What happened
After landing at a private airstrip, the flight instructor exited the helicopter while the student pilot remained in the aircraft with the engine running. The instructor then reboarded the aircraft, after which the student performed a takeoff.
While flying on the downwind leg of the traffic pattern, a witness observed a loud noise emanating from the engine. Following this noise, the main rotor was seen to slow significantly, and the helicopter entered a very steep descent. The aircraft impacted the ground approximately 1,000 feet south of the runway. There were no fatalities reported.
The investigation
Investigators examined the engine and found that the rod bolts on the number four cylinder had fractured. A metallurgical examination of one of the rod bolts revealed cracking through 25 percent of the bolt.
An examination of the flight controls showed no evidence of failure prior to the impact. At the time of the accident, the engine had recorded 813 hours since its last major overhaul and 36 hours since its annual inspection.
Findings
- The fracture of the engine rod bolts was caused by fatigue.
- The fatigue was attributed to insufficient preload on the bolt.