WEST BEND, WI, USA
N2976R
BELLANCA 8KCAB
THE RIGHT WING SEPARATED FROM THE AIRCRAFT DURING AN INSTRUCTIONAL AEROBATIC FLIGHT. METALLURGICAL EXAMINATION REVEALED THAT THE RIGHT WING'S LIFT STRUT-TO-WING SPAR ATTACHMENT FITTING HAD FAILED DUE TO FATIGUE PROGRESSION IN BOTH SIDE PLATES OF THE FITTING. THE INITIATION AREAS WERE IMMEDIATELY ADJACENT TO THE ENDS OF TWO WELDS. IN THE WELD HEAT AFFECTED ZONE, THE MICROSTRUCTURE TRANSITIONED FROM A SPHEROIDIZED PEARLITE REGION NEAR THE BASE METAL INTO A COARSE TEMPERED MARTENSITE REGION ADJACENT TO THE FUSION ZONE.
INFLIGHT FAILURE OF THE RIGHT WING STRUT-TO-SPAR ATTACHMENT FITTING DUE TO FATIGUE INITIATED BY IMPROPER WELDING TECHNIQUES INVOLVING INADEQUATE POST-WELD COOLING CONTROL.
Source: NTSB Aviation Accident Database
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