What happened
On November 27, 2022, an S-58JT helicopter, registration HL9678, was conducting a scheduled wildfire patrol flight over the Yeongdong region of Gangwon Province. The aircraft, operated by TransHeli Co., Ltd., had been in the air for approximately 76 minutes when it experienced a sudden loss of stability. While flying over a mountainous area in Yangyang-gun, the aircraft slowed significantly and entered a rapid right-hand rotation. The helicopter completed roughly four spins before impacting a forested area near Eoseongje-ri. The impact was followed by a severe fire and multiple explosions that completely destroyed the airframe. All 5 people on board, including the pilot, maintenance staff, and passengers, were killed.
The investigation
The ARAIB conducted an extensive investigation involving engine teardowns, fuel testing, and acoustic analysis. Investigators utilized CCTV footage from nearby locations and performed a sound spectrum study to reconstruct the final moments of the flight. This acoustic analysis was critical in determining that while the main rotor and engines continued to operate normally until the moment of impact, the tail rotor's effectiveness had vanished. The investigation also examined the aircraft's maintenance history, the pilot's credentials, and the operational procedures of the operator.
Findings
Technical analysis confirmed that the tail rotor functional loss was the primary cause of the accident. Specifically, the investigation found that the tail rotor's thrust ceased, leading to a loss of anti-torque control. This caused the aircraft to enter a spiral descent with a yaw rate exceeding 90 degrees per second, creating a load factor that likely exceeded structural limits and led to a near-vertical crash. A significant contributing factor was the insufficient recovery measures taken by the crew to manage the tail rotor failure. Additionally, while the aircraft's maintenance and airworthiness documentation were in order, the investigation noted that there were more individuals on board than were listed in the original flight plan.