What happened
On February 17, 2024, an Agusta AW139 helicopter, registration JA03FD, operated by the Saitama Disaster Prevention Air Squadron, was conducting a mandatory load test at Honda Airport in Kawajima Town. The flight was intended to verify the integrity of a recently replaced external hoist cable.
During the procedure, the crew was testing the hoist's ability to handle a 136 kg load. As the aircraft reached a hovering altitude of approximately 340 feet and the operator began reeling out the cable to its maximum length of 300 feet, the cable unexpectedly detached from the drum. The cable and the attached weight fell to the ground near Taxiway E4. Fortunately, the ground crew had evacuated the immediate area, and there were no injuries and no damage to ground property.
The investigation
The Japan Transport Safety Board (JTSB) examined the hoist, the new cable, and the maintenance records of the replacement process. The investigation focused on the installation of the new cable, which had been installed the previous day by mechanics.
Investigators found that while the mechanics had performed a tension check, the installation of the cable end into the drum slot was flawed. Specifically, the investigation compared the new cable to the old cable and found that the new cable lacked the characteristic conical dent on its strands that occurs when the securing screw is properly tightened against the cable strands. Furthermore, the investigation revealed that the screw had only been turned six times, whereas a correct installation requires eight turns to ensure the screw contacts the strands rather than the welded end of the cable.
Findings
- The primary cause of the incident was that the cable was not properly secured to the drum during the replacement process.
- The installation error was likely caused by a deviation from the prescribed maintenance procedure. Instead of removing the variable speed controller to fully unwind the drum, the mechanic stopped unwinding when the limit switch activated, leaving four wraps of cable on the drum. This reduced the workability and made it difficult to properly seat the cable end in the slot.
- The securing screw was likely pressing against the welded end of the cable rather than the strands, preventing the necessary depth of insertion.
- While the mechanics performed a tension check, the cable appeared sufficiently tight because the screw was temporarily wedged against the cable's welded end, masking the underlying instability.