What aircraft was involved and where did it happen?

The accident on 1976-09-18 involved a LET, N.P. L-13, registration HB-1246, at Schaffhausen Flugplatz (LSPF), SH, CH.

What was the probable cause of the accident?

The accident was caused by the failure of the right aileron control linkage due to excessive aerodynamic loads, which led to a loss of lateral control. This was compounded by improper control inputs during an unauthorized aerobatic maneuver.

Control Failure Leads to Fatal Glider Crash in Schaffhausen

A training flight involving a Blanik L-13 ended in a fatal spiral dive after a mechanical failure in the control linkage caused a loss of aileron control.

What happened

On September 18, 1976, a flight instructor and a student were conducting aerobatic training maneuvers near the Schaffhausen airfield. During the third training flight of the day, the Blanik L-13, registered as HB-1246, performed a series of maneuvers including loops and a vertical pull-up. Following a maneuver described as a 'looping' or 'stall' figure at approximately 500 meters, the glider drifted backward for 50 to 100 meters. During this phase, the aircraft underwent a violent pitch-over. Shortly after recovering to a level attitude, a loud bang was heard by witnesses, and the aircraft entered a steep, high-speed right-hand spiral. The glider continued through several increasingly tight turns before impacting the ground at a high speed with a heavy bank angle. The impact resulted in two fatalities and the total destruction of the aircraft.

The investigation

Investigators examined the wreckage and the control systems of the Bliele L-13. The investigation focused on the mechanical state of the aileron linkage and the flight path of the aircraft. Physical evidence showed that the right wingtip had struck the ground first, causing a portion of the wing to tear away. Examination of the control rods revealed that a light-alloy clevis connection on the middle control rod had failed due to a tensile break. The investigation also reviewed the flight history of the instructor and the student, as well as the aircraft's maintenance and weight/balance records. Witnesses reported seeing the ailerons moving during the backward drift, and the investigation looked into whether the flight maneuvers performed were within the permitted flight manual specifications.

Findings

The primary cause of the accident was the loss of control over the aircraft due to the failure of the right aileron caused by a break in the control linkage.
The failure of the clevis connection was the result of overstressing the components, likely caused by aerodynamic forces acting on the ailerons during an improper control input.
The occurrence of a failed maneuver (specifically a 'loop' or 'stall' figure) contributed to the accident, as these specific maneuvers were not authorized by the aircraft's flight manual.
Improper control handling during an abnormal flight attitude likely contributed to the structural overstress of the control rods.
The aircraft's weight and center of gravity were within permissible limits at the time of the accident.