What happened
On December 8, 2023, a Kawasaki BK117 helicopter, registration B-55507, operated by TransAsia Airways, was performing a specialized external sling operation in Nantou County. The flight crew, consisting of two pilots, departed from the Tongfu temporary landing site at 07:28 for a supply mission along the Chenyoulan River.
During the climb, the crew noticed significant vibrations in the flight controls caused by the movement of the flat webbing sling used for the cargo. While performing engine power checks at approximately 07:38, the crew realized that the external load was no longer visible. Suspecting the cargo had detached, the crew decided to abort the mission and return to the landing site. The aircraft landed safely at the Tongfu site at 07:48 with no injuries and no damage to the aircraft.
The investigation
An investigation by the Taiwan Transportation Safety Board (TTSB) examined the mechanical and operational aspects of the detachment. Investigators reviewed the aircraft's cargo hook release system, manual mechanical devices, and the integrity of the shackles used. The investigation also scrutinized the flight crew's adherence to Standard Operating Procedures (SOP) and Crew Resource Management (CRM) principles.
Technical analysis included an evaluation of the flat webbing sling, which was found to cause significant oscillations during flight. The investigation also looked into the consistency of various company manuals, including the Aircraft Operating Manual (AOM), the Helicopter External Load Manual (HEM), and the Flight Crew Training Manual (FCTM), noting discrepancies in their descriptions of sling operations.