Incident
While conducting a sightseeing flight eastbound over a river, the pilot and two passengers in a Bell 47G helicopter heard a 'clunk.' Following the sound, the engine began to overspeed and rotor RPM decayed. The pilot initiated an autorotation to a nearby field.
Emergency Landing
During the approach to landing, the helicopter snagged the wires of two low-profile, four-foot-high electrical fences, which the pilot did not see. Immediately prior to touchdown, the helicopter rolled and yawed to the right, making a hard landing and coming to rest on its right side.
Post-Crash Examination
Examination of the engine and transmission after the accident revealed no mechanical malfunctions. However, the oil jet—a threaded hollow plug that normally screws into the forward end of the engine crankshaft—was found lying loose beneath the clutch housing. The interior surfaces of the clutch housing showed a large amount of oil. The clutch assembly itself was within normal wear limits and had no mechanical disparities. The release of the oil jet allowed engine oil to flow through the 3/8-inch threaded orifice into the lower transmission and clutch housing at a significantly higher rate than through the small-diameter orifice in the end of the oil jet. This resulted in disengagement of the clutch. The small orifice in the jet was intended to provide a mist or light spray of oil to lubricate the lower mast bearing.