N3893S
Bell 412
One of the tail rotor blades separated from the helicopter during cruise flight as a result of a fatigue crack. The pilots reported hearing a 'loud bang,' and the helicopter 'immediately and violently tucked down and left then rolled inverted and [was] spinning to the right.' The flying pilot righted the helicopter, and the non-flying pilot inflated the emergency floats prior to impacting the ocean. The outboard section of one of the tail rotor blades (which one could not be determined) was found and examined. A fatigue crack initiated in the spar in an 'area of round-bottomed corrosion pits 0.001 inch deep.' The two tail rotor blades installed on the helicopter had accumulated 4,644 and 4,913 hours, respectively. The tail rotor blades are life limited at 5,000 hours.
On June 9, 1999, approximately 1535 central daylight time, a Bell 412 helicopter, N3893S, was substantially damaged when a tail rotor blade separated while in cruise flight near Eugene Island 313, an offshore platform located in the Gulf of Mexico. The flying commercial pilot and the non-flying airline transport pilot, sole occupants, were not injured. The helicopter was owned and operated by Petroleum Helicopters, Inc., of Lafayette, Louisiana. Visual meteorological conditions prevailed and a company flight plan was filed for the 14 Code of Federal Regulations Part 91 repositioning flight. The flight originated from Garden Banks 172, an offshore platform, at 1524, and was destined for Morgan City, Louisiana. According to written statements provided by the pilots, the helicopter was in cruise flight at 5,500 feet msl with a power setting of 70-75 percent torque, when they heard a "loud bang." The flying pilot reported that the helicopter "immediately and violently tucked down and left then rolled over inverted and [was] spinning to the right." The helicopter entered a dive with the nose pitched "straight down and still turning." The flying pilot stated that he "rolled the throttles to idle and bottomed the pitch," along with placing the cyclic to the right and applying full left pedal. Approximately 1,000 feet above the ocean, the helicopter "leveled out." Just prior to setting down in the water, the non-flying pilot inflated the emergency floats. The helicopter came to rest upright in the water, and the two pilots exited the helicopter through the right side cargo window and entered a life raft. From the life raft, both pilots observed that the tail rotor, 90 degree gear box, and vertical fin cowling were missing, along with a 12-16 inch section of tail rotor drive shaft. Subsequently, the helicopter rolled over to the left inverted, due to the ocean waves, and sank. The flying pilot reported that, at the time of the accident, there were scattered clouds at 3,500 feet, the visibility was at least 20 miles, and there were a few isolated thunderstorms in the distance. On June 15, 1999, the helicopter was recovered from the ocean and transported to the operator's facility in Lafayette, Louisiana. Inspection of the helicopter by the FAA inspector, operator, and a manufacturer's representative revealed that the 90 degree gear case and most of its support fitting were missing from the top of the vertical fin. Also missing were the complete tail rotor hub and blade assembly, and the top portion of the #6 tail rotor driveshaft. There was no evidence that the departing tail rotor hub and blade assembly contacted the airframe. The top section of vertical fin and the remaining section of the #6 tail rotor driveshaft were removed and sent to the Bell Helicopter Textron, Inc., facility for further metallurgical examination in the presence of the NTSB investigator-in-charge (IIC). Examination of the parts on June 23, 1999, revealed that the driveshaft segment had "fractured at a band of heavy scoring and deformation. Fractures in the gearbox support and the driveshaft segment were a result of overstress." On June 29, 1999, the outboard section of a tail rotor blade was found on a beach near Cameron, Louisiana. The section of tail rotor blade was examined on July 1, 1999, at the manufacturer's facility under the supervision of the NTSB IIC. The recovered section did not have a data plate; however, it was similar to the tail rotor blades (Part Number 212-010-750-105) installed on the Model 412 helicopter. The piece measured 17.5 inches long, and displayed a flat straight fracture. The fracture was a "result of fatigue cracking that originated in the spar of the tail rotor blade." The fatigue origin was on the "topside of the blade spar," which was on the outboard surface (side away from the tailboom) as installed on the helicopter. Examination of the fracture with a scanning electron microscope revealed that the fatigue originated in an "area of round-bottomed corrosion pits 0.001 inch deep." Hardness tests indicated that the spar material met engineering specifications. See the enclosed manufacturer's engineering laboratories report for more information. According to the operator's maintenance records, two tail rotor blades with the serial numbers A7586 and A9466 were installed on December 7, 1998, on the accident helicopter. The tail rotor blades had accumulated 4,643.55 and 4,912.75 hours, respectively. The tail rotor blades are life limited at 5,000 hours. The identification, as to which of the two tail rotor blades was found, could not be determined. The accident helicopter was maintained in accordance with a continuous maintenance inspection program. The most recent inspection (a Zone 4 inspection) was completed on May 18, 1999, with no discrepancies noted.
The separation of a section of the tail rotor blade, and subsequent separation of the tail rotor system, as a result of fatigue cracking due to corrosion in the tail rotor blade spar.
Source: NTSB Aviation Accident Database
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