What happened
On 3 March 2011, a Bell 206L-4 Longranger IV, registration G-PTOO, was conducting a private flight when the pilot detected high-frequency vibrations spreading through the airframe and flight controls. In response to the instability, the pilot declared an emergency and executed a precautionary landing on the grass adjacent to a runway at London City Airport. There were no injuries to the pilot or the single passenger on board.
The investigation
Following the landing, an inspection of the helicopter identified that a tail rotor balance weight assembly had detached from one of the blades. The detached assembly, along with a broken section of its securing bolt, was located on the ground near the aircraft. The investigation determined that the assembly had likely remained attached to the blade until the final stages of the flight.
Examination of the hardware revealed that the bolt, which secures the balance weight assemblies to the blade, had suffered damage to its cadmium plating due to contact with the blade structure. While the remaining portion of the bolt stayed in place, the broken section showed evidence of a crack that had progressed through the shank. The investigation also noted damage to the tail rotor gearbox mountings and the tail boom structure. Records indicated the tail rotor hub assembly had last undergone an overhaul 1,691 flying hours prior, though specific maintenance details for the gearbox hub were unavailable.
Findings
- The vibration was caused by the detachment of a tail rotor balance weight assembly.
- The failure was triggered by a fatigue crack that formed within the shank of the securing bolt.
- The crack originated at a point where the bolt's cadmium plating had been worn away by friction against the tail rotor blade.
- The manufacturer stated there had been no previous reported failures of this specific bolt design in the Bell 206L-4 or related models.