What happened
On 20 December 2012, a Bell UH-1H helicopter, registration HB-RXC, was performing a flight from Balzers heliport to St. Gallen-Altenrhein airport. After picking up three passengers in Buchs/SG, the aircraft was flying alongside a motorway when the occupants experienced intense vibrations and a sudden, sharp yaw to the right. A loud bang was heard by one of the passengers, and the pilot struggled to maintain directional control.
Faced with the loss of stability, the pilot initiated an emergency landing. The helicopter struck the ground heavily, causing the main rotor blades to strike a garden shed. The impact caused the aircraft to roll approximately 270 degrees before coming to rest on its right side. While all occupants managed to exit the wreckage, one passenger sustained serious injuries.
The investigation
Investigators examined the mechanical state of the tail rotor transmission and the maintenance history of the aircraft. The inquiry focused on the structural integrity of the tail rotor components and the regulatory oversight of aircraft registered under the 'historic' subcategory. The investigation also reviewed the qualifications of the personnel responsible for performing maintenance on the complex systems of the Bell UH-1H.
Findings
Technical analysis revealed that the accident was caused by a failure of two bolts and a retaining nut at the crosshead, which created a massive imbalance on the tail rotor shaft. This imbalance led to the failure of the tail rotor gearbox and the subsequent loss of tail rotor thrust.
Contributing factors included inadequate maintenance work performed on the tail rotor system. Furthermore, the investigation identified systemic issues regarding the oversight of historic aircraft, noting that the registration of the aircraft in a special category had led to the loss of certain operational restrictions that would have otherwise been applicable under its original type certificate.