What happened
On March 7, 2008, a Kamov 32A11BC helicopter, registration CS-HMK, was engaged in a forest fire suppression mission in the Alto Minho region of Portugal. The aircraft was operating over the Touvedo reservoir on the Rio Lima, utilizing a 5,000-liter underslung bucket for water collection.
After performing a reconnaissance pass of the area, the crew determined the landing site was clear of obstacles and proceeded to hover over the water to refill the bucket. During the takeoff phase, the crew detected three electrical cables in close proximity to the aircraft. Despite the detection, no evasive maneuvers were performed, and the tether lines of the water bucket struck the cables, causing them to break. The helicopter sustained no damage, and there were no injuries to the two crew members.
The investigation
The investigation focused on the visibility of the electrical lines, which were part of a 15 kV medium-voltage line owned by EDP. These cables spanned 273 meters across the reservoir, suspended only 10 meters above the water surface. The investigation examined the physical characteristics of the lines, noting they were 40mm thick, grey in color, and constructed of aluminum reinforced with steel.
Investigators evaluated the visibility of the infrastructure, noting that the support poles were located within dense vegetation, making them difficult to spot. Furthermore, the investigation reviewed the regulatory requirements for obstacle marking under CIA No. 10/03. While the span length and height did not individually trigger the obstacle classification, the fact that the line crossed a reservoir wider than 80 meters categorized the line as an obstacle that required marking.
Findings
- The primary cause of the incident was that the crew failed to detect the presence of the cables in a timely manner.
- The grey color of the cables provided very low contrast against the water and the surrounding shoreline vegetation.
- The cables lacked any form of aerial marking or lighting.
- The placement of the support poles within vegetation hindered the crew's ability to use the poles as visual indicators of the line's presence.
Safety action
It is recommended that the National Civil Aviation Authority (INAC) coordinate with the utility provider, EDP, to ensure that the medium-voltage line feeding the AVV 005 transformer station is properly characterized and marked as an aviation obstacle.