Kapaau, HI, USA
N4064F
Eurocopter AS350BA
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.
HISTORY OF FLIGHT On July 12, 2005, about 0645 Hawaiian standard time, a Eurocopter AS350BA helicopter, N4064F, experienced the separation of its tail rotor blade assembly and tail rotor gearbox during cruise flight. The accident occurred about 6 miles southeast of Kapaau, on the Big Island of Hawaii. Sunshine Helicopters, Inc., Kahului, Hawaii, operated the helicopter during the positioning flight under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed, and a company flight plan was filed. The helicopter was substantially damaged, and the commercial pilot was not injured. No passengers were on board. The flight originated from a remote off-airport site in a valley near the north shore of the Big island about 0640. The pilot transported three passengers to private property located in a remote valley on the north side of the island. The landing zone was an unimproved landing area adjacent to a stream and rock dam. The pilot offloaded the passengers without shutting down the helicopter, and had departed the landing zone when the accident occurred. Pilot Statement The pilot reported to a National Transportation Safety Board investigator that he did not observe any damage to the helicopter during his preflight inspection. The pilot stated that he had visually examined the tail rotor assembly before the flight, in accordance with Sunshine company policy and pursuant to the applicable Federal Aviation Administration (FAA) airworthiness directives. In summary, the pilot indicated that shortly after departing the landing zone, and 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 stated that after recovering from the unusual attitude, he realized that he had lost tail rotor control. After slowing to about 60 knots, he reestablished adequate control to fly to the Kona International Airport (KOA) where he performed an autorotative descent to an uneventful landing. Passenger Statements According to a statement provided by one of the passengers, he was seated in the right rear seat during the previous flight. As they approached the landing area, the pilot asked the passengers which way he should land. The passenger in the left rear seat instructed the pilot to face up river. The front seat passenger informed the pilot that "the other helicopter" landed facing downstream (according to the statement, the "other helicopter" was smaller then the one they were utilizing the day of the accident). The pilot landed the helicopter with the nose facing downstream and the tail facing upstream. Before the passengers disembarked they noted that the tail rotor guard was resting on a rock and the rotor blades were in the water of the stream (the pilot did not shut down the helicopter as the passengers disembarked). They informed the pilot that the guard was on a rock, and he responded that he would "hold it steady" while the passengers deplaned. After the passengers exited the helicopter the pilot took off, and a small object hit the passenger's hand, which he thought was a pebble from the stream. The passenger in the left rear seat also provided a statement. According to his account, as they approached the landing site, the pilot asked how the other pilots land. The passenger stated that they landed with the tail facing downriver. The front seat passenger responded by telling the pilot that they landed with the tail facing upriver. The left rear passenger repeated his statement indicating that they land with the tail facing downriver. The pilot listened to the front seat passenger and landed with the tail facing upriver. The left rear seat passenger asked the pilot twice, "is the back alright?" as the helicopter would not land flat; however, the pilot did not hear him. The passengers disembarked and the left rear seat passenger noted that the tail rotor guard was resting on a stone in the river and the tail rotor blades were in the water. As the pilot started to takeoff, the passengers attempted to wave him down, but the pilot didn't see them. The left rear seat passenger had a hand-held radio and tried to contact the pilot to let him know what they observed, but they were unable to contact him. The passenger added that they could hear a vibration noise as the helicopter lifted off, and little stones flew by and hit one of the other passengers. PERSONNEL INFORMATION The pilot held a commercial certificate with a helicopter and instrument helicopter rating. He was issued a second-class medical certificate on April 26, 2005, with no limitations. According to the submitted Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2), he accumulated approximately 8,000 hours of total flight time, of which about 5,000 hours were acquired in the accident helicopter make and model. According to the director of operations, he took the pilot into the landing site a few days before the accident flight as the accident pilot had not been there before, nor had he flown this mission before. AIRCRAFT INFORMATION The helicopter (serial number 3087) accumulated 7,373.8 hours of operation at the time of the accident. It underwent a 100-hour inspection on June 30, 2005, at a helicopter total time of 7,333.1 hours. The mechanics were instructed to "check the tail rotor gearbox attachment bolts for proper torque" during the 100-hour inspection. The maintenance manual delineates the torque requirements for the gearbox attachment bolts. This inspection was marked as completed in the 100-hour checklist. Two written statements were provided to Safety Board investigators. The statements indicated that the mechanics checked the torque on the tail rotor gearbox attachment bolts during the 100-hour inspection and then safety wired the bolts following the torque check. During that 100-hour inspection, two tail rotor bearings (part number 350A33-2153-00) were replaced. The helicopter was also under the inspection requirements established by two tail rotor related airworthiness directives (AD). One AD required the inspection of the tail rotor blade trailing edge tab for cracks. This AD was signed off as complied with on the morning of the accident. The other AD required that the tail rotor pitch change control rod be checked for play. This AD was also signed off as complied with on the morning of the accident. WRECKAGE AND IMPACT INFORMATION The helicopter was examined at the operator's maintenance facility in KOA. The entire tail rotor system was missing including the tail rotor gearbox. The bottom portion of the vertical stabilizer was cut in two during the event, but the lower portion remained attached to the tail rotor guard. The aft end of the tail boom (where the tail rotor gearbox would mount) was removed from the helicopter and was shipped to the Safety Board Materials Laboratory, Washington, DC, for further examination. TESTS AND RESEARCH The tail rotor gearbox normally attaches to the tail boom with a u-shaped bracket in the front of the gearbox and two mounting pads in the rear. The forward attach bracket was torn and the right portion was missing. The tail boom aft of and below the forward right attach point was deformed inward and was severely gouged. The right and left nut plates for the tail rotor gearbox aft attachment bolts were fractured from the inside of the tail boom. The aft left attachment bolt for the tail rotor gearbox was fractured, and a piece of the bolt shank was retained in the left nut plate. The entire aft right attachment bolt was missing. The areas of the tail boom around the tail rotor gearbox aft attachment bolt holes were damaged. Paint was missing from the areas, and the underlying metal contained numerous overlapping impact marks. The metal deformation and missing paint around the right aft attachment bolt hole was more severe than around the left hole. Inside the holes, spiral impressions were observed corresponding to contact with the attachment bolt threads. The left bracket of two brackets located aft of the tail rotor gearbox aft attachment holes was bent to the left. Each nut plate was fractured where its outboard attachment tab intersected the barrel of the nut plate. The fractured tab piece from the right nut plate remained attached to the tail boom rib flange. The fractured tab piece from the left nut plate was missing. The right nut plate was cleaned with soapy water, removing a black greasy substance from the threads. Most of the fracture surface had relatively flat fracture features with curving arrest lines, features consistent with fatigue. The fatigue features emanated from origin areas at the surface in the corner between the inner (non-contact) surface of the tab and the surface of the barrel. Thread damage was observed on the right nut plate. The outermost thread was deformed outward (relative to the tail boom centerline) around approximately 90 degrees of the circumference. The location and plane of the fracture in the left nut plate was similar to that of the right nut plate. Most of the fracture surface was smooth and shiny, consistent with post-fracture damage. However, an area of the surface near the edge of the fracture was relatively undamaged, and in this area, flat fracture features with crack arrest lines were observed, consistent with fatigue. Relatively rough fracture features with curving arrest lines were observed on the fracture surface on the right aft attachment bolt, consistent with fatigue under relatively high stresses. The fatigue features were present around the circumference emanating inward from the surface of a thread root. ADDITIONAL INFORMATION At the time of this report's writing, the tail rotor components have not been recovered.
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.
Source: NTSB Aviation Accident Database
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