What happened
On April 20, 2010, a training flight involving an instructor and a student commenced at the Königsdorf glider airfield. The flight began with a winch launch from runway 28. During the initial climb, at an altitude of approximately 100 meters above ground level, the cockpit canopy suddenly opened with a loud bang.
Upon the canopy opening, the instructor took control of the aircraft, released the winch cable, and initiated a right turn toward the airfield. The crew attempted to close the canopy while in flight, but these efforts were unsuccessful. The instructor subsequently performed a landing on a field north of the airfield. The Alexander Schleicher ASK 13 touched down in a nose-down attitude, causing the skid to strike the ground. The aircraft slid for approximately ten meters before rotating 90 degrees to the right and coming to a halt. The student pilot, seated in the front, sustained one serious injury to the thoracic spine.
The investigation
The BFU examined the aircraft's canopy locking mechanism to determine why the canopy opened unexpectedly. The investigation focused on the function of the two locking points and the forces required to engage and disengage them. Physical inspection of the aircraft revealed a tear in the skid fairing.
Findings
Investigation of the locking mechanism revealed that when the canopy was placed onto the fuselage frame, the locking bolts would slip approximately one to two millimeters into the locking eyes. This displacement prevented the canopy from being lifted, yet it also meant that a proper, secure engagement of the locks was not achieved.
While the mechanism could be closed by applying approximately 78 N of force, and the force required to open the locks was 123 N, the improperly seated locking bolts meant the canopy was not correctly secured. Although the mechanism was capable of preventing accidental opening when properly locked, the specific condition of the bolts during the pre-flight phase prevented a secure closure.