Accident Overview
During a hover training flight, a helicopter sustained a main rotor hub fracture, resulting in a rollover and subsequent fire. The flight instructor and student pilot were not injured.
Flight Details
The certified flight instructor (CFI) reported that he was assigned to fly with the student on the morning of the accident. Before the flight, he consulted with the student's regular CFI to discuss the student's progress and abilities. The preflight preparations included a weight and balance check, a weather assessment, and a thorough preflight inspection of the helicopter using the checklist. Operational checks of the helicopter were also performed.
Hover Training
The helicopter was taxied to a designated spot on the airport, where the CFI began instructing the student on hovering techniques. Initially, the student was allowed to operate each control independently, and later the student practiced hovering with corrective input from the CFI. Approximately 15 to 20 minutes into the instructional flight, while the helicopter was hovering, the CFI recalled that the helicopter moved aft, followed by a violent jolt. The helicopter then rolled onto its left side and caught fire. Both occupants exited the helicopter from the right side and were uninjured.
Post-Recovery Inspection
After the helicopter was recovered, an inspection revealed that the main rotor hub was fractured. The associated main rotor blade was found in two pieces: one piece was located about 30 yards away and the other about 60 yards away. The fracture surface of the main rotor hub exhibited features consistent with cantilever bending overstress. The separated section of the main rotor blade showed compression wrinkles.