Accident Summary
On June 30, 2017, a Sikorsky UH-60M helicopter, registration NA-703, operated by the National Airborne Service Corps (NASC) of the Ministry of the Interior, was conducting a sea rescue hoisting training exercise off the north breakwater of Taichung Harbor. The aircraft carried nine occupants: a foreign flight instructor, a domestic flight instructor, a pilot, a foreign mechanic, two domestic mechanics, and three rescue personnel (two from the Coast Guard and one from the Fire Department).
At approximately 1105 hours local time, the hoist hook detached during a recovery operation. The helicopter was hovering at about 70 feet above the water when rescue personnel were deployed. During the recovery of two rescuers at around 50 feet altitude, the hook assembly separated from the hoist. One Coast Guard officer and one Fire Department rescuer fell into the sea, sustaining minor and serious injuries respectively. The helicopter crew confirmed the safety of both individuals on a guard boat and landed safely at Taichung Ching Chuan Kang Airport at 1120 hours.
Investigation Findings
The Aviation Safety Council (ASC) conducted the investigation and found that the hoist hook failure resulted from improper maintenance during a recent hoist cable replacement. When the hook assembly was disassembled, the hook bearing likely sustained an impact, causing it to shift and become tilted within the bearing housing. During reassembly, the retaining nut could not be fully tightened against the bearing housing due to the stuck bearing, leaving a 1.5 mm gap between the nut and housing. Consequently, the two locking screws could not be inserted into the castle slots on the bearing housing, as they are intended to prevent loosening.
The inspection personnel did not detect that the locking screws were not properly seated in the castle slots. During the training exercise, as the hoist rotated during personnel recovery, the threads connecting the bearing housing and the nut loosened under load, causing the hook assembly to detach from the hoist system.
Probable Cause
The probable cause was the improper assembly of the hoist hook during maintenance. The bearing was displaced, preventing full tightening of the retaining nut. The locking screws were not engaged in the castle slots due to a gap, leading to thread loosening and detachment of the hook assembly under operational loads.