Overview
On December 22, 2021, at approximately 11:00 Japan Standard Time, a Bell 412 helicopter (registration JA9584) operated by Aero Asahi Corporation unintentionally released a slung load of ready-mixed concrete while in flight over Kiryu City, Gunma Prefecture. The aircraft was flying at an altitude of about 1,150 ft (approximately 350 m) when the concrete fell from the bucket. No injuries were reported, and no damage to persons or property on the ground was caused. The aircraft itself sustained no damage, and flight characteristics remained normal until landing.
Flight History
The helicopter was conducting repeated transport of ready-mixed concrete from the Kiryu Outdoor Activities Center (elevation about 1,000 ft) to an unloading site approximately 6.2 km away. The crew consisted of a pilot in the right seat and an onboard mechanic (responsible for guiding the aircraft and operating the bucket) in the left rear seat. For the 10th transport (the fifth using that particular bucket), the ground operator had prepared a bucket of ready-mixed concrete weighing about 1,000 kg, confirmed by the load gauge in the cockpit. The onboard mechanic visually inspected the bucket and reported no abnormalities. The aircraft climbed to about 1,150 ft and flew toward the unloading site at about 65 kt. Shortly after, the pilot checked the cargo mirror and observed that the bucket was empty and the bottom plate had an opening. The load gauge indicated about 190 kg (bucket weight 135 kg). The crew confirmed the bucket door was slightly open, and the pilot concluded the concrete had fallen. The helicopter returned to the operation site, landed, and later conducted an aerial search. The company identified the fall location on December 24, 2021, in a mountainous area about 540 m south-southwest of the operation site. The fallen concrete weighed about 800 kg.
Bucket Mechanism
The bucket was suspended from the aircraft's cargo hook via a sling assembly and hook, with a total length of about 20 m. The bucket door was opened and closed using a switch at the onboard mechanic's station. The mechanism used carbon dioxide pressure to operate opening/closing and locking cylinders. In the closed position, an over-center locking mechanism secured the door. The door opened when the locking cylinder retracted and the opening cylinder extended. After the incident, the cylinder was in the CLOSE position, but the door was approximately 3 cm open. No abnormalities were found during visual inspection, continuity checks, operation checks, or gas leakage tests.
Investigation Findings
The investigation included validation tests on the bucket. When approximately 870 kg of stone gravel was placed in the bucket and the locking cylinder was set to UNLOCK, the door did not open, even with vibration and impact, while locked by the over-center mechanism. Switching to OPEN for one second caused the door to open slightly, then fully under the weight of the gravel, which then flowed out. With less gravel, the door closed. Further testing assumed a clogged exhaust port in the pneumatic system. When the exhaust port was clogged at temperatures above 10 °C, the door did not open. The outside air temperature at the time of the incident was 8 °C.