FTW98LA260

The helicopter experienced a tail rotor drive shaft failure while on final approach to landing. Approximately 45 to 50 feet AGL, the pilot heard a 'loud snapping sound,' which was followed by a 'loss of tail rotor control.' An autorotation was initiated, and subsequently, the helicopter landed 'hard' on the grassy staging field and came to a stop upright. Examination of the helicopter revealed that two of the tail rotor drive shaft segments were fractured. Additionally, one of the two bolts connecting the aft end of one of the fractured segments to a disc coupling (Thomas Coupling) was missing. The missing bolt was not recovered, although a nut and two beveled washers were found on top of the tail boom beneath the driveshaft. An examination of the remaining bolt revealed that it exhibited fretting, wear and deformation to only the last two and one-half threads. Review of the maintenance records revealed no entries indicating that any of the tail rotor drive shaft segments were removed in the two years preceding the accident.

On June 7, 1998, at 1744 central daylight time, a Bell 206B helicopter, N16814, registered to and operated by Houston Helicopters, Inc., was substantially damaged during a hard landing following the failure of the tail rotor drive shaft in Corpus Christi, Texas. The commercial pilot and the three passengers were not injured. Visual meteorological conditions prevailed and no flight plan was filed for the Title 14 CFR Part 135 sightseeing flight. The local flight originated from a hotel parking lot in Corpus Christi at 1646.

According to the pilot, the flight was returning to the hotel after observing a boat race. While on final approach to the staging area, approximately 45 to 50 feet AGL, the pilot heard a "loud snapping sound." He stated that the helicopter experienced a "loss of tail rotor control," and he initiated an autorotation. Subsequently, the helicopter landed "hard" on the grassy staging field and came to a stop upright.

An FAA inspector examined the helicopter at the site. He reported that the tail boom was bent, and the skids were spread. The number two and number three tail rotor drive shaft segments (first and second segments aft of the oil cooler) were fractured. One of the two bolts attaching the aft end of the number three tail rotor drive shaft segment to the disc coupling (Thomas coupling) was missing, and the other bolt was loose. A nut and two beveled washers were found resting on the tail boom beneath the driveshaft; the missing bolt was not found.

Review of the helicopter's maintenance records revealed no entries indicating that any of the tail rotor drive shaft segments were removed in the two years preceding the accident. According to the maintenance records, the helicopter's most recent annual inspection was completed on April 21, 1998, at 10,054.1 hours. Review of the checklist for the April 21st inspection indicated that the drive shaft (disc) couplings were inspected for condition and security. At the time of the accident, the helicopter had accumulated a total of 52.5 hours since this inspection.

The number two, number three, and number four tail rotor drive shaft segments, the hardware between the number three segment and the number four segment (including the disc coupling), and the nut and two beveled washers were examined under the supervision of the NTSB investigator-in-charge at the Bell Helicopter Engineering Laboratories, Hurst, Texas. The examination revealed that the number two segment (first drive shaft segment aft of the oil cooler) was fractured 6 inches from the forward end. The number three segment (second segment aft of the oil cooler) was fractured 14.5 inches from the forward end. According to the Bell metallurgist, both fractures exhibited signatures typical of bending and torsional overload.

The remaining bolt joining the aft section of the number three drive shaft segment to the disc coupling was removed and examined. The bolt threads exhibited fretting, wear, and deformation to only the last two and one-half threads, and the bolt was bent along the shank. The discs of the coupling assembly in the area where the bolt was attached were deformed and spread open. The inside diameter of the bolt hole in the discs was worn, fretted and deformed. No torque paint was found on the bolt, washers, or nut. The opposite hole in the discs, from which the bolt was missing, was not "extensively" damaged. The discs surrounding this hole were "relatively flat," with no evidence of permanent deformation or spread.

The nut and two beveled washers found resting on the tailboom were examined. The internal threads on the nut and the washer mating surfaces were "relatively undamaged." No torque paint was found on the washers or nut.

Thread and head imprints from a bolt were found on the third drive shaft segment approximately one inch aft of the fracture surface. The imprints were adjacent to the disc coupling from which the bolt was missing and corresponded to the size of a AN174-7A bolt, which was the size of the missing bolt.