What happened
On June 6, 1965, at approximately 15:51 CET, a flight instructor and a student were conducting aerobatic training at the Bellechasse airfield in Switzerland. The aircraft, a DFS 108-30 Kranich with registration HB-477, was performing a series of maneuvers. During the third flight of the afternoon, the instructor took control of the aircraft at an altitude of 620 meters to demonstrate a loop to the student.
As the aircraft reached the bottom of the loop at roughly 500 meters above ground level, the pilot began pulling out of the maneuver. At an angle of approximately 30 degrees from the horizon, the right wing failed at the main spar fittings. The aircraft entered an uncontrolled descent, crashing into a grain field approximately 420 meters from the airfield. While the student was able to exit the cockpit and deploy a parachute, the instructor remained in the seat and was killed upon impact.
The investigation
Investigators examined the wreckage and the aircraft's technical records to determine the cause of the structural failure. Analysis of the debris by the Swiss Federal Institute of Materials Science (EMPA) revealed that the failure originated as a tension failure in the lower main spar web of the right wing, specifically near the outermost connection bolts. This was followed by a shear failure along the wood fibers, leading to the complete separation of the lower spar from the fitting. The upward rotation of the right wing was so violent that it also caused the upper spar to break and crushed the rear cockpit area.
Technical inspections of the aircraft's weight and balance revealed that the aircraft was significantly overweight for the intended maneuver. The total weight was approximately 495 kg, which exceeded the maximum allowable weight for aerobatic flight by 45 kg. Additionally, the center of gravity was found to be 24 mm forward of the allowable forward limit. The investigation also noted that the aircraft was not equipped with an accelerometer, which is a requirement for the permitted aerobatic maneuvers.
Findings
- The primary cause of the accident was the exceedance of the structural load limit during the pull-out from the loop.
- The aircraft was overloaded for aerobatic operations, which reduced the allowable load factor from +4g to approximately +3.6g.
- The structural strength of the aircraft was further compromised by the weight exceeding the aerobatic limit by 45 kg.
- The absence of an accelerometer prevented the pilot from monitoring G-loads during the maneuver.
- The instructor's inability to escape the aircraft was likely due to the cockpit structure being crushed by the upward movement of the wing, which also caused significant head injuries during the breakup.