What happened
On July 3, 1966, a mid-air collision occurred at Altenrhein Airfield involving two light aircraft. The first aircraft, a Cessna 150 with registration HB-CMV, was performing a passenger flight. The pilot decided to execute a direct approach to runway 28 to land as quickly as possible for the comfort of a passenger. Simultaneously, a Cessuna 172, registration HB-CBS, was completing a standard circuit after a training session.
As the two aircraft descended toward the runway, they collided approximately 10 meters above the ground. The impact caused the two planes to become interlocked. Despite the severe impact, the combined wreckage remained somewhat flightworthy and descended to the ground. The occupants of the Cessna 150 sustained injuries, while the pilot of the Cessna 172 escaped without injury. The Cessna 150 was a total loss, and the Cessna 172 suffered heavy damage.
The investigation
Investigators reconstructed the flight paths of both aircraft to determine the proximity and visibility during the final minutes of flight. The reconstruction suggested that while the aircraft were initially within each other's view, they moved into blind spots created by the aircraft structures (such as door pillars and wing roots) as they approached.
Testing conducted in Bern with similar aircraft models confirmed that unfavorable factors—including the distance between the planes, the sun's position, and the dark background of the terrain—made detection extremely difficult even with active scanning. The investigation also examined the local airfield regulations, noting that the rules regarding approach procedures were ambiguous at the time of the accident.
Findings
- The pilot of the Cessna 150 utilized a direct approach rather than joining the standard circuit, which reduced the time available for traffic observation.
- Both pilots failed to detect the other aircraft despite performing airspace surveillance.
- The low sun angle and the visual difficulty of spotting aircraft against a dark landscape contributed to the lack of detection.
- The airfield regulations at Altenrhein were ambiguous regarding whether aircraft departing the field were required to fly a partial circuit before landing.
Safety action
Following the accident, the Swiss Federal Office of Civil Aviation clarified the regulations. A new rule was implemented stating that if no two-way radio communication is established with the airfield traffic control, an aircraft must fly at least a half-circuit before attempting to land, ensuring better visibility and traffic integration.