What happened
On September 10, 2009, at 12:45 UTC, an Eurocopter AS350 B2 helicopter, registration CS-HEY, was operating on behalf of the National Authority for Civil Protection (ANPC) near Resende, Portugal. The aircraft, operated by HELIBRAVO, was performing firefighting duties near the banks of the Douro River, specifically targeting a forest fire near the village of Barrô.
During the approach to drop water from a Bambi Bucket, the pilot encountered significantly reduced visibility due to heavy smoke from the ongoing wildfires. As the pilot maneuvered to position the aircraft for the drop, the helicopter struck two medium-voltage electrical cables. The impact was severe: one cable struck the right skid, tearing it from its mounts, while a second cable struck the upper cabin and slid toward the main rotor. This second cable became entangled in the rotor control linkages, causing a partial loss of engine power and a reduction in rotor RPM.
Faced with compromised flight controls, the pilot executed an emergency autorotation. The aircraft landed in a valley covered with high vegetation and scattered trees, which helped stabilize the landing. The pilot was able to exit the aircraft uninjured.
The investigation
The GPIAAF investigation focused on the circumstances surrounding the collision and the visibility conditions at the time of the accident. Investigators noted that while high-voltage lines in the area were marked with aviation obstruction lighting, the medium-voltage lines were not. The investigation examined the flight path, noting that the pilot's maneuvering to avoid high-voltage lines, combined with the smoke, led to a lack of visual references for the unlit medium-voltage poles.
Findings
- The primary cause of the accident was the collision with medium-voltage electrical cables under low-visibility conditions caused by wildfire smoke.
- The impact with the cables resulted in the loss of flight control effectiveness and the detachment of the right skid.
- The pilot's visual recognition of the drop zone was impaired because the maneuver to approach the fire placed the primary support pole outside the pilot's field of view.
- The presence of heavy smoke camouflaged the grey electrical cables, making detection nearly impossible until the aircraft was only five meters away.
- The aircraft was not equipped with a cable-cutting device, which could have mitigated the consequences of the strike.
Safety action
Following the high frequency of collisions involving aircraft and power lines during firefighting operations, the GPIAAF recommended that the National Civil Aviation Authority (INAC) launch an awareness campaign for pilots and aerial work operators. The recommendation specifically encourages the installation of cable-cutting devices on aircraft used in wildfire suppression to minimize the impact of such encounters.