Accident Summary
During an agricultural spraying operation over a tomato field, the pilot of a helicopter encountered a strong vibration throughout the airframe, immediately followed by a severe yaw. Pedal inputs did not affect the yaw, and the helicopter began to spin. The pilot reduced power and lowered the collective, which stopped the yaw, and the helicopter subsequently landed hard.
Post-Accident Examination
An FAA airworthiness inspector examined the wreckage at the accident site. One tail rotor blade was missing and was later found approximately 100 feet away. The inspector reported that the tension torsion (TT) plates of the separated blade were broken at the hub attach bolts. No leading edge damage was observed on the blade.
Laboratory Analysis
The fractured TT plates were sent to the NTSB lab, where representative plate fractures were selected for detailed examination. Features typical of progressive fatigue cracking were observed. A material analysis revealed that the straps were made from the manufacturer's specified material. No evidence of foreign element contamination was found on the fracture faces.
Component History
The tail rotor TT bar has a 12,500-hour life limit. The fractured TT bar had accumulated a total time in service of 3,710 hours. No prior incidents were found in the FAA Service Difficulty Report database.