What happened
On 8 October 2012, an AS350B2 helicopter, registration G-ORKY, was completing a cargo transit flight within the Cairngorms National Park, Scotland. While flying at 80 knots, the aircraft encountered localized severe turbulence. During this period, the helicopter experienced a rapid descent of approximately 60 to 80 feet. The pilot heard a loud bang and noted that a chain lifting sling, which had been suspended beneath the aircraft, was no longer visible in the cargo mirrors. Despite the noise, the pilot reported no unusual vibrations or uncommanded yawing, and the aircraft responded normally to control inputs. The pilot proceeded to land the aircraft safely at a reduced speed. Upon landing, ground crew reported that the chain sling had become wrapped around the tail boom, and an inspection confirmed significant damage to the tail rotor system.
The investigation
An internal investigation by the helicopter operator determined that the incident was caused by the sling moving into the tail rotor area. The investigation established that the combination of high airspeed, a descent, and a nose-up attitude during the turbulence caused the 7-meter sling to shift. The investigation also noted that the sling was covered in a cloth sheath, which influenced its flight characteristics. Following the event, the operator conducted flight trials which revealed that sheathed slings tended to angle further back during flight compared to unsheathed versions.
Findings
- The primary cause of the incident was the sling striking the tail rotor system due to high airspeed and the aircraft's attitude during turbulence.
- The 7-meter lifting sling was covered in a cloth sheath, which contributed to its movement toward the tail.
- The aircraft was flying at the maximum permitted airspeed for that specific configuration when the turbulence occurred.
Safety action
- The operator increased the maximum allowed sling length to 10 meters.
- The operator removed the cloth sheaths from most of their slings.
- A safety bulletin was distributed to all pilots and ground crews, emphasizing the importance of adhering to the 80-knot speed limit and reducing speed further during unfavorable weather conditions.