Dickinson, AL, USA
BELL 206L 3
During an aerial application flight, the helicopter was flying between 70 and 120 ft above trees, and the pilot heard a "pop" sound followed by an immediate loss of tail rotor thrust. He lowered collective and descended into trees, and the helicopter came to rest on its left side. Examination of the helicopter determined that the loss of tail rotor thrust was precipitated by a disconnect in the tail rotor drive system. A self-locking nut securing one of the two bolts that attached the aft end of the first tail rotor driveshaft (shaft S1) aft of the oil cooler blower to the disc pack coupling was missing. The nut and the bolt that it secured were not recovered at the accident site. The separated hardware allowed misalignment of the driveshaft with the axis of rotation, which led to the fracture of shaft S1 and of the second tail rotor driveshaft (shaft S2) aft of the oil cooler blower. The prevailing or tare torque on two out of the remaining three self-locking nuts securing the forward and aft ends of shaft S1 to the disc pack couplings were below the minimum torque specified in the helicopter manufacturer's standard practices manual. The prevailing torque is a measure of the turning resistance of a lock nut, and the turning resistance generally decreases as nuts are reused. About 490 hours earlier, the operator had performed a modification to the helicopter that required in part, removal of shaft S1 and the tailboom assembly. Since this modification, the helicopter had been subjected to four 100-hour inspections. Three lots of defective nuts of the type used to secure components of the tail rotor drive system had been identified; however, the operator could not determine whether they had previously had any of the suspect nuts in stock. Although the possibility of a defective nut resulting in the driveshaft disconnect could not be eliminated, given the insufficient prevailing torque on 2 of the remaining 3 self-locking nuts securing shaft S1 to the disc pack couplings, it is more likely that the separated nut also had insufficient prevailing torque. And it was that insufficient prevailing torque of the missing self-locking nut that had been removed and improperly re-installed by the operator's maintenance personnel, which allowed the self-locking nut to back-off. This accident and six previous accidents involving the same make and model of helicopter illustrate an in-service issue with hardware used to secure the tail rotor driveshafts. As a result of these accidents, Transport Canada issued a safety alert notifying maintenance personnel of the need to check the prevailing or tare torque of hardware securing the tail rotor drive system.
On October 15, 2015, about 1500 central daylight time, a Bell 206L-3, N206CJ, experienced a loss of tail rotor thrust and contacted trees during an emergency descent near Dickinson, Alabama. The commercial pilot was not injured, and the helicopter was substantially damaged. The helicopter was registered to CB Couch, Inc., and operated by Couch Helicopter Service, Inc., as an aerial application flight under the provisions of 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed in the area at the time, and no flight plan was filed for the local flight that originated about 5 minutes earlier from a nearby field.The operator reported that, toward the end of the flight, during a left turn while flying at an airspeed less than 15 knots and between 70 and 120 ft above trees, the pilot heard a "pop" sound followed by a loss of tail rotor thrust. The pilot added full left anti-torque pedal input but that was not effective. He then lowered the collective but did not reduce throttle. As the helicopter descended, the main rotor blades, mast, and a portion of the tailboom contacted trees and fractured. The helicopter descended to the ground and came to rest on its left side. Examination of the helicopter by a Federal Aviation Administration operations inspector revealed that the tailboom with attached tail rotor had separated but was found near the main wreckage. The tail rotor blades were intact and showed little damage except for scratches from contact with trees. Examination of the tail rotor drive system revealed that the first driveshaft assembly (shaft S1) aft of the oil cooler blower, and the second driveshaft assembly (shaft S2) aft of the oil cooler blower, were both fractured about midspan. One of the fasteners used to attach shaft S1 to the disc pack coupling at the shaft's aft end was missing, and the securing hardware was not located. The third tail rotor driveshaft (shaft S3) aft of the blower was not fractured. The forward flange of the fourth tail rotor driveshaft (shaft S4) aft of the blower was fractured on one side, and the fractured section remained attached to the disc pack coupling. The opposite side bolt remained trapped in the disk pack coupling, and the bolt was bent and fractured. Components of the tail rotor drive system consisting of fractured shafts S1 and S2, non-fractured shaft S3, and the fractured section of the forward end of shaft S4 with disc pack couplings and securing hardware were sent to the NTSB Materials Laboratory located in Washington, DC. According to the NTSB Materials Laboratory report, the fracture surface of shaft S1 showed irregular fracture features with postfracture damage and inward deformation consistent with an overstress fracture, and the fracture features on shaft S2 were on slant planes and matte gray, which is consistent with overstress. There was no evidence of preexisting damage to either fracture surface. Examination of the fractured section of shaft S4 revealed fracture features consistent with an overstress fracture under shear loading. Examination of the flange of the aft portion of shaft S1 associated with the missing hardware revealed wear in the hole bore corresponding to contact with the threads of the missing fastener. The outside face of the flange of the aft end of shaft S1 diametrically opposite from the flange with the missing fastener displayed circumferential sliding contact marks, and no torque paint was observed on the flange, washer, or bolt threads. At the missing attachment location, the forward plate in the disc pack coupling was cracked across the width of the plate between the inner and outer diameter, and the crack intersected the middle of the attachment hole. Contact marks were observed at the aft end of shaft S1 at the sides approximately orthogonal to the flanges. The shape and location of the contact marks were consistent with contact with the heads of the bolts attaching shaft S2 to the disc pack coupling and shaft S1 as it rotated about the remaining attachment bolt. The prevailing or tare torque for one nut at the forward end and of the remaining nut at the aft end of shaft S1 were below the minimum torque specified in Bell Helicopter's Standard Practices Manual (SPM), and the prevailing or tare torque for the remaining fasteners were above the minimum torque specified in the SPM. (The prevailing torque is a measure of the turning resistance of a lock nut, and the turning resistance generally decreases as nuts are reused.) Returning torque measurements were conducted on each of the coupling attachments, which revealed that only the two fasteners on the forward end of shaft S1 were within limits. According to the helicopter's maintenance records, on March 15, 2015, at helicopter total time 6,059.9 hours, the helicopter was modified in accordance with Bell Helicopter Technical Bulletin 206L-07-226 dated October 8, 2007. The modification included installation of an improved upper left longeron/fitting assembly, P/N 206-031-314-217B, and aft fuselage bulkhead, P/N 407-030-027-103. According to the technical bulletin accomplishment instructions, preparation included, in part, removal of: the tail rotor driveshaft between the engine and oil cooler, the oil cooler and oil cooler blower assemblies, the engine, the tail rotor drive shaft segment aft of the oil cooler blower, and the tailboom assembly. The maintenance record entry returning the helicopter to service after the modification did not specify whether new hardware was used during reinstallation of the tail rotor drive shaft segments or if the old hardware was re-installed. Since the modification was performed, the helicopter had undergone four 100-hour inspections, the latest of which was completed on September 10, 2015. The helicopter total time at its last 100-hour inspection was 6,456.5 hours, and the total time at the time of the accident was reported to be 6,550 hours. In May 2002, Bell Helicopter issued Revision A to Operations Safety Notice (OSN) 206L-02-43, which required a 100-hour recurring torque check for the tail rotor disc pack coupling hardware as well as the application of torque stripes after torqueing. The OSN also indicated that the 206L maintenance manual (MM) would be revised to incorporate the new torque check and torque stripe requirement and reminded mechanics of the need to include tare torque of the nut in the torque value. Postaccident review of the 206L maintenance manual revealed no reference to adding torque stripes to the disc coupling fasteners following torque check. As a result, Bell Helicopter changed the MM to incorporate application of torque stripes after torque check of the disc pack coupling fasteners. On May 25, 2015, Transport Canada issued Revision 1 to Civil Aviation Safety Alert (CASA) 2013-04, which identified certain defective MS21042-4 nuts, which are the type of nuts used to secure components of the tail rotor drive shaft system. The alert indicated that failures of nuts due to hydrogen embrittlement had occurred, and the defective nuts were from 3 lots produced by a single manufacturer in 2009 and 2010. According to the operator, they could not determine whether they had ever had in their inventory any nuts from the suspected lots of nuts specified in Revision 1 of CASA 2013-04. On March 29, 2017, Transport Canada issued CASA 2017-02, titled "Loss of Hardware – Tail Rotor Drive Shaft Couplings," which called attention to six previous NTSB investigations of Bell 206 helicopters involving in-flight loss of tail rotor authority. The recommended action section of the alert specified using a calibrated torque wrench when installing hardware pertaining to the tail rotor drive shaft system, avoiding re-use of the hardware, verifying that tare or run-on torque of nuts meets the minimum specification, requiring torque checks per the maintenance manual, and finally, applying torque stripe material to the hardware after torque check. Review of the six NTSB investigations cited in CASA 2017-02 revealed that all identified separation of a fastener securing one of the tail rotor driveshafts to a disc pack coupling. Of the six fastener separations, only one separated nut was found. The report prepared by Bell Helicopter concerning the separated nut indicated that the prevailing or tare torque was greater than the minimum specified in the SPM. The NTSB metallurgy reports for two of the investigations indicated that the remaining fasteners of the provided parts were not checked with an instrument of sufficient accuracy; therefore, it could not be determined whether they met the minimum prevailing or tare torque value specified in the SPM. The accident helicopter was manufactured in 1992, but certificated in accordance with Civil Air Regulations (CAR) 6, dated December 20, 1956, Amendments 6-1 thru 6-4, CAR 6.307(b) and 6.637 of Amendment 6-5, special conditions dated October 2, 1962, as revised February 8, 1966. Review of CAR 6.303, titled "Standard Fastenings," revealed that it stated, "Self-locking nuts shall not be used on bolts which are subject to rotation in operation." According to Bell Helicopter and Transport Canada personnel, the bolts used to secure the tail rotor driveshaft disc pack couplings are in rotation, but do not serve as the axis of rotation itself. Therefore, CAR 6.303 was not applicable.
The improper installation by company maintenance personnel of the securing hardware of a tail rotor driveshaft section, which resulted in that section of tail rotor driveshaft disconnecting and a complete loss of tail rotor thrust.
Source: NTSB Aviation Accident Database
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