What happened
On June 13, 2009, at approximately 15:35 UTC, a Bell 206B3 helicopter, registration CS-HFB, was performing aerial photography for a commercial automotive advertisement in the Serra da Estrela mountains, Portugal. The aircraft was flying at a low altitude of approximately 150 feet above the ground and a speed of 35 knots, following a vehicle along a winding mountain road near Lagoa Comprida.
As the pilot transitioned from following the first vehicle to approaching a second vehicle, the pilot initiated a right turn using the cyclic and right pedal. During this maneuver, the aircraft suddenly entered an unanticipated right yaw. The pilot attempted to correct the movement with full left pedal, but the aircraft continued to rotate, completing four 360-degree turns. The helicopter subsequently collided violently with the steep terrain, coming to rest suspended on a rock on a mountainside.
The impact caused substantial damage to the Bell 206B3, including a fractured tail boom, damaged vertical stabilizer, and broken main rotor mast. The two occupants—the pilot and a camera operator—sustained serious injuries, including spinal fractures and broken limbs.
The investigation
The GPIAAF investigation examined the flight dynamics, meteorological conditions, and aircraft configuration. The investigation established that the aircraft was operating at a low translational speed and high engine power setting. The crew was experienced, and the aircraft was well-maintained with a valid certificate of airworthiness.
Investigators analyzed the aerodynamic phenomena affecting the tail rotor during the turn. They determined that as the aircraft turned right, the tail rotor encountered several aerodynamic challenges. Specifically, the investigation identified that the tail rotor entered a vortex ring state and was subsequently affected by main rotor disc vortex interference. These factors, combined with weathercock stability and the loss of translational lift, created a sudden, uncommanded right yaw acceleration.
Findings
- The primary cause of the accident was the pilot's loss of control following an unanticipated right yaw.
- The aircraft was flying at a low altitude, which left the pilot with insufficient margin to execute the necessary recovery techniques, such as pushing the cyclic forward to increase airspeed.
- Contributing factors included flying at a low translational speed and operating at a high engine power setting.
- The aircraft's mass was near the maximum allowable for the given altitude.
- Aerodynamic factors, including tail rotor vortex ring state and main rotor disc vortex interference, significantly contributed to the sudden yaw acceleration.