What happened
On December 22, 2021, a Bell 412 helicopter, registration JA9584, operated by Aero Asahi Corporation, was performing a routine transport of ready-mixed concrete in Kiryu City, Gunma Prefecture. The mission involved transporting concrete from an outdoor activities center to an unloading site approximately 6.2 km away using a specialized bucket attached via a sling assembly.
During the tenth transport of the day, the aircraft was cruising at an altitude of roughly 1,150 ft. The pilot noticed a significant drop in the load gauge, which indicated the weight had plummeted from approximately 1,000 kg to just 190 kg. Upon inspection via the cargo mirror, the crew observed that the bucket door was slightly ajar and the concrete had been discharged. The spilled material, weighing about 800 kg, fell into a forested area. Fortunately, the drop occurred over uninhabited terrain, resulting in no injuries and no damage to ground objects or the aircraft.
The investigation
The Japan Transport Safety Board (JTSB) examined the mechanical components of the bucket, specifically the carbon dioxide-powered opening and closing mechanism. The investigation focused on the solenoid valve, the air operation valve, and the locking and opening cylinders.
Testing revealed that while the bucket's locking mechanism was designed to remain secure under load, the system was vulnerable to pressure changes within the valve assembly. Investigators performed validation tests using stone gravel to simulate the load and subjected the bucket to various temperatures and vibrations. A disassembly of the solenoid valve uncovered solidified grease and unidentified minute foreign objects adhering to the spool.
Findings
- The bucket door opened unintentionally because the air operation valve's spool shifted to the "OPEN" position.
- This shift was likely triggered by an increase in internal pressure within the exhaust path.
- The clogged exhaust path, caused by the ingress of foreign matter and solidified grease, caused carbon dioxide gas to leak into the exhaust side of the valve.
- The manufacturer's manual lacked instructions for periodic inspections of the internal valves and pipes to prevent such blockages.
Safety action
Following the incident, the operator suspended the use of all gas-type buckets and implemented enhanced training for sling operations. In coordination with the manufacturer, repairs were made to the bucket's air supply and exhaust paths, including the addition of independent exhaust paths for each valve. The company also updated its inspection protocols to include checks for debris in air filters and inspection windows.