine revealed that the #3 cylinder connecting rod bearing cap bolts had fractured resulting in a catastrophic failure of the bearing cap and piston. The connecting rod arm, cap and corresponding nuts and fractured bolts from cylinder #3 were forwarded to the Safety Board's materials laboratory for further metallurgical examination. NTSB Metallurgist's Factual Report No.95-98 states "Scanning electron microscopic examination of all bolt fractures revealed that the fractured areas that were located adjacent to the outside diameter contained mechanical damage that obliterated the original fracture features. However, the remaining portion of the fracture surface at the center of each bolt contained no damage and exhibited ductile dimple fracture features that are typical of an overstress separation." (All references to cylinder #4 in the report should correctly read cylinder #3.) The nuts and connecting rod bolts had markings, chemical compositions, microstructure, hardness, and thread configurations consistent with those specified by the manufacturer. The aircraft and the tested components were returned to the operator following completion of the investigation. (This narrative was rewritten March 18, 1997 by Joel K. Ryan, Senior Air Safety Investigator, Regional Operations and General Aviation Division, Office of Aviation Safety. The original narrative contained details of a metallurgical examination of components from another accident, and should be disregarded. This error was brought to the Safety Board's attention in a letter from the U.S. Department of Agriculture (USDA) dated February 19, 1997, and the letter has been added to the report.)
THE LOSS OF ENGINE POWER DUE TO OVERSTRESS FRACTURES OF THE #3 CYLINDER CONNECTING ROD CAP BOLTS. THE ORIGIN OF THE FRACTURES COULD NOT BE DETERMINED.
Source: NTSB Aviation Accident Database
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