Incident Overview
A helicopter, conducting dual instruction, landed at a private airstrip. The instructor deplaned, leaving the student pilot alone in the helicopter with the engine running. The certified flight instructor (CFI) then reboarded, and the student performed a takeoff.
Accident Sequence
Later, while on the downwind leg of the traffic pattern, a witness reported hearing a loud noise from the engine. The main rotor was observed to slow noticeably, and the helicopter entered a very steep descent. It impacted the ground approximately 1,000 feet south of the runway.
Examination Findings
Examination of the engine revealed that the rod bolts on the number four cylinder had fractured. Inspection of the flight controls showed no evidence of preimpact failure. One of the fractured rod bolts was subjected to metallurgical analysis, which disclosed cracking through approximately 25 percent of the bolt. The fatigue cracking was attributed to insufficient preload on the bolt.
Engine History
At the time of the accident, the engine had accumulated 813 hours since a major overhaul and 36 hours since its most recent annual inspection.