What happened
On April 7, 2010, a Piper PA-28-161, registration LN-TOS, was conducting a low-level sightseeing flight in Kåfjorddalen, Norway. The pilot was flying a passenger over a childhood home in the narrow valley. After performing several low passes at approximately 600 feet, the pilot initiated a climb at full power. During this ascent, the aircraft struck a cable car wire at an altitude of roughly 416 feet.
The impact was severe, tearing away portions of the right wing and the tail section. Despite the loss of structural integrity, the pilot maintained control of the aircraft. The pilot managed to navigate the damaged plane back to Tromsø Airport Langnes, landing without further incident. During the emergency flight, the pilot noted that the aircraft was difficult to fly in a coordinated manner, eventually using 20 degrees of flaps to stabilize the flight.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the flight planning, the aircraft's condition, and the effectiveness of the local Obstacle Collision Avoidance System (OCAS). The investigation found that the pilot had only partially mapped potential hazards in the area. While the pilot had noted power lines in the area on their aeronautical chart, the specific cable car wire was not identified.
Investigators also analyzed the warning systems in place. The cable car was equipped with an OCAS system designed to provide visual and aural warnings. However, the pilot did not receive any warning via the aircraft's radio or visual indicators before the collision occurred.
Findings
Several contributing factors were identified in the collision. The pilot was flying at low altitudes within a narrow valley, which increased the risk of encountering obstacles. The cable car wire itself was difficult to spot from the air, and the lighting conditions—characterized by bright sunlight and white snow—reduced the contrast of the system's warning lights.
A critical finding involved the failure of the aural warning system. The investigation revealed that the OCAS aural alerts are only transmitted on six specific frequencies. Because these frequencies do not necessarily match the standard communication frequencies used by pilots in the area, the pilot's radio was not tuned to the correct channel to receive the alert. Furthermore, the investigation concluded that the warning lights were largely ineffective due to their placement and the environmental lighting.
Safety action
The NSIA issued several recommendations to address the systemic failures identified. The authority recommended that the Norwegian Civil Aviation Authority find compensatory measures for obstacles where OCAS installations are out of service. Additionally, the NSIA called for significantly improved information regarding the location and frequency settings of OCAS systems to ensure they reach the intended users. Finally, the board suggested that future obstacle warning technologies should transition toward more accessible methods, such as integrating obstacle databases into GPS and electronic flight maps.