What happened
On August 28, 1974, a Cessna 150 L, registration HB-CDM, was conducting a training flight near Ecuvillens, Switzerland. After performing several circuit patterns and simulated engine failures, the instructor took control of the aircraft to demonstrate obstacle clearance maneuvers. While navigating the Saane Gorge, the aircraft's propeller struck a 60 kV high-voltage power line.
The impact severed one of the copper conductors. The momentum of the cable caused it to whip across the upper surface of the left wing, where it acted like a saw, tearing through the leading edge and damaging the main spar. As the aircraft continued through the wires, the vertical stabilizer struck a second conductor, triggering a short circuit. The electrical discharge caused localized burning on the left wing and the upper part of the tail, though no in-flight fire erupted.
Despite the severe structural damage and the loss of rudder control due to the stabilizer shifting backward, the instructor managed to return to the airfield. However, because the rudder pedals were jammed, the pilot could not control the nose wheel during the landing roll. Driven by a crosswind, the aircraft veered off the runway and came to rest in a nearby field. Both occupants survived the incident without injury.
The investigation
Investigators examined the flight path, the mechanical state of the aircraft, and the visibility conditions at the time of the accident. The inquiry established that the aircraft was well-maintained and that the crew was properly licensed and medically fit. The investigation also focused on the visibility of the power lines, noting that the copper cables were oxidized to a dark brown color and were largely obscured by the dense foliage of the gorge. The investigation further revealed that the flight was conducted at an altitude below the legal minimums required for such maneuvers.
Findings
- The primary cause was the collision with high-voltage power lines while flying below the regulatory altitude limit.
- The low light levels of late afternoon, combined with the dark, oxidized color of the cables and the presence of trees obscuring the pylons, made the wires nearly invisible to the crew.
- The high engine RPM at the moment of impact allowed the propeller to strike the cable with enough force to sever it.
- The structural damage to the vertical stabilizer caused the rudder mechanism to jam, preventing directional control during the landing roll.