Incident Overview
A pilot was applying a chemical to a field. After completing a pass, as aft elevator was applied for a straight-ahead pull-up, a loud 'boom' was heard, and the outboard section of the right wing separated. The airplane became 'uncontrollable' and impacted in an uncultivated field.
Metallurgical Findings
Metallurgical examination revealed that the separation of the right wing forward spar lower attachment fitting was the result of fatigue cracking in the bracket lugs. The fatigue cracking initiated from areas of severe corrosion damage in the lugs. Evidence of extensive corrosion pitting was found in brackets from all other examined front spar attachment fittings, particularly on the internal surfaces of the lugs.
Inspection History
The Service Manual and applicable service bulletins call for fluorescent penetrant or magnetic particle inspection of wing attachment fittings during the 3,000-hour inspection for extension of the airframe life to 6,000-hours. Aircraft records show that during the last annual inspection, completed approximately 596.5 hours prior to the accident at an airframe time of 4,245.5 hours, all time life items were complied with. However, the metallurgical examination disclosed no trace of dye penetrant residue on any of the examined components.