What happened
During a flight, the pilot experienced the onset of mild vibrations within the airframe. This vibration increased in severity as the flight progressed, eventually leading to a precautionary landing. Following the landing, a post-flight inspection by the pilot revealed a crack in one of the main rotor blades. This crack was consistent with the vibrations felt during the flight.
The investigation
The investigation focused on the condition of the rotor blades and the circumstances leading to the crack. Examination of all four blades revealed damage, though no specific operational events, such as ground handling incidents or blade strikes, were identified as the source of the stress. While no manufacturing or material defects were found, the crack in the affected blade passed through the site of a previous repair. The manufacturer noted that cracks appearing at prior repair sites are not uncommon, suggesting the repair may have influenced the rate of cracking in that specific area.
Because the vibration was only noted toward the end of the preceding flight, it is believed the crack was present but smaller at the start of the occurrence flight, making it difficult to detect during a standard pre-flight check. Furthermore, the time elapsed since the last detailed periodic inspection made it unlikely that the damage would have been identified during that interval.
Findings
- The primary cause of the vibration was a cracked main rotor blade.
- The crack was located at a site of a previous blade repair, which likely contributed to the development of the crack.
- The pilot did not report the mild vibration from the previous flight to maintenance, as the vibration was below their threshold for concern and was attributed to the helicopter's loading condition.
- No specific operational or manufacturing factors were found to explain why all four blades showed concurrent damage.
- The failure mechanism is considered progressive, meaning significant vibration typically serves as a warning to the pilot before a critical failure occurs.