What happened
On August 20, 2017, at approximately 11:20 UTC, an Eurocopter AS 350 B3, registration OE-XTM, was engaged in a forest firefighting mission near Cabril, Castro Daire, in the Viseu district of Portugal. While performing water drops at low altitude, the helicopter's tail rotor struck high-voltage power lines. The impact caused the immediate separation of the tail rotor assembly and the vertical stabilizer from the aircraft. This resulted in an inevitable loss of control, causing the helicopter to enter an abrupt, spinning descent. Upon impact with the ground, a violent fire erupted, which completely consumed the aircraft. The pilot sustained one fatality.
The investigation
The GPIAAF investigation examined the aircraft's maintenance records, the flight conditions, and the environmental factors present at the time of the accident. The investigation confirmed that the aircraft was airworthy, properly maintained, and within its prescribed weight and balance limits. The pilot was found to be experienced in this aircraft type, holding a valid license and medical certificate, and specifically trained for firefighting operations using a bucket system.
Investigators also evaluated the effectiveness of the aircraft's Wire Strike Protection System (WSPS). While the aircraft was equipped with a cable cutter, the investigation established that this system is designed to protect the aircraft during forward horizontal strikes with sufficient airspeed and is not designed to protect the tail rotor from strikes. Furthermore, while the pilot was aware of the power lines in the area, the investigation could not determine if the lack of aerial markers on the lines was a contributing factor, as current regulations only require signaling under specific conditions.
Findings
- The tail rotor strike with high-voltage power lines was the primary cause of the accident.
- The wildfire had progressed through the hilly terrain, crossing the power lines and creating a complex flight environment where the only viable exit route was to climb over the lines.
- The pilot's focus on completing the firefighting mission led to an inadequate risk assessment, specifically regarding the danger of flying near live high-voltage lines in complex orography.
- It was noted that smoke from the wildfire may have potentially impaired the pilot's situational awareness, though the lack of flight recorders prevented a definitive conclusion.
- The aircraft was in a fully airworthy condition prior to the strike.