What happened
On July 6, 1978, an AS-202 Bravo, registration HB-HEU, departed from Birrfeld airfield for a scheduled aerobatic training session. The flight was intended to finalize the training of a student pilot in preparation for an upcoming examination. Approximately 18 minutes into the flight, witnesses observed the aircraft perform a series of maneuvers, including a vertical climb and a spin. As the crew attempted to recover from the spin, the right wing detached from the aircraft at an altitude of roughly 500 meters.
The aircraft entered a steep, 70-degree dive and crashed into a field approximately one kilometer south of the airfield. The impact resulted in two fatalities, involving both the instructor and the student. The aircraft was destroyed, though damage to the surrounding terrain was minimal.
The investigation
Investigators examined the wreckage and conducted extensive metallurgical testing through the Swiss Federal Institute of Materials Science (EMPA). The investigation focused on the structural integrity of the wing spar and the manufacturing quality of the components.
Technical analysis of the fracture site on the lower spar flange revealed a pre-existing fatigue crack. Metallurgical testing of the aluminum alloy (707 and 5) indicated that while the chemical composition met standards, the mechanical properties—specifically the yield strength and tensile strength—only partially met required specifications. Furthermore, the investigation found that the manufacturing of the spar flange did not conform to the original design drawings, specifically regarding the transition radius at the taper. This deviation created a stress concentration point that could facilitate crack formation.
Findings
- The primary cause of the accident was the separation of the right wing during a recovery maneuver.
- A pre-existing fatigue crack was present on the lower flange of the right wing spar.
- The manufacturing of the lower spar flange was non-compliant with design specifications, lacking the required transition radius.
- The mechanical properties of the material used in the spar flange were inconsistent with standard requirements.
- Contributing environmental and operational factors included wind gusts and potential structural overstressing during or prior to the flight.
- The aircraft was operating at its maximum allowable weight for aerobatic flight, which may have limited the ability to carry parachutes for the crew.