What happened
On July 22, 1979, during a glider training camp near Zweisimmen, Switzerland, two gliders were involved in a mid-air collision. The first aircraft, a B4 with registration HB-1139, was flying at an altitude of approximately 2,300 meters alongside five other gliders near the Gandhore ridge. At approximately 16:15 local time, the Astir CS with registration HB-133 and 2 entered the same area from the south, heading toward an updraft zone.
As the pilot of the Astir CS completed a wide right-hand turn, they observed other gliders flying at lower altitudes but failed to notice the B4 approaching at a similar level. Simultaneously, the B4 was executing a right-hand turn toward the north. The pilot of the Astir CS attempted an evasive maneuver to the left, but it was too late. The left wing of the Astir CS struck the left wing and the horizontal stabilizer of the B4.
Following the impact, the pilot of the Astir CS managed to land safely at the Zweisimmen airfield with only minor damage to the aircraft. The pilot of the B4 chose to land at the Saint-Stéphane military airfield because the aircraft had become difficult to maneuver and required a longer runway.
The investigation
The investigation examined the flight paths, the weather conditions, and the medical fitness of the pilots. At the time of the accident, the weather was characterized by a high-pressure system with visibility exceeding 20 km.
Investigators reviewed the medical records of the pilot of the Astir CS. It was discovered that the pilot's visual acuity was below the required regulatory limits for flight operations. Specifically, while the pilot's right eye was functional, the left eye had significantly impaired vision. This medical deficiency had not been communicated to the medical examiner, and the pilot was not required to wear corrective lenses during flight.
Findings
- The primary cause of the collision was insufficient observation of the airspace by the pilots.
- The pilot of the Astir CS likely failed to detect the B4 because the aircraft was flying at a similar altitude and may have been obscured by a blind spot created by the instrument panel or masked against a background of grey-white clouds.
- The pilot's ability to estimate the distance to the B4 was severely compromised due to highly restricted binocular vision.
- A contributing factor was the pilot's unrecorded visual impairment, which meant the pilot was flying without the necessary corrective lenses required to meet the minimum standard of 1.0 visual acuity.