Accident Overview
On October 15, 2015, about 1500 central daylight time, a Bell 206L-3, registration 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 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight that originated about 5 minutes earlier from a nearby field.
Flight and Pilot Actions
According to the operator, 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
Examination 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 showed 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 fastener 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 were sent to the NTSB Materials Laboratory.
Laboratory Findings
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, 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 opposite 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, 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.
Torque measurements revealed that the prevailing or tare torque for one nut at the forward end and for the remaining nut at the aft end of shaft S1 were below the minimum torque specified in Bell Helicopter's Standard Practices Manual (SPM). The prevailing or tare torque for the remaining fasteners were above the minimum. Returning torque measurements on each coupling attachment showed that only the two fasteners on the forward end of shaft S1 were within limits.
Maintenance History
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 and aft fuselage bulkhead. The technical bulletin accomplishment instructions included 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 old hardware was reinstalled. Since the modification, the helicopter had undergone four 100-hour inspections, the latest completed on September 10, 2015, at total time 6,456.5 hours. The total time at the time of the accident was reported to be 6,550 hours.
Related Safety Notices
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 and the application of torque stripes after torqueing. The OSN also indicated that the 206L maintenance manual would be revised to incorporate the new torque check and torque stripe requirement. 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 used in 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 three lots produced by a single manufacturer in 2009 and 2010. According to the operator, they could not determine whether they had any nuts from the suspected lots.
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 specified using a calibrated torque wrench, avoiding re-use of hardware, verifying tare torque meets minimum specification, requiring torque checks per the maintenance manual, and applying torque stripe material 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, only one separated nut was found; the Bell Helicopter report indicated its prevailing torque was greater than the minimum specified in the SPM. NTSB metallurgy reports for two investigations indicated that the remaining fasteners were not checked with an instrument of sufficient accuracy, so it could not be determined whether they met the minimum prevailing torque value.
Certification Context
The accident helicopter was manufactured in 1992 but certificated in accordance with Civil Air Regulations (CAR) 6, dated December 20, 1956, with amendments. Review of CAR 6.303, titled "Standard Fastenings," 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.
Probable Cause
Not stated in the source.