What happened
On February 21, 2008, a BELL 212 helicopter, registration EC-IAV, was engaged in aerial firefighting operations near Grado, Asturias. The aircraft, operated by TRAGSA, was transporting a firefighting crew and a Bambi Bucket to combat a forest fire.
During the mission, the pilot performed two water collection maneuvers from the Nalón River. After the second water load, as the helicopter began to climb, the aircraft experienced an uncontrolled yaw to the right and a lateral drift. The helicopter struck trees on the riverbank before ultimately crashing into the ground. The pilot was able to exit the aircraft safely, though the engines continued to run until the fuel was exhausted.
The investigation
The investigation focused on the mechanical failure of the tail rotor system and the sequence of events during the water loading process. Investigators examined the wreckage, noting significant damage to the main rotor blades, the tail cone, and the tail rotor drive shaft.
Physical evidence showed that the tail rotor drive shaft had suffered torsional deformation and had disconnected from the intermediate gearbox. Furthermore, investigators identified scrape marks on the right skid, indicating that the Bambi Bucket's suspension cables had come into contact with the aircraft's structure. The investigation also analyzed the flight manual, which warns that rapid turns near the water surface while lifting a heavy bucket can cause the cables to snag on the skids, potentially leading to a dynamic rollover or an unbalanced aircraft attitude.
Findings
- The primary cause of the accident was the uncontrolled rightward yaw resulting from a total loss of tail rotor thrust due to the disconnection of the power transmission shaft from the intermediate gearbox.
- The disconnection of the shaft was caused by torsional deformation, which occurred because of an external load that increased the drag on the tail rotor blades.
- This increased load was likely caused by the tail rotor blades striking the water surface.
- The tail rotor immersion was a consequence of an aircraft imbalance caused by the bucket cables snagging on the right skid during the water loading process, which shifted the center of gravity and induced a heavy tilt.