What happened
On January 5, 1999, a Bücker Bü-133 BM Antarès, registration HB-MKM, was performing a series of aerobatic maneuvers near the village of Abtwil, Switzerland. The flight was intended to be a celebratory flight for a Swiss Army company, involving a formation flight with a Boeing Stearman.
During the performance, the pilot executed a sequence of maneuvers, including a series of left-hand snap rolls performed in an inverted position. As the aircraft moved through these maneuvers, its flight path drifted toward rising terrain. While flying inverted, the pilot lost situational awareness regarding the approaching landscape. Upon leveling the aircraft, the left wing struck a pear tree, causing the aircraft to spin and collide violently with the facade of an old dairy building. The impact caused the aircraft to burst into flames. Although bystanders and the building owner managed to pull the pilot from the wreckage before the fire consumed the plane, the pilot later succumbed to his injuries.
The investigation
The investigation by the Swiss Transportation Safety Investigation Board (SUST) focused on the pilot's flight authorization and the environmental factors at the crash site. Investigators established that while the pilot held a special permit for low-altitude aerobatics, this permit was strictly limited to specific zones such as airfields, bodies of water, or designated public event areas, and required the area to be free of spectators. The flight in question took place over terrain that did not meet these criteria, and no specific authorization from the Federal Office of Civil Aviation (FOCA) had been obtained for this location.
Investigators also examined the pilot's preparation. It was found that the pilot had not performed a ground reconnaissance of the area prior to the flight. Furthermore, while the pilot had practiced the maneuvers two days earlier, the investigation noted that the terrain's undulating and wooded nature made altitude estimation difficult during high-intensity maneuvers.
Findings
- The pilot was highly experienced in aerobatics and held valid licenses, including an aerobatic endorsement.
- The aircraft, HB-MKM, was airworthy and showed no signs of mechanical failure.
- The pilot performed the maneuvers at an altitude significantly lower than the legal minimum of 500m AGL, likely even dropping below 100m AGL.
- The primary cause of the accident was the execution of aerobatic maneuvers at an insufficient altitude over complex terrain.
- The lack of ground reconnaissance meant the pilot was unaware of the rising, wooded terrain directly in his flight path.
- The aircraft's flight path drifted approximately 30 degrees during the snap rolls, moving the aircraft toward the obstacle.
- Limited visibility while flying in an inverted position contributed to the pilot's inability to react to the approaching trees.