What happened
On 21 March 2015, a Sikorsky S-76C+ helicopter, registration PK-FUP, operated by PT. Hevilift Aviation Indonesia, crashed into the Delta Mahakam River in East Kalimantan, Indonesia. During the flight, the aircraft entered an uncommanded attitude, leading to a total loss of control. The impact resulted in fatalities and significant damage to the aircraft, including a broken tail boom and a dislodged main rotor head.
The investigation
The Indonesian National Transportation Safety Committee (KNKT) conducted an extensive examination of the wreckage and flight recorders. Investigators analyzed the engine systems, finding no evidence of propulsion failure or progressive damage to the engines. Sound spectrum analysis of the flight data also indicated no significant instability in the rotor speed prior to the accident.
Focusing on the main rotor flight control system, the investigation scrutinized the servo actuators and push rod assemblies. Physical examination of the forward push rod revealed worn internal threads and asymmetric deformation. Investigators also examined the lateral push rod, noting that the torque paint across the joint between the push rod, jam nut, and rod end bearing had been discontinued, suggesting movement.
Findings
- The primary cause of the uncommanded attitude and subsequent loss of control was the separation of the forward rod end bearing from the push rod.
- This separation was triggered by the loosening or insufficient initial torque of the jam nut on the forward rod end bearing.
- The continuous operation of the aircraft with an improperly secured connection led to the deterioration of the engagement between the push rod and the rod end bearing, eventually causing the components to detach.
- The investigation noted that the maintenance-related visual inspections of the push rod were insufficient to detect the internal thread wear or the loosening of the jam nut.