What happened
On Wednesday, March 9, 1988, a Piper PA-28-161 Warrior II bearing registration HB-PIH was engaged in an unauthorized aerial photography mission along the Rhône River in the Chablais region of Switzerland. The flight departed from La Côte aerodrome around 11:30 local time with two occupants: a pilot and a passenger, both civil engineers working for a firm contracted by the "Hydro-Rhône" project.
The crew had independently planned the route to document riverbanks from Lake Geneva toward Saint-Maurice. They carried a photocopy of a national map with specific sites marked in red. After flying south along the right bank and then north along the left bank, the aircraft approached the area of Vouvry near the Chavalon thermal power plant around 12:20. The pilot appeared to be targeting a point on the map located approximately 450 meters north of a 220 kV high-voltage transmission line connecting the plant to the Rhône plain.
As the aircraft passed the rocky escarpments known as "Porte du Scex," it flew dangerously close to the terrain. Approximately 200 meters later, the right wing struck the upper conductor of the southern bundle of the power line. The impact sheared the wing off at the root, sending it tumbling down a rock face about 15 meters below the wires. The fuselage and left wing crossed the remaining lines without further contact but lost all control. The aircraft spiraled downward, crashing into a poplar forest roughly 300 meters horizontally and 200 meters vertically from the point of impact. Both occupants died instantly from the collision forces. There was no fire, and damage to the power line or surrounding property was minimal.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) conducted a thorough inquiry into the accident. Investigators examined the wreckage, flight records, and meteorological data. The pilot held a valid private pilot license with over 320 hours of total experience, including more than 76 hours on the Warrior type. The aircraft was airworthy, properly maintained, and within weight and balance limits at departure.
Meteorological reports indicated clear skies, good visibility (30 km), and light winds from the north-northwest. The sun was behind the pilots, which typically aids visibility. However, the investigation noted that the high-voltage line was difficult to see against the dark brown background of the winter forest. The specific section of the line involved had not been marked with warning lights or other visual aids, despite being listed on aeronautical obstacle charts.
Medical examinations confirmed that neither occupant was under the influence of alcohol or suffering from pre-existing medical conditions that could have contributed to the accident. The flight recorder was not installed, but analysis of the wreckage showed the altimeter was set correctly, and the fuel selector was on the left tank, which had been breached.
Findings
The investigation established that the primary cause of the accident was a collision with a high-voltage power line. Several contributing factors were identified:
- Proximity to Terrain: The flight path brought the aircraft too close to the rocky cliffs and the power line, leaving insufficient margin for error.
- Poor Visibility of Obstacles: Despite good weather conditions, the high-tension wires were nearly invisible against the dark forest background. The lack of visual markers on the line exacerbated this issue.
- Planning Oversight: Although the crew prepared a map with marked sites, the power line itself was not highlighted or emphasized in red. It is unclear if the pilot fully appreciated the proximity of the line to their intended flight path relative to the marked site.
- Lack of Visual Aids: The specific span of the 220 kV line where the collision occurred lacked any physical warning devices, such as aviation safety balls or lights, which might have made the wires more conspicuous.
Safety action
The report highlighted that while the line was documented on aeronautical charts, the absence of visual marking contributed to the accident. The investigation noted that previous considerations for marking the line had been tied to a proposed airport at Noville that was never built. No specific safety recommendations were issued in this final report beyond the implicit need for better obstacle visibility and careful route planning near high-tension infrastructure.