What happened
On June 7, 2018, a Eurocopter AS350B3, registered JA350D, was performing a transport flight from Naha airport to Aguni airport. While cruising at approximately 1,000 feet, the aircraft experienced a sudden engine-related issue. The pilot observed a red GOV warning light and heard warning tones, accompanied by an abrupt veering to the right and a drop in the main rotor speed (NR).
In an attempt to stabilize the rotor speed, the pilot transitioned to manual fuel control. However, the pilot was unable to restore the NR to the normal green arc range, and the aircraft began a continuous descent. As the helicopter approached the sea surface at roughly 300 feet, the pilot initiated emergency ditching procedures, which included sending a distress message, activating the emergency locator transmitter, and extending the emergency float. The aircraft struck the water at an excessive speed and descent rate, causing the airframe to sustain damage and subsequently sink.
The investigation
The Japan Transport Safety Board (JTSB) investigated the incident, involving representatives from the French Republic as the state of design and manufacture. The investigation focused on the sequence of engine performance degradation and the pilot's emergency response. Investigators examined the aircraft's maintenance history, which showed no prior issues with the governor or hydraulic systems. The investigation also reviewed the pilot's training, noting that while he had practiced the procedures verbally, he had not undergone specific emergency control training for a governor warning failure.
Technical analysis of the flight parameters suggested that while the engine output was insufficient to maintain level flight, a more effective recovery might have been possible through deceleration to a lower airspeed, which would have reduced the power required for level flight.
Findings
- The primary cause of the accident was a reduction in the main rotor speed and a subsequent inability to maintain altitude, which led to the ditching.
- The reduction in rotor speed was likely caused by a malfunction within the engine system, though the specific component failure could not be determined because the aircraft sank.
- The helicopter ditched at an excessive descent rate and airspeed, which resulted in structural damage to the airframe and the detachment of the emergency float.
- The pilot was not wearing a life jacket at the time of the accident, which increased the risk during the period between the ditching and rescue.