What happened
On March 20, 1966, at approximately 15:25 CET, a mid-air collision occurred at the Beromunster airfield in Switzerland involving two Piper Super-Cub aircraft, registrations HB-OYB and HB-OYC.
The incident began during a glider towing operation. The pilot of the towing aircraft, HB-OYB, successfully released the tow cable after a flight pattern. Following instructions, the pilot attempted a shortened left-hand pattern to prepare for landing. During this maneuver, the pilot failed to observe an aircraft already on its final approach.
Simultaneously, the pilot of HB-OYC was executing a standard left-hand circuit for landing. As the pilot of HB-OYC prepared to land, they observed ground personnel waving a red signal flag, prompting an attempt to go around. At an altitude of approximately 10 to 15 meters, the two aircraft collided and became interlocked. The impact caused both planes to enter a steep left-hand bank before crashing near the runway threshold. The pilot of HB-OYC sustained serious injuries, while the pilot of SB-OYB suffered minor injuries. Both aircraft sustained heavy damage, estimated at 80% of their value.
The investigation
Investigators examined the flight paths and visibility conditions at the time of the accident. Reconstructions of the flight trajectories indicated that the pilot of HB-OYB had the other aircraft within their field of view for approximately 40 seconds prior to the collision. The pilot of HB-OYC also had the opportunity to see the towing aircraft for roughly 40 seconds, though the visibility of HB-OYB was likely obscured by the wing of HB-OYC during the final 8 to 10 seconds of the approach.
The investigation also looked into the recent medical history of the pilot of HB-OYB, who had sustained a head injury from a flying stone four days prior to the accident. While it was noted that the pilot had experienced headaches on the morning of the accident, it could not be definitively determined if this affected their performance.
Findings
- The primary cause of the collision was objectively inadequate airspace surveillance, particularly by the pilot of the towing aircraft, HB-OBC.
- The pilot of HB-OYB likely focused too heavily on the execution of the shortened flight pattern, neglecting to ensure the landing sector was clear.
- The pilot of HB-OYC, while having the right of way, was primarily focused on ground operations and the landing itself, which hindered their ability to monitor the critical sector.
- Sunlight positioned perpendicular to the left of the approach axis may have further complicated the pilot's ability to monitor the incoming aircraft during the final seconds of the approach.