What happened
On April 4, 2011, a Ka-3lyT helicopter, registration HL9400, operated by Kim's Solution Co., Ltd., crashed into Machasan Mountain in Gyeonggi-do while performing cargo transport duties. The aircraft was shuttling construction materials for power-line towers between a helipad and a tower site. During the fourth shuttle flight of the afternoon, the aircraft's engines shut down almost simultaneously, causing the helicopter to strike the mountain at approximately 70 percent of its elevation. The impact destroyed the aircraft and resulted in two fatalities.
The investigation
The ARAIB investigation focused on the fuel system and the sequence of events leading to the engine failure. The investigation established that the aircraft had undergone two refueling sessions that day. During the second refueling, which occurred while the engines were running, the aircraft mechanic added fuel to several tanks. However, the investigation found that the fuel transfer pumps for tanks No.1 and No.3+4 were inoperative.
Investigators examined the fuel transfer pump protection circuit, which is designed to shut down pumps to prevent motor failure if fuel depletion is detected. Because the engines were running during the refueling process, the power supply to the pumps remained constant, preventing the pilots from performing the necessary reset procedure (cycling the fuel pump switch) to reactivate the pumps. Furthermore, a malfunction in the fuel quantity transmitter for the left tank meant that the crew did not receive a low-level warning for that specific tank, leaving them only aware of the shortage in the right tank.
Findings
- The engines shut down because fuel in the No.2 service tanks, which supply fuel directly to the engines, was depleted.
- Although fuel remained in tanks No.1 and No.3+4, it could not be transferred to the service tanks because the fuel transfer pumps were inoperative.
- The fuel transfer pumps had been deactivated by a protective circuit following a previous fuel depletion event.
- The crew was unable to reset the pumps because the engines were running during refueling, which prevented the necessary power interruption required to reset the relay.
- A malfunctioning fuel quantity transmitter prevented the pilot from detecting that the left tank was also running low, leading the crew to believe sufficient fuel remained.