CEN15LA412

The private pilot was conducting a personal flight in the experimental, amateur-built helicopter. The pilot reported that, while he was maneuvering the helicopter about 400 ft above ground level, it yawed left, then suddenly "snapped" right, and then began spinning. The pilot attempted to regain control of the helicopter and was able to briefly stop the spin, but the spin then continued. The pilot subsequently conducted a forced landing to a cornfield, during which the helicopter impacted terrain and rolled over. Examination of the helicopter revealed that the tail rotor driveshaft had failed during the flight, which resulted in a loss of control. The fracture surface of the driveshaft was mechanically and thermally deformed, and the failure mode could not be established. The pilot's medical and flight review were not current; however, the pilot's actions were not a contributing factor in the accident.

On September 12, 2015, about 1130 central daylight time, a Guisinger Baby Belle experimental amateur-built helicopter, N135PA, impacted terrain following a failure of the tail rotor drive system while maneuvering near Stuart, Iowa. The private pilot sustained minor injuries, the passenger was not injured, and the helicopter sustained substantial damage. The helicopter was registered to and operated by a private individual as a 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that visual meteorological conditions prevailed at the time of the accident, and no flight plan was filed. The flight originated from an off airport location near Guthrie, Iowa, about 1045. According to the 61-year old pilot, while maneuvering about 400 feet above ground level, the helicopter yawed to the left, and then suddenly snapped to the right. The helicopter began spinning, and the pilot attempted to regain control of the helicopter. The pilot was able to briefly stop the spin, but the spin then continued. The pilot conducted a forced landing to a corn field. During the forced landing, the helicopter impacted the terrain and rolled over. Examination of the helicopter revealed substantial damage to the fuselage, tail boom, and main rotor blades. A friend of the pilot who responded to the accident site stated it appeared the tail rotor driveshaft had failed. The tail rotor driveshaft flexible coupling assembly, which included a portion of the driveshaft, was retained for further examination. The NTSB's Materials Laboratory examined the tail rotor driveshaft flexible coupling assembly. Examination of the coupling showed a portion of the driveshaft exhibited a smeared fracture surface. The fracture surface of the driveshaft was mechanically and thermally deformed and the failure mode could not be established. A review of the maintenance records revealed on September 13, 2012, at a total airframe time of 209.4 hours, the pilot/owner installed a rebuilt tail rotor assembly, which included new titanium rotor blades. The most recent conditional inspection was completed on January 15, 2015, at a total airframe time of 238.5 hours. At the time of the accident, the airframe had accumulated 242.5 hours. During a telephone conversation with the NTSB investigator-in-charge, the pilot stated that he experienced an issue with the main rotor system in 2004 that caused a severe vibration which damaged the tail boom. The damage was repaired; however, the issue remained present. With the assistance of the kit manufacturer, the main rotor issue was corrected. A maintenance logbook entry, dated May 26, 2004, showed a total airframe time of 64.9 hours. The pilot recalled an event back in 2009 that during an inspection of the helicopter, he noticed the nylon drive star exhibited excessive wear and replaced it with a new nylon drive star. After installation, a small gap was observed between the steel star coupling and nylon drive star. The pilot contacted the kitmanufacturer and the manufacturer provided guidance on shimming the system to remove the gap. A 2009 aircraft logbook entry showed a total airframe time of 175.5 hours. After considering the details of the accident flight and the failed tail rotor driveshaft, the pilot speculated that the failed driveshaft may have been the result of a small crack in the driveshaft during the severe vibration issue in 2004. The 61-year old pilot's most recent 3rd class medical certificate was issued on March 13, 2013. A review of the pilot's logbook showed his last flight review was completed on February 27, 2002.