What happened
On May 21, 2011, a UH-1H helicopter, registration NA-511, operated by the National Airborne Service Corps, was conducting routine life-saving hoist training in Lingang Township, Pingtung County. The flight crew, which included the pilot, co-pilot, and crew chief, was accompanied by three members of the Special Rescue Team.
After landing at Longxiang Park to disembark the rescue personnel, the crew performed several successful hoist training cycles. During the third cycle of the co-pilot's operation, the helicopter was maintaining a stable hover at approximately 60 feet. As the crew chief began recovering the steel cable after the rescue personnel had been hooked to the hoist head, the mechanism suddenly stalled and began to slip. At an altitude of approximately 30 feet, the crew chief observed that the cable could no longer be controlled despite the controller being in the upward position. This mechanical failure caused the rescue personnel to fall to the ground. The crew chief immediately ordered the helicopter to stop forward progress and continued to lower the cable. The pilot then landed the aircraft to retrieve the injured individual, who was subsequently transported to Tainan Municipal Hospital. There were one injury and no damage to the aircraft.
The investigation
The investigation focused on the failure of the rescue hoist mechanism. Investigators examined the maintenance history of the hoist, specifically looking at recent overhaul work performed by Asia Airways. The investigation included a metallurgical analysis of the planetary gear shaft and a review of the overhaul procedures and documentation used during the component's last maintenance cycle.
Findings
- The primary cause of the accident was a mechanical failure of the hoist coupling caused by an excessive axial clearance. During the overhaul process, an error in measuring the axial clearance led to an incorrect assembly. As the driving gear length decreased during operation, the wall stress exceeded the yield strength, causing the driving gear to fail and the driven wheel to rotate in reverse, resulting in the uncontrolled descent of the personnel.
- The overhaul technicians used a manual screwdriver as a positioning tool, which caused the bearing to seat incorrectly, leading to inaccurate measurements.
- The overhaul records for the specific hoist units involved contained inaccurate data regarding shim thickness.
- The overhaul manual and work orders contained discrepancies between text and illustrations, which contributed to maintenance errors.
- The crew's specific hoisting pattern—advancing the aircraft at speed while simultaneously recovering the cable—was not explicitly outlined in the standard operating procedures.