What happened
On 25 September 2011, a Bell 412 helicopter, registration PK-OCV, operated by PT. Airfast Indonesia, crashed in a forested area near Lemurung, Sumbawa. The aircraft was performing external load operations for a mining contract, transporting supplies between the Dodo and Lemurung sites.
After completing several supply drops, the helicopter departed Dodo to return to Lemurung with an empty long-line hook. Shortly after departure, the crew issued a "MAYDAY" call, after which all communication ceased. Search efforts, including aerial reconnaissance and ground teams, eventually located the wreckage at an elevation of approximately 850 meters. The crash resulted in two fatalities, claiming the lives of both the pilot and the load master.
The investigation
The investigation focused on the physical evidence found at the wreckage site and the operational procedures in place at the time. Investigators examined the long-line assembly and the tail rotor components, which were initially difficult to locate as they were scattered in the surrounding rainforest.
Analysis of the wreckage revealed that the tail rotor blades had struck the steel long-line cable while the aircraft was in flight. This impact caused the tail rotor system to enter an extreme out-of-balance state. The resulting mechanical loads were severe enough to fracture the mounting bulkhead of the 90-degree gearbox, causing the tail rotor assembly to separate from the airframe. This separation caused a significant shift in the aircraft's center of gravity, rendering the helicopter uncontrollable.
Findings
- The tail rotor blades made contact with the sling cable during flight.
- The impact caused the tail rotor gearbox mounting to fail, leading to the loss of the tail rotor assembly.
- The aircraft was operating with an empty long-line hook, which did not meet the weight requirements specified in the operator's standard operating procedures (SOP). The SOP required at least 25 pounds of weight on the end of the cable to prevent whipping.
- The pilot had been performing duties across multiple time zones with limited rest periods prior to the accident.
Safety action
The NTSC issued recommendations to the Indonesian Directorate General of Civil Aviation to improve safety oversight regarding operational procedures and pilot fatigue monitoring. Additionally, recommendations were made to the operator to ensure strict compliance with long-line weight procedures and to implement better monitoring of crew duty levels to prevent fatigue-related risks.