What happened
On September 5, 1997, an SA 330 J Puma, registration HB-XVI, was performing a vertical takeoff from a forest location in Vira Gambarogno, Val Trodo, Switzerland. The aircraft, operated by Heli-TV, was engaged in external load operations, transporting workers to a job site.
As the pilot initiated a climb to clear the treetops at approximately 50 meters above the ground, the aircraft experienced a sudden loss of yaw control. The pilot reported feeling vibrations through the pedals accompanied by an abnormal noise from the tail section. The helicopter immediately began to rotate uncontrollably around its vertical axis. In an attempt to manage the torque and steer the aircraft toward the valley, the pilot reduced power and maneuvered the aircraft, but the helicopter struck trees and crashed into the ground. The impact resulted in two fatalities, four serious injuries, and two minor injuries.
The investigation
Investigators examined the wreckage and the tail rotor drive unit (BTA). The examination of the BTA revealed a significant accumulation of black sludge mixed with lubricant, and the magnetic plug contained heavy deposits of black debris. The oil level indicator glass was also heavily blackened, indicating highly degraded oil quality.
Technical analysis by the Federal Laboratory for Materials Science and Research (EMPA) determined that the failure of the tail rotor drive unit was caused by the loosening of the 90-degree bevel gear plate's fastening. This loosening led to progressive destruction of the splines on the rotor shaft and the bevel gear. The investigation also found that the tail rotor was unbalanced, as three of the five blades were fitted with balancing masses, exceeding the manufacturer's limit of two.
Findings
- The primary cause of the accident was the loss of control due to a failure in the tail rotor drive unit (BTA), which caused the loss of tail rotor drive.
- The failure originated from the progressive wear and total destruction of the splines, triggered by a loss of clamping force in the gear assembly.
- Maintenance deficiencies contributed to the accident, including inadequate inspection of the B/T unit and failure to properly balance the tail rotor according to manufacturer instructions.
- The lack of a cockpit indicator for oil quality or metal particle detection prevented the crew from identifying the degrading condition of the transmission oil.