OAK GROVE, LA, USA
N7929J
Bell 206B
During an external load operation, the pilot was hovering the helicopter at about 60 feet above the boat, when the helicopter began to 'spin' to the right. He 'rolled off the throttle' and the spin stopped. He then maneuvered the helicopter to the right as it descended in an effort to clear the boat; however, the main rotor blades struck the front of the boat. Subsequently, the helicopter settled into the water and sank. Examination of the tail rotor drive system revealed that the #5 tail rotor drive shaft segment P/N 206-040-931-9 had separated at the bonded joint between the shaft tube and the flange fitting. Examination of this tail rotor drive segment by a metallurgist revealed that the separated joint revealed evidence of pitting which was indicative of plastic blast media impingement and the remaining adhesive had the consistency of powder/talc with no apparent structural integrity. According to the metallurgist the 'blast media impingement may effectively compromise the moisture barrier at the joint and allow the moist chloride environment to invade the joint and chemically degrade the adhesive.' Blast media impingement is not approved by Bell Helicopter.
On August 23, 1997, at 0900 central daylight time, a Bell 206B helicopter, N7929J, registered to Evergreen Equity, Inc., and operated by Evergreen Helicopters International, Inc., as a Title 14 CFR Part 133 flight, was substantially damaged during water impact, following a loss of directional control while at an out-of-ground effect hover over the flat boat Echo, located in the Gulf of Mexico, about 3 nautical miles south of Oak Grove, Louisiana. Visual meteorological conditions prevailed, and a company flight plan was filed for the local external load operation. The airline transport rated pilot, sole occupant of the helicopter, was not injured. The flight originated from a field site near Cameron, Louisiana, about 10 minutes prior to the accident. During a telephone interview with the investigator-in-charge, the pilot reported that during an external load operation, he was hovering the helicopter at about 60 feet above the boat when he heard a change in engine noise. He checked the engine torque, and noticed it was increasing rapidly. The helicopter began to "spin" to the right, and the application of left anti torque pedal had no effect in stopping the spin. He reduced collective, but this also had no effect in stopping the spin. He then "rolled off the throttle" and the spin stopped. He maneuvered the helicopter to the right as it descended in an effort to clear the boat; however, the main rotor blades struck the bow of the boat. The helicopter impacted the water right skid first. As the helicopter settled into the water, it rolled over onto its right side and sank. After the pilot egressed from the helicopter, witnesses dove into the water and assisted him into the boat. The operator reported that after the helicopter was recovered from the Gulf of Mexico, it was washed down immediately with fresh water. An examination of the helicopter revealed that the main rotor head was separated from the mast, the main transmission was leaning forward, the box beam was bent, and the tail boom was twisted just forward of the tail rotor gearbox. Examination of the tail rotor drive system by an FAA inspector revealed that the #5 tail rotor drive shaft segment P/N 206-040-931-9 (S/N 720) had separated at the bonded joint between the shaft tube and the flange fitting. This tail rotor drive segment was installed on N7929J at the time of an April, 1994, incident when the tail rotor drive shaft segment S/N 863 failed at a similar flange fitting bonded joint. Tail rotor drive segment S/N 720 was removed from the helicopter and inspected per the requirements of Bell helicopter, Inc., (BHI) 206B-3 Component Repair and Overhaul Manual, deemed serviceable, and returned to storage as a spare part. Tail rotor segment S/N 720 was reinstalled on N7929J in September of 1995. On November 18, 1997, an examination of the tail rotor drive shaft segment S/N VNMH 00720 (S/N 720) was conducted at Materials Analysis, Inc., Dallas, Texas. In addition to the involved shaft, five other shaft segments S/N VNMH 00863 (S/N 863), S/N VNMH 00695 (S/N 695), S/N VNMH 06307 (S/N 6307), S/N VNMH 00706 (S/N 706), and S/N VNMH 00709 (S/N 709) were provided by Evergreen Helicopters for comparison examination. Disassembly of the tail rotor drive shaft segment S/N 720 flange fitting from the tubular shaft revealed very little residual adhesive still visible on the bonded end of the shaft and inside the flange fitting. The remaining adhesive within the separated joint members had the consistency of powder/talc with no apparent structural integrity. There was no indication or visual evidence of a forced separation through the remaining adhesive. Evidence of dark corrosion on the inside diameter surface of the aluminum flange fitting confined along the edges of the lap joint was noted. There was no evidence of general aqueous corrosion on the exposed bonded surfaces on the shaft tube or inside the flange fitting, which according to the metallurgist, suggests that the salt water may not have penetrated into the lap joint during the helicopter's immersion into the Gulf of Mexico. A stereomicroscopic visual examination of the surface in and around the separated joint revealed evidence of pitting that, according to Bell Helicopter and Evergreen Helicopter representatives, was indicative of plastic blast media impingement on the outside surfaces of the flange fitting and tube. An examination of the tail rotor drive shaft segment S/N 863 which failed in April 1994 revealed pitted surface of the tube adjacent to the edge of the overlap of the joint which suggests that it had also been subjected to plastic bead blast media impingement prior to joint separation similar to S/N 720. Plastic blast media impingement is not approved by Bell Helicopter. Samples of the remaining degraded adhesive in the failed joint from S/N 720 was submitted for Fourier Transform Infrared (FTIR) spectroscopy along with samples of the adhesive from the intact spline fitting joint in S/N 720, intact spline fitting joint in S/N 706, and intact spline fitting joint in S/N 709. FTIR was conducted in order to identify the chemical makeup of the adhesive samples and to determine if any chemical differences existed between the degraded and intact adhesive samples. The deteriorated adhesive from the failed flange joint was missing a critical amine component responsible for the structural integrity of the adhesive. Exposure of an epoxy adhesive to an acid such as hydrochloric acid can create an environment which will allow the adhesive to react to moisture. Samples of the intact adhesive from the tail rotor drive shaft segment S/N 709 flange bonded joint were exposed to hydrochloric acid and after 45 minutes of exposure, the amine component was effectively eliminated from the adhesive. According to the metallurgist, "if chloride bearing salt water did not penetrate the joint after structural failure of the joint, then chemical deterioration of the adhesive was probably due to marine environment exposure during service. Blast media impingement may effectively compromise the moisture barrier at the joint and allow the moist chloride environment to invade the joint and chemically degrade the adhesive." See the enclosed Materials Analysis, Inc. report for further details.
The disconnect of the #5 tail rotor drive segment due to the debonding of the joint between the shaft tube and the flange fitting. Factors were the improper bead blasting of the tail rotor drive segment by an unknown maintenance personnel, and the lack of altitude and suitable terrain for the forced landing.
Source: NTSB Aviation Accident Database
Aviation Accidents App
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