What happened
On June 30, 2017, a Sikorsky UH-60M helicopter, registration NA-703, operated by the National Airborne Service Corps (NASC), was conducting maritime search and rescue training near the northern dike of Taichung Port. The flight crew included two flight instructors, a pilot, and three maintenance personnel, along with three rescue personnel (two from the Coast Guard and one from the Fire Department).
During the sixth training iteration of the mission, the aircraft was hovering at approximately 50 feet above the sea to recover a rescue technician. As the crew was performing the recovery, the rescue hoist assembly began to rotate. Suddenly, the threads connecting the hook bearing housing and the bushing nut loosened, causing the hook assembly to detach from the rescue hoist system. The sudden detachment caused two rescue personnel to fall into the sea. Both individuals sustained injuries, with one suffering a cervical spine fracture and the other sustaining head trauma and a concussion. The crew immediately notified a nearby Coast Guard vessel, which successfully recovered the personnel from the water. The aircraft returned to Taichung Ching Chuan Kang Airport and landed safely at 11:20 local time.
The investigation
The Taiwan Transportation Safety Board (TTSB) investigated the mechanical failure and the maintenance history of the hoist system. The investigation focused on the recent replacement of the hoist cable and the assembly of the hook components. Investigators examined the maintenance logs, the physical condition of the bearing housing, and the technical manuals used by the maintenance and inspection personnel.
Findings
The primary cause of the accident was the failure of the threads connecting the hook bearing housing and the bushing nut to remain secured, leading to the detachment of the hook assembly.
Key findings included:
- During the replacement of the hoist cable, the hook bearing may have been subjected to external impact, causing it to shift and become wedged within the bearing housing.
- Due to this wedged bearing, maintenance personnel were unable to install the bushing nut in the correct position, leaving a 1.5 mm gap between the upper edge of the housing and the lower edge of the nut. This gap prevented the securing screws from properly engaging the castellated grooves.
- The inspector failed to identify that the two securing screws were not correctly seated in the castellated grooves during the post-maintenance inspection.
- Maintenance and inspection personnel lacked a clear method for visually verifying that the screws had fully seated in the grooves, mistakenly believing that the presence of the pin holes indicated proper installation.
- There were systemic risks identified regarding the management of maintenance manuals, specifically a lack of coordination between aircraft and hoist maintenance manuals, and a lack of formal English language proficiency requirements for personnel interpreting English-language technical instructions.
Safety action
The TTSB issued several recommendations to the NASC and the Ministry of the Interior, including the establishment of a dedicated, professional flight safety management unit and the implementation of more robust external safety oversight mechanisms for government aircraft.