What happened
On June 10, 1998, a Bell 205 helicopter was conducting a cargo transport mission from Ahlen to Eisenach. After delivering materials to an automotive plant, the crew departed Eisenach-Kindel for the return flight to Ahlen. Approximately 30 minutes into the flight, while cruising at 700 ft AGL and 100 knots, the crew heard a loud bang originating from the rear of the aircraft.
Immediately following the noise, the helicopter experienced severe flight instabilities, including a simultaneous yaw to the right, a roll to the left, and a heavy nose-down attitude. The pilot reported rhythmic striking noises and felt the tail rotor pedals become blocked. In an attempt to stabilize the aircraft, the pilot initiated an autorotation. Despite a second attempt to restart the engine to regain control, the aircraft continued to descend rapidly. As the helicopter entered a large right-hand turn, the pilot encountered a group of bushes and performed an emergency landing into the vegetation. The aircraft sustained heavy damage, but the crew escaped the wreckage without injuries.
The investigation
The BFU investigation focused on the tail rotor drive system and controls, prompted by the crew's accounts of unusual noises and flight dynamics. Initial inspections at the crash site revealed that the tail rotor drive shaft had undergone torsional deformation and had slipped out of its splines, indicating a sudden mechanical seizure within the drivetrain.
Investigators, supported by the manufacturer, disassembled and examined several components, including the tail rotor gearbox, the 42° gearbox, the drive shaft, bearings, and the tail rotor blades. Metallurgical analysis was performed on the drive shaft and bearings in Braunschweig. While the investigation confirmed that a sudden blockage had caused the shaft to deform and slip, the specific trigger for this seizure could not be identified. Furthermore, testing of the tail rotor control hydraulics showed that while some deviations from standard values were present, the system was functioning correctly.
Findings
- The primary cause of the accident was a sudden mechanical seizure within the tail rotor drivetrain.
- This seizure caused the tail rotor drive shaft to undergo torsional deformation, resulting in it slipping out of the splines.
- The investigation could not determine the exact origin of the blockage.
- Evidence of bird strike was ruled out as highly unlikely.
- The pilot's professional handling of the emergency prevented fatalities despite the loss of tail rotor control.