Incident Overview
On December 4, 2020, at approximately 10:55 Japan Standard Time, a Toho Air Service Co., Ltd. Airbus Helicopters AS350B3 (registration JA504D) dropped some of the withered pine trees it was transporting by cargo sling onto a fallow field in Miyato Island, Higashi-Matsushima City, Miyagi Prefecture. There was no damage to the helicopter and no injuries to persons on board or on the ground.
Flight History
The helicopter had been tasked with transporting dead pine trees (withered by pine weevils) using a 23-meter-long sling from four locations in Miyato Island to the Naruse No. 2 helipad. It took off from the helipad at around 09:01 and completed 23 transports before the incident. For the 24th transport, the helicopter slowly approached lifting place (4) from the north, avoiding vessels at work in the bay. When the captain was lifting a cluster of dead pine trees (about 350 kg) with some branches cut, a signal person radioed, "There is something unstable." The captain continued the maneuver slowly, watching the load on a mirror, and headed east toward the helipad at a ground speed of about 25 knots. At around 10:55, a mechanic in the left rear seat notified the captain that some of the dead trees had dropped onto the field. The captain did not see the fallen object on the mirror and, seeing no abnormality in the sling equipment or remaining load, continued to the helipad and unloaded. The captain then flew back to the drop area, confirmed no abnormality, and landed. At about 11:00, the captain reported the dropped object (dead branches weighing approximately 40 kg) to the flight operations manager.
Observations
The signal person reported that before lifting, he had cut wisteria vines entangled with the dead pine trees using a chainsaw and judged the lifting feasible. When the trees were lifted, he visually recognized that a mass of dead branches was entangled with vines and radioed the captain. The mechanic in charge noted that the dead trees were entangled with vines but appeared stable during hover and initial forward flight. He did not see the moment of dropping but observed the trees falling and reported it.
Analysis
The investigation found no signs of abnormality in the airframe or sling system; the hook at the sling tip remained closed. The engine control unit data showed a rise in outside temperature at around 10:55 when the helicopter was flying at low speed after lifting, indicating that downwash (airflow from the main rotor) had moved just under the airframe. The downwash, combined with a northwest wind of about 15 knots, likely increased wind pressure on the dead trees. The company's procedures did not specify packaging methods for dead or damaged trees; customary practices were not formally documented. Although the signal person noted instability, this did not prevent the incident.
Probable Cause
In this serious incident, it is highly probable that some of the dead trees dropped due to wind pressure including downwash because measures to prevent the slung dead trees from dropping were not sufficient.
Safety Actions
On December 9, 2020, the company added to its standard operating procedures the methods for packaging dead or damaged trees and procedures to suspend slinging work to prevent dropping. These were communicated within the company and safety education was implemented.