What happened
On December 4, 2020, an Airbus Helicopters AS350B3, registration JA504D, was engaged in an external cargo operation in Higashi-Matsushima City, Miyagi Prefecture. Operated by Toho Air Service Co., Ltd., the aircraft was tasked with transporting withered pine trees using a 23-meter long sling.
During the 24th transport cycle of the day, the helicopter performed a slow approach to a lifting site on Miyato Island. While lifting the load, which weighed approximately 350 kg, a signal person notified the pilot via radio that the load appeared unstable. Despite this warning, the pilot continued the maneuver, heading toward the Naruse Helipad at a ground speed of roughly 25 knots. During the flight, a mechanic observing the load from the rear seat noticed that several dead branches, weighing about 40 kg, had fallen from the sling and landed in a fallow field.
The investigation
The JTSB investigation examined the aircraft's flight data, engine control unit (ECU) records, and the physical condition of the sling and cargo. Investigators analyzed the meteorological conditions, noting a northwest wind of approximately 15 knots.
Engine data provided critical insight into the aircraft's movement; a rise in recorded outside temperature indicated that the rotor downwash had moved directly under the airframe during the low-speed flight. The investigation also reviewed the company's existing procedures, finding that while protocols existed for general goods, there were no specific instructions for the packaging of dead or damaged trees. Additionally, the cockpit recorder was found to have malfunctioned, preventing a full review of the audio and video from the cabin.
Findings
- The investigation determined that the airframe and the sling system itself were functioning correctly and showed no signs of mechanical failure.
- The primary cause of the incident was that the measures taken to prevent the cargo from dropping were insufficient.
- The instability of the load was exacerbated by wind pressure. As the helicopter moved, the combination of the northwest wind and the rotor downwash increased the pressure on the trees, causing the loosely bound branches to detach.
- Although the signal person identified the instability during the initial lift, the pilot's decision to continue the flight without repositioning or repackaging the load contributed to the loss of cargo.
Safety action
Following the incident, the operator updated its Standard Operating Procedures to include specific packaging methods for dead or damaged trees. The company also implemented new protocols for suspending sling operations when instability is detected and conducted safety education for its personnel.