What happened
On April 7, 1972, at approximately 18:30, a pilot initiated a flight from Grenchen Airport in a Bücker-Jungmeister, registration HB-MKE, for the purpose of a technical inspection and low-level aerobatic practice. After departing from runway 25, the pilot performed a steep climb to roughly 100 meters and proceeded with several aerobatic maneuvers.
During the sequence, the pilot executed a half-loop. Due to insufficient airspeed at the apex of the maneuver, the aircraft entered an inverted descending flight. In an attempt to recover, the pilot applied significant elevator and left rudder input. This caused the aircraft to snap into a normal flight attitude while simultaneously deviating approximately 30 degrees to the right of the runway axis. The aircraft's longitudinal axis was pitched forward at an angle of approximately 40 degrees.
In an effort to arrest the descent and return to the runway centerline, the pilot initiated a left turn. However, the aircraft continued to descend at a high rate with low forward velocity. At approximately 18:35, the Bücker-Jungmeister struck the ground at a high angle of attack with a 15-degree left bank, subsequently flipping over. The impact resulted in the death of the pilot and the total destruction of the aircraft. No third-party damage was reported.
The investigation
SUST investigators examined the aircraft, the pilot's records, and the flight conditions. The investigation confirmed that the Bücker-Jungmeister was properly maintained and that all control linkages, cables, and pulleys showed no signs of prior defect. The engine had functioned correctly throughout the flight.
The pilot held valid professional and aerobatic licenses, including a special permit for low-level aerobatics. Medical examinations performed after the accident confirmed the pilot was sober and had no underlying health conditions that would have unexpectedly impaired flight performance, though investigators noted a physiological sensitivity to G-forces due to low blood pressure. Flight tests using similar aircraft demonstrated that the Bücker-Jungmeister has a natural tendency to roll from an inverted position into a normal flight attitude when airspeed is low, often resulting in a nose-down pitch.
Findings
- The primary cause of the accident was that the aircraft entered an excessive flight state during low-level aerobatics.
- The altitude available during the maneuver was insufficient to allow the pilot to recover the aircraft in time.
- The airspeed at the top of the half-loop was likely near the stall speed for inverted flight.
- The pilot's corrective inputs (elevator and rudder) contributed to the aircraft's loss of airspeed and subsequent descent.
- The pilot's recent lack of aerobatic training and physiological sensitivity to G-forces may have contributed to a potential momentary loss of consciousness, though this could not be definitively proven.