What happened
On May 1, 2020, at approximately 12:06, an S-76B helicopter, registration HL9646, operated by Sejin Aviation Co., Ltd., crashed approximately 400 meters south of Jirisan Cheonwangbong Peak in Sancheong-gun, Gyeongsangnam-do. The aircraft was engaged in a mountain rescue mission, attempting to hoist an emergency patient from a sloped area.
While the aircraft was hovering at a height of roughly 5 to 6 meters to perform the hoist operation, it began to sink. As the helicopter turned to the right, the descent continued until the aircraft struck the terrain. The impact resulted in the deaths of the patient being hoisted and the caregiver on the ground, and caused substantial damage to the S-76B.
The investigation
The ARAIB investigation examined the aircraft's mechanical condition, maintenance history, and the crew's operational decisions. Investigators performed a disassembly inspection of both engines in collaboration with the Canadian Transportation Safety Board and the US NTSB.
Technical analysis of the engines revealed significant internal contamination, corrosion, and erosion caused by environmental deposits. Specifically, the No. 1 engine showed a severed temperature sensing probe boss, while the No. 2 engine exhibited coating loss on the compressor turbine blades and heat discoloration. Furthermore, the investigation found that the Engine Electronic Control (EEC) data was inconsistent with the cockpit instrument readings.
Regarding the flight operations, the investigation reviewed the meteorological conditions, the terrain characteristics, and the crew's adherence to emergency procedures. The investigation also scrutinized the maintenance records of Sejin Aviation, specifically regarding engine washing protocols.
Findings
- The primary cause of the accident was that HL9646 crashed after exceeding its gross hover weight limits during the rescue operation.
- The flight crew failed to consult the hover performance charts for mountainous terrain prior to the mission.
- Inadequate engine maintenance, specifically the failure to perform required engine washes, led to internal contamination and reduced engine performance.
- The rescue site's terrain prevented the use of ground effect, and the combination of altitude, temperature, and total weight placed the aircraft beyond its operational capabilities.
- Discrepancies existed between the actual engine parameters and the values displayed on the cockpit instruments due to unadjusted EEC trim.
- The crew was unable to execute standard emergency procedures, such as disconnecting the hoist, because the patient was still suspended, making such actions too hazardous.