NTSB Identification: LAX06GA254.
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Accident occurred Friday, August 04, 2006 in Happy Camp, CA
Probable Cause Approval Date: 07/25/2007
Aircraft: Aviation International Rotors CH-54A, registration: N6156U
Injuries: 2 Fatal.

: NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this public aircraft accident report.

The helicopter experienced an in-flight separation of a tail rotor blade and subsequent separation of the tail rotor gearbox and remaining tail rotor blades while conducting fire suppression operations for the United States Forest Service (USFS). A witness located near the accident site reported that he observed the helicopter over the dip site when the witness heard a "loud bang." He observed the helicopter flying over a stone riverbed toward a bridge when he noticed a large piece fall off the helicopter, later identified as the tail rotor gearbox with three of the four tail rotor blades attached to the hub. The helicopter pitched nose low at an approximate 45-degree nose down attitude while rotating around its vertical axis before impacting the side of the river/shoreline. The tail rotor blade associated with the red spindle separated from the tail rotor. The red tail rotor spindle failed due to a fatigue fracture in the spindle. Among three machined grooves, located on the spindle under a spacer, the fatigue initiated in the groove located closest to the blade (the outboard groove). Fatigue initiated at a single origin associated with a shot-peen pit. Circumferential scratches were observed in the groove areas and on the adjacent journal surface; however, the fatigue crack origin was not associated with these scratches. The journal diameter was also undersize by 0.007 inch, and fretting was observed on the journal surface. The origin was not associated with any particular scratch or other apparent material defect. The groove peak between the middle and inboard grooves was sheared at the trailing side of the red spindle and displaced toward inboard, damage consistent with a tail rotor strike. The total 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 underwent an overhaul about 233.6 hours prior to the accident. During the overhaul, the spindles are to be inspected via fluorescent penetrant; however, the overhaul manual did not require the removal of the spacer that covered the groove area where the fatigue cracking initiated unless the spacer was damaged or worn beyond its approved limits. According to the operator, the spacer was not routinely removed at overhaul unless it was damaged or out of dimensional tolerance.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The fatigue failure of tail rotor blade spindle, which resulted in the separation of the blade and tail rotor gearbox.

Full narrative available

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