NTSB Identification: LAX05FA229.
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Accident occurred Tuesday, July 12, 2005 in Kapaau, HI
Probable Cause Approval Date: 12/28/2006
Aircraft: Eurocopter AS350BA, registration: N4064F
Injuries: 1 Uninjured.
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 aircraft accident report.
The helicopter's tail rotor blade assembly and tail rotor gearbox separated while in cruise flight. The pilot transported three passengers to a remote landing site located adjacent to a stream and rock dam. He had off-loaded the passengers and had departed the landing zone when the tail rotor assembly separation took place. The pilot indicated that while cruising over a valley at 500 feet above ground level and at 90 knots, he experienced a high frequency vibration that lasted about 5 seconds. This was followed by a "bang," whereupon the helicopter violently reacted by yawing left and pitching nose down, and with a right rollover 90 degrees. The pilot recovered from the unusual attitude after slowing to about 60 knots, and performed an autorotative descent to an uneventful landing. Two of the three passengers that accompanied the pilot on the flight to the landing zone reported that the pilot was not certain of how to land the helicopter at the landing zone, as it was his second time landing in that area. One passenger instructed the pilot to land with the tail boom facing down stream, but the other told him to land with the tail boom facing up stream. The pilot followed the latter instructions and accidentally placed the tail rotor guard on a rock in the stream. The passengers stated that after they disembarked, they noticed the tail rotor blades were turning in the water. They attempted to notify the pilot, but were unsuccessful. The separated components were not located during the course of the investigation. Laboratory examination of the aft end of the tail boom revealed that the gearbox and tail rotor assembly separated from the tail boom due to a loss of clamp-up force in one or possibly both of the aft attachment bolt and nut assemblies for the tail rotor gearbox. At the right aft attachment location for the gearbox, thread damage localized to only a part of the outermost thread of the right nut plate indicated that the right aft attachment bolt unscrewed from the nut plate. The presence of fatigue cracking in the left and right nut plates also indicates insufficient clamp-up. The aft attachment bolts for the gearbox should have lock wire that secure them to the gearbox; however, since all parts containing the lock wire are missing, there is no way to determine if the lock wire was in place. According to maintenance records, a 100-hour inspection was performed 12 days (40.7 hours time in service) before the accident. As part of the inspection procedures, the aft attachment bolts for the gearbox are checked for proper torque. The mechanics that performed the inspection and checked the torque provided statements indicating that the torque was checked and the bolts were safety wired following the inspection. One possible source for excessive vibration could be an inadequately balanced drive shaft from a tail rotor blade strike.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: the in-flight separation of the tail rotor system as a result of a loss of clamp force and fatigue fracturing of the attachment nut plates. The cause of the loss of clamp force was likely due to an inadvertent tail rotor blade strike during the previous landing, which resulted in an imbalance and a high frequency vibration that both induced fatigue in the nut plates and caused one bolt to back out. The pilot's unfamiliarity with the landing area and his poor decision to land with the tail boom facing upstream were factors. Full narrative available
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