What happened
On 13 January 2022, a Bell 412 helicopter, registration EC-GSK, was conducting a flight training exercise at the Amador and reservoir in La Vila Joiosa, Spain. The flight was a line check for a pilot undergoing assessment, with a verifying pilot on board. The crew was performing maneuvers simulating an emergency scenario involving a jammed tail rotor pitch control mechanism during water loading.
During the maneuver, the crew landed the aircraft lightly on the ground with the Bambi bucket loaded with water. As the aircraft was still moving forward and the skids were in light contact with the terrain, the crew noticed an unexpected and violent change in the aircraft's behavior. The pilot attempted to adjust power to stabilize the aircraft, but the helicopter immediately became uncontrollable. The aircraft experienced intense vibrations, struck the bank of the reservoir, and plunged into the water. The aircraft sank to a depth of approximately 6 meters. Both crew members successfully evacuated through the left door and swam to the shore; no injuries were reported.
The investigation
The investigation focused on the sudden loss of control and the intense vibrations reported by the crew. Examination of the wreckage revealed significant damage to the airframe, including fractures in the main rotor blades and the vertical stabilizer. Crucially, the investigation found that the main transmission support case arms had broken, and the main driveshaft had disconnected from the transmission, causing the rotor to lose power almost instantaneously.
Laboratory analysis of the transmission mounts showed that while all components met the manufacturer's stiffness specifications, there was a significant disparity in stiffness between the mounts. This was due to the fact that one mount had been recently replaced, while the others had been in service since 2016. The investigation determined that this asymmetry likely induced high-intensity vibrations during the light-on-skids phase of the landing, leading to a static overload that fractured the support case attachments.
Findings
- The accident was caused by a structural failure of the main transmission support case attachments to the helicopter's main structure.
- A significant difference in stiffness values between the transmission mounts created an asymmetry in the pitch and roll behavior of the pylon.
- The aircraft was in a "light on the skids" state during the onset of the failure, which contributed to the intensification of vibrations.
- The disengagement of the main driveshaft resulted in a total loss of rotor power, leading to the subsequent impact with the reservoir bank.