What happened
On February 16, 2020, an Agusta AW139 operated by the Sapporo City Fire Department Air Corps was conducting rescue training at the Ishikari Temporary Airfield in Hokkaido. During the sixth of six scheduled hoist training runs, the helicopter was approaching the airfield from the west side.
As the crew prepared for the maneuver, a rescue member positioned weights onto the hoist hook. These weights, totaling 4.54 kg, are used to stabilize the hoist hook and prevent unwanted swinging during operations. As the weights were released outside the aircraft, they detached from the hoist hook and fell from an altitude of approximately 55 meters. The weights landed in a nearby national forest, roughly 250 meters west of the airfield. There were no injuries to personnel and no damage to property on the ground.
The investigation
The JTSB investigation examined the mechanical function of the weight hook and the procedures used during the training session. The investigation included a review of video footage recorded during the weight release, which captured the moment of the incident.
Investigators analyzed the design of the weight hook, noting that it was not a standard part of the aircraft's design but rather specialized climbing gear. The mechanism relies on a spring-loaded system where the hook closes when an object pushes against a guide. The investigation also reviewed the crew's recent experience with this specific equipment, as the weights had been acquired only about a month prior to the incident.
Findings
Technical analysis revealed that the weight hook was likely not fully closed when the weights were attached to the hoist. Video evidence showed the rescue member's hands were on the hook's lever during the attachment process, which prevented the hook from being properly secured. Furthermore, the rescue member did not perform a tension test to verify the connection.
Because the hook's internal mechanism makes it difficult to visually confirm whether it is closed, the investigation found that the weights were able to hang while one side of the hook remained open. When the weights were released outside the aircraft, the resulting sway created a turning force that caused the remaining side of the hook to open, leading to the unintentional drop of the weights.
Safety action
Following the incident, the Sapporo City Fire Department Air Corps implemented several safety measures:
- Suspended all use of the specific weights involved in the incident.
- Revised procedures for positioning weights on the hoist hook.
- Mandated that weight installation be performed inside the aircraft by a jumpmaster.
- Required a formal confirmation of the connection by applying tension.
- Established a double-check protocol involving both the jumpmaster and the hoist operator before the fall prevention rope is removed.