What happened
On 11 March 2022, a Bell B206L-1 helicopter, registered VH-BHF, departed Jindabyne aerodrome in New South Wales to perform a weed survey for the National Parks and Wildlife Service. The aircraft was carrying the pilot and four passengers. During a low-level descent near the Guthega power station, the aircraft began an uncommanded yaw to the right. Although the pilot managed to stop the yaw by reducing engine power, the aircraft could not be prevented from striking the terrain. The impact destroyed the helicopter. The accident resulted in 3 serious injuries and 2 minor injuries.
The investigation
Investigators examined the flight conditions and the aircraft's configuration during the survey. To provide passengers with a better view of the vegetation, the pilot had yawed the aircraft approximately 45 degrees to the right of the flight track. At the time of the incident, the aircraft was operating at a high density altitude of approximately 4,500 ft and was near its maximum all-up weight. Flight data showed the groundspeed had dropped below 20 knots just before the event.
Technical analysis revealed that the combination of low airspeed and the specific yaw angle placed the tail rotor in a region of main rotor disc vortex interference. The investigation found no evidence of mechanical failure; instead, the reduction in engine power stopped the yaw, suggesting the motion was driven by engine torque. Furthermore, the aircraft was operating within the avoid area of the height-velocity diagram, leaving insufficient altitude for a safe recovery.
Findings
- The primary cause of the uncommanded yaw was a loss of tail rotor effectiveness (LTE), triggered by the combination of low airspeed, high gross weight, high density altitude, and the aircraft's yawed position.
- The altitude at which the LTE occurred was too low to allow the pilot to recover before the collision.
- The presence of an extra passenger, who was not essential to the mission, increased the risk profile of the low-level operation.
- Operational procedures for the client did not clearly define the roles of essential personnel during aerial tasks.
- The operator's safe work method statements failed to identify specific operational factors that could impact helicopter control, and there was no requirement for a pre-flight risk review for these low-level surveys.