Accident Overview
During fire suppression operations for the United States Forest Service, a helicopter experienced an in-flight separation of a tail rotor blade. Subsequently, the tail rotor gearbox and the remaining tail rotor blades separated from the aircraft. The helicopter then pitched nose-down at approximately 45 degrees while rotating about its vertical axis and impacted the side of a river shoreline.
Witness Observations
A witness near the accident site reported hearing a loud bang while observing the helicopter over a dip site. The helicopter was flying over a stone riverbed toward a bridge when a large piece, later identified as the tail rotor gearbox with three of its four blades attached to the hub, fell from the aircraft.
Post-Accident Examination
Examination revealed that the tail rotor blade associated with the red spindle had separated from the tail rotor. The red tail rotor spindle failed due to a fatigue fracture. The fatigue initiated at a single origin associated with a shot-peen pit, located in the outboard of three machined grooves on the spindle under a spacer. Circumferential scratches were observed in the groove areas and on the adjacent journal surface, but the fatigue crack origin was not associated with these scratches. The journal diameter was found to be undersize by 0.007 inch, and fretting was observed on the journal surface. The groove peak between the middle and inboard grooves was sheared on the trailing side of the red spindle and displaced inward, consistent with damage from a tail rotor strike.
Maintenance History
The total operating time of the spindle could not be determined as it was not a life-limited component that required serial number and time tracking. The tail rotor assembly had undergone an overhaul approximately 233.6 hours prior to the accident. During the overhaul, spindles are inspected via fluorescent penetrant. However, the overhaul manual did not require removal of the spacer covering the groove area where fatigue cracking initiated unless the spacer was damaged or worn beyond its limits. The operator reported that the spacer was not routinely removed during overhaul unless it was damaged or out of dimensional tolerance.