What happened
On the afternoon of July 25, 1988, at approximately 15:28 local time, a Piper PA-28-181 Archer II, registered as HB-PKO, departed from Sion Airport in Switzerland. The aircraft was carrying three passengers on what was intended to be a private flight following a commercial operation. Immediately after liftoff on runway 26, while climbing through an altitude of roughly 2,000 feet and passing the end of the runway, the pilot noticed a significant drop in engine power.
Witnesses on the ground reported hearing irregular engine sounds and observing the aircraft struggling to gain altitude. The pilot initiated an immediate left turn to attempt a forced landing back toward the opposite end of the airfield. During this steep maneuver, the left wingtip struck the guard cable of a high-voltage transmission line running along the southern perimeter of the airport. The impact severed the cable, causing a short circuit that ignited the aircraft. The plane crashed into an adjacent orchard and was completely destroyed by fire.
The pilot and the front-seat passenger sustained serious injuries, while the two rear passengers escaped uninjured. Emergency responders arrived quickly, extracting the unconscious pilot from the wreckage. The high-voltage line was damaged and temporarily taken out of service for over two hours.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) conducted a thorough examination of the accident. Investigators analyzed the wreckage, noting that the propeller was not rotating at impact and the cabin door had opened due to the force of the crash. They also reviewed the aircraft's maintenance logs, which indicated normal operation prior to the flight, including a recent 50-hour inspection.
To determine the cause of the power loss, the engine was removed and completely disassembled. Every component that could be tested underwent rigorous inspection and bench testing. Fuel samples were taken and found to meet all standards. The investigation team also consulted with the engine manufacturer but found no similarities to known failure modes for this model.
Despite these extensive efforts, the investigation was hindered by the severe destruction of the engine group in the fire. No mechanical failure or defect could be identified in the aircraft systems.
Findings
The inquiry established that the pilot held valid licenses and medical certification, with sufficient experience on the aircraft type. The weather conditions were favorable, with good visibility and no risk of carburetor icing. The aircraft was within weight and balance limits.
The primary finding was that the accident resulted from a collision with a high-voltage line during a forced landing attempt. This attempt was necessitated by a sudden and unexplained loss of engine power immediately after takeoff. Although the pilot's report and witness accounts confirmed the power loss, the root cause of the engine failure could not be determined due to the destruction of the evidence.
Safety action
No specific safety recommendations were issued in this final report, as the cause remained undetermined and no systemic regulatory deficiencies were identified.