What happened
On August 24, 1968, a Piper J3C, registration HB-OSL, was performing a glider tow operation departing from Sion Airport. The aircraft was towing a Bocian glider, HB-596, which carried a pilot and a passenger. During the climb, the tow aircraft followed the ridge of the Mont d’Orge.
As the aircraft ascended, the speed of the tow formation dropped significantly, falling below 70 km/ decelerated to approximately 70 km/h. Fearing the low airspeed and proximity to the terrain, the glider pilot executed a release. Shortly after the glider disconnected, the Piper J3C lost airspeed, entered a spin, and struck a rocky outcrop near the Mont d’Orge. The impact caused the aircraft to burst into flames, and the pilot was killed.
The investigation
The investigation examined the mechanical condition of the aircraft, the pilot's health, and the flight parameters. Investigators found that the engine was in good working order and no mechanical failures were identified. The aircraft was trimmed heavily aft, a common practice among tow pilots to reduce control pressure, but this configuration contributed to the aircraft's tendency to pitch up at low speeds.
Toxicological analysis of the pilot revealed traces of solvents, including acetone and ethanol, which the pilot had been exposed to while working in a hangar earlier that day. While these levels were not high enough to cause loss of consciousness, investigators could not rule out that they impaired the pilot's reaction speed.
Findings
- The primary cause of the accident was improper towing tactics, which led to a loss of airspeed and a subsequent spin.
- The aircraft was trimmed in a fully aft position, which, combined with the low speed, caused the aircraft to pitch up and stall.
- The pilot may have been experiencing a reduced level of responsiveness due to the inhalation of toxic solvent vapors prior to the flight.
- The Piper J3C was being used for towing operations despite not being officially certified for such use.
Safety action
- The investigation noted that the aircraft was being operated for towing without the required authorization, despite having the necessary hardware (release mechanism and indicator) installed.