What happened
On 6 June 2013, an AgustaWestland AW109SP, registration HB-ZRS, conducted a technical flight at Zurich Airport to perform a functional check of its rescue hoist following maintenance. The procedure involved lifting a 250 kg test load to verify the system's operation. During the flight, the helicopter was hovering at a low altitude while a mechanic on the ground attached the weight to the hoist hook.
As the pilot began to lift the load, the pilot and hoist operator experienced a sudden change in the aircraft's attitude and heard a loud bang. The hoist cable had snapped under the tension of the lift. Following the break, the pilot maintained a hover to assess the situation before landing safely at the RegaCenter apron. There were no injuries and no damage to the aircraft other than the broken cable and hoist components.
The investigation
Investigators examined the rescue hoist attachment assembly and the broken cable. Microscopic analysis of the cable's breaking point revealed traces of red paint, which matched the paint on the hoist's handle assembly. Furthermore, the investigation found that the edges of the aluminum brackets on the handle assembly showed clear imprints of the cable's surface structure, indicating that the cable had been in contact with these sharp edges under load.
Testing of an intact section of the cable confirmed that its breaking strength met manufacturer specifications for shock-free loading. However, the investigation established that the cable had become snagged behind a nut on the attachment assembly, which significantly reduced its load-bearing capacity at the point of contact.
Findings
- The primary cause of the incident was that the rescue hoist cable became snagged behind a nut on the handle assembly, leading to its failure when the load was lifted.
- The design of the rescue hoist attachment assembly allowed the cable to catch on protruding hardware.
- The hoist operator's ability to detect the snag was limited by restricted visibility caused by the aircraft's cabin structure and his helmet.
- The investigation noted that similar patterns of abrasion had been observed on the door frames of other aircraft in the operator's fleet.