NTSB Identification: CHI90FA117.
The docket is stored on NTSB microfiche number 42857.
Accident occurred Tuesday, April 24, 1990 in WEST BEND, WI
Probable Cause Approval Date: 01/23/1992
Aircraft: BELLANCA 8KCAB, registration: N2976R
Injuries: 2 Fatal.

NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.

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.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

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.

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