Incident Overview
During an approach, the helicopter yawed to the right. The pilot reduced power and executed an emergency landing. Before shutdown, it was noted that the tail rotor was stationary while the main rotor continued to turn.
Investigation Findings
Investigation revealed that the tail rotor drive shaft had separated due to thermal/mechanical damage incurred by contact with the engine firewall. Further investigation determined that a firewall modification had been designed which included a doubler strip at the forward and aft firewall section connection. The presence of the doubler would decrease clearance between the firewall and the tail rotor shaft.
Contributing Factors
It was determined that during installation, the forward firewall section, to which the doubler is attached, was mispositioned. As a result, clearance was reduced to nil, and components came into contact during operations.
Conclusion
The separation of the tail rotor drive shaft was caused by thermal/mechanical damage from contact with the engine firewall, exacerbated by a firewall modification that reduced clearance and mispositioning of the forward firewall section.