What happened
On September 2, 2003, a Hughes 369D helicopter, registration SE-HSI, was performing a reindeer herding mission near Store Gåsvatn in Norway. While the pilot was maintaining a low-altitude hover into the wind, a yellow caution light for tail rotor gearbox chips illuminated, accompanied by an unusual noise coming from the rear of the aircraft.
Within approximately two seconds, the pilot lost all control of the tail rotor. The aircraft began to rotate around its main rotor axis. Despite the pilot applying maximum pedal deflection, the rotation could not be corrected. To prevent a more violent uncontrolled state, the pilot reduced the throttle and performed a forced landing in the terrain. The impact was severe; the main rotor blades struck the tail boom, severing it, and subsequently struck the sloping ground, causing extensive damage to the blades and the airframe.
Both the pilot and the passenger evacuated the aircraft without injury.
The investigation
Investigators examined the tail rotor gearbox to determine the source of the mechanical failure. Metallurgical analysis conducted by the Norwegian Defence Research Establishment revealed that the input gear had suffered a fatigue crack. This crack eventually caused a single tooth to break off, leading the gear to split and the entire gearbox to fracture into two pieces.
Technical examinations found no evidence of improper lubrication, gear misalignment, or material inclusions that would typically initiate such a crack. However, investigators discovered that this specific gearbox had a history of involvement in a previous accident in 1998, when it was installed in another aircraft belonging to the same operator. That previous incident involved a sudden stoppage during a crash landing.
Findings
- The primary cause of the failure was a fatigue crack in the input gear of the tail rotor gearbox.
- The fatigue crack was likely initiated by an overload event caused by a sudden stoppage during the gearbox's previous involvement in a 1998 accident.
- The loss of tail rotor control led to the uncontrolled rotation of the aircraft and the subsequent hard landing.