What happened
On September 18, 1976, a Blanik L-13 glider, registration HB-1246, was performing aerobatic training maneuvers near the Schaffhausen airfield. The flight, which included a flight instructor and a student, involved several maneuvers, including loops and rolls. During the sequence, the aircraft performed a vertical climb that resulted in the glider stalling and sliding backward for approximately 50 to 100 meters.
Following this stall, the aircraft underwent a violent pitch-over. Witnesses reported hearing a loud bang, after which the aircraft entered a steep, high-speed right-hand spiral. The glider continued in this descending spiral for several turns before striking the ground at a high velocity with a heavy bank angle of 75 to 80 degrees. The impact caused the destruction of the aircraft and resulted in two fatalities.
The investigation
The investigation focused on the mechanical integrity of the control surfaces and the flight dynamics leading to the spiral. Investigators examined the wreckage and found that the right wingtip and a portion of the aileron had been torn away upon impact. Crucially, the investigation identified a broken connection in the right aileron control linkage. Specifically, a light-alloy clevis at the end of the middle control rod had failed due to extreme stress.
Investigators also reviewed the flight history and training protocols. It was noted that the aircraft's weight and center of gravity were within permissible limits and the weather conditions were favorable. The investigation also looked into the possibility of improper pilot input during the failed maneuver and examined previous similar incidents involving broken control linkages in other Blanik aircraft.