What happened
On July 18, 2000, a Bell 206B helicopter was conducting a training flight for type rating purposes near Augsburg. While cruising at 3,000 ft MSL at 110 mph, the crew experienced sudden vibrations accompanied by a humming noise. The flight instructor immediately took control of the aircraft. After attempting to reduce power without success, the crew suspected a tail rotor drive issue and decided to perform an emergency landing in a field.
During the autorotation, the instructor noted that the tail rotor was non-functional. As the aircraft touched down in a turnip field at approximately 18:50 local time, the landing gear caught in the soft ground. This sudden deceleration caused the main rotor disk to tilt excessively, resulting in a main rotor blade striking the tail boom. The impact caused the tail rotor, rear stabilization surfaces, and parts of the landing gear to separate from the aircraft.
The investigation
The BFU investigation examined the wreckage at the site and the aircraft after it had been moved to a hangar. Investigators analyzed the distribution of components at the crash site, the fracture surfaces of the tail rotor control rods, and the damage to the main rotor and tail boom.
Technical examination of the main drive shaft revealed that the shaft had broken at the coupling facing the engine. The investigation found that the elastomer seal was missing and the coupling lacked lubrication, leading to dry running. Evidence of overheating was present, indicated by blue discoloration on the fracture surface. Furthermore, the investigation looked into a special inspection performed 18 flight hours prior to the accident, which had addressed similar vibrations but failed to identify the internal degradation of the coupling.