What happened
While operating in a low hover, a Robinson helicopter experienced a sudden structural failure of its tail assembly. The event began when a tip cap detached from one of the tail rotor blades. This detachment triggered intense, high-frequency vibrations throughout the tail rotor drive system. The resulting abnormal loading caused the tailcone casting to fracture, leading to the complete separation of the empennage assembly. This separation allowed the tail rotor blades to strike the horizontal stabilizer, causing further damage to the drive system. The pilot's prompt actions prevented the incident from resulting in a more serious outcome.
The investigation
Investigators performed a detailed fractographic analysis of the fractured tailcone casting. The study found no manufacturing defects that would cause failure under normal conditions, suggesting the fracture was caused by extreme stress. This stress was traced back to the imbalance created by the missing tip cap.
Regarding the adhesive failure, the investigation found that approximately 85% of the original bonded area at the tip cap had failed. Evidence suggested that moisture had compromised the bond. On one blade, a crack in the leading-edge skin—likely caused by erosion and pitting—may have acted as a pathway for moisture to reach the adhesive. While the blades were part of the same set and showed similar wear, the degradation was more advanced on one specific blade.
Findings
- The in-flight detachment of the tail rotor blade tip cap was the primary driver of the structural failure.
- Moisture-induced adhesive failure significantly weakened the bond between the tip cap and the blade.
- A maintenance inspection conducted 21 flight hours prior to the event failed to detect the existing adhesive disbonding.
- The use of tap testing, while a standard method for checking bond integrity, proved insufficient in this instance, as extensive adhesive failure may not produce a distinct acoustic change for an inspector to identify.