History of the Flight
On 3 March 2011 at 1512 UTC, a Bell 206L-4 Longranger helicopter, registration G-PTOO, was in cruising flight when the pilot experienced the onset of a high frequency vibration through the airframe and flight controls. The pilot declared a PAN and made a precautionary landing on an area of grass beside the runway at London City Airport.
Damage and Discovery
After landing, an inspection of the helicopter revealed that a tail rotor balance weight assembly had detached from one of the tail rotor blades. The balance weight assembly, together with a fractured section of its securing bolt, was discovered close to the helicopter. Additional damage to the tail rotor gearbox mountings and the associated structure of the tail boom was also found.
Investigation Findings
Examination determined that the bolt securing the balance weight assemblies to the tail rotor blade had failed due to the formation of a crack in the bolt shank, which propagated in fatigue. The failure occurred approximately 12 mm from the end of the shank and included the securing nut and associated split pin. The remaining portion of the bolt had stayed in place, retaining the second balance weight assembly. The initiation point of the crack coincided with a section of the bolt shank where the cadmium plating had been worn away by contact with the tail rotor blade structure, though no cause for the initiation of the crack could be identified.
Prior to removal of the securing bolt from the remaining tail rotor blade, the break-away torque of the nut was measured and found within expected values. Inspection of that bolt showed a similar but less pronounced wear pattern on the cadmium plating; no evidence of crack initiation was found upon sectioning.
Maintenance History
The tail rotor hub assembly fitted to the Bell 206L-4 is subject to a 2,500 flying hour overhaul requirement. The helicopter’s records indicated that the last overhaul of the tail rotor assembly had been carried out 1,691 flying hours prior to the incident while the helicopter was on another national register. The records did not contain detailed information regarding the overhaul of the tail rotor gearbox hub, making it impossible to verify the history of the failed balance weight bolt.
Manufacturer's Confirmation
The helicopter manufacturer confirmed that this was the first reported occurrence of this nature relating to this design of tail rotor system. The tail rotor hub assembly is also fitted to the Bell 407 and 427 helicopter types, and no failures of these bolts had been reported on those types either.