What happened
On March 20, 1966, a Minicab GY-20-1, registration HB-SOX, departed from Birrfeld airfield for a private flight with one passenger on board. During the initial climb, the aircraft struggled to maintain altitude and descended rapidly. At an altitude of approximately 15 to 20 meters, the aircraft entered a steep dive, striking a path before crashing into a meadow. The impact resulted in two serious injuries, and the passenger subsequently succumbed to their injuries one month later.
The investigation
The investigation examined the aircraft's technical condition, the pilot's experience, and the flight parameters. Technical inspections of HB-SOX revealed no mechanical failures, noting only a minor fuel valve leak that did not contribute to the accident. The investigation focused on the aircraft's weight, as the takeoff weight was calculated at approximately 545 kg, which exceeded the maximum allowable weight of 515 kg by 30 kg.
Investigators also reviewed the pilot's recent flight history, noting that while the pilot had significant total experience, his training in the preceding months had been minimal. Furthermore, the investigation looked into the flight path, noting that the pilot failed to correct a leftward drift during the takeoff roll, leading the aircraft toward a wooded area known as Bleicherhölzli.
Findings
Several contributing factors led to the loss of control. The aircraft was overloaded, which significantly impacted its climb performance. The investigation established that the pilot utilized an inappropriate flight tactic by failing to maintain an adequate speed reserve. Specifically, the pilot did not account for the fact that the optimal climb speed must be significantly higher than the minimum stall speed.
As the aircraft entered the turbulent lee of the nearby trees, the lack of airspeed caused the plane to enter a "sink" condition. The combination of the excess weight, the pilot's failure to maintain the runway centerline, and the insufficient airspeed during the climb resulted in the fatal loss of altitude.