What happened
On the afternoon of August 26, 1966, a collision occurred at the Sion Aerodrome involving a Piper J-3 C, registration HB-OAV, and a Greif I glider, registration HB-579. The glider pilot, an instructor's student, was completing a solo flight after failing to find sufficient thermals. Meanwhile, the instructor was operating the Piper J-3 C to conduct training maneuvers with another student.
As the Piper J-3 C taxied toward the threshold of the grass runway to begin a takeoff, the Greif I glider was on its final approach for landing. The two aircraft's flight paths intersected approximately 15 to 20 meters above the runway, roughly 400 meters past the threshold. The impact caused both aircraft to be destroyed. The instructor in the Piper J-3 C and the glider pilot both succumbed to their injuries, while the student pilot in the powered aircraft suffered serious injuries.
The investigation
The investigation included a flight reconstruction conducted at the Sion Aerodrome in February 1967. Investigators examined the airfield's operational regulations, which had recently been modified to include a trial period for different traffic patterns. The inquiry also reviewed the flight data from the glider's barograph, which showed a steady descent rate increasing significantly in the final seconds before impact. Witnesses were interviewed, and the visibility conditions—noting that the glider pilot was flying directly toward the sun—were analyzed.
Findings
- The primary cause of the collision was that the pilot of the Piper J-3 C failed to yield right-of-way to the glider which was in the process of landing.
- The airfield was operating under a new, experimental traffic pattern that utilized different runway thresholds for powered aircraft and gliders.
- A specific derogation allowed for mixed traffic on the grass runway, which created a dangerous intersection of flight paths during takeoff and landing.
- The glider pilot's visibility was likely compromised by sun glare and light refraction through the aircraft's canopy during the approach.
- The instructor, while observing student maneuvers from the ground, failed to maintain sufficient surveillance of the approaching glider.