Accident Sequence
On Saturday, 18 September 1976, at 16:26, a flight instructor and an aerobatics student took off in a LET Blanik L-13 glider (registration HB-1246) from Schaffhausen airfield (runway 07) on an aerobatic instruction flight. After a normal aerotow, the glider released at approximately 1500 m MSL (about 1000 m AGL) near the airfield. The pilot performed several renversements and a "Männchen" (loop-like maneuver). While pulling vertical at about 500 m AGL, the glider remained vertical and then slid backward 50 to 100 m. During this phase, a witness observed that the left aileron was deflected, while another witness saw both ailerons moving. The glider then experienced a violent pitchover; due to the high rotation rate, witnesses could not determine whether it was forward or backward. After this upset, the glider returned to a normal attitude. At approximately this moment, witnesses located near the old hangar and the eastern end of the airfield heard a loud noise. One witness described: "With this sound, something changed on the aircraft. In my opinion, it was flaps. Nothing fell off." The HB-1246 then flew straight for about two seconds before entering a right spiral with a steep bank at about 300 m AGL. Witnesses observed that the landing flaps were extended and possibly the airbrakes were deployed. After two or three steep turns, the rotation rate decreased temporarily but then increased again. Following about two more irregular, increasingly steep spirals with rising speed, the glider struck the ground at 16:37. The impact site was 400 m north of the eastern end of the airfield, at an elevation of 438 m MSL. The glider came to rest upside down, with the rear fuselage twisted about 90° and the forward fuselage rotated 90° clockwise. The first ground contact was made by the right wing tip at a bank angle of 75-80°, tearing off the outer wing section.
Damage and Injuries
The two occupants were killed by the impact. The glider was destroyed. Minor damage to the ground (crops) occurred.
Investigation Findings
The right wing was correctly attached and secured. The Fowler flaps on the left wing were extended, while those on the right wing were almost retracted. The airbrakes on both sides were slightly open. Examination of the right wing's control system revealed a fracture in the yoke (Gabelstück) connecting the aileron pushrod to the bellcrank; the threaded portion of the yoke had broken. Additionally, the steel joint at the aft end of the pushrod was bent to the left relative to the flight direction. No pre-existing defects were found in the flap and airbrake actuation components. The glider's weight and center of gravity were within limits.
Probable Cause
The investigation concluded that the probable cause was loss of control due to failure of the right aileron because of a break in the control linkage. A contributing factor was inappropriate control inputs in an abnormal flight attitude.