Airworthiness Directives; Air Tractor, Inc. AT-400, AT-500, and AT-800 Series Airplanes

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This document proposes to supersede Airworthiness Directive (AD) 2001-10-04 R1, which lowered the safe life for the wing lower spar cap on certain Air Tractor, Inc. (Air Tractor) AT-400, AT-500, and AT-800 series airplanes. AD 2001-10-04 R1 resulted from numerous reports of cracks in the \3/8\-inch bolthole of the wing lower spar cap on the affected airplanes. This proposed AD would maintain the safe life and add a requirement for you to eddy--current inspect the wing lower spar cap immediately prior to the replacement/modification in order to detect and correct any crack in a bolthole before it extends to the modified center section of the wing and repair that crack or replace the wing section. This proposed AD would also further reduce the safe life for certain AT-400 and AT-500 series airplanes that incorporate or have incorporated Marburger Enterprises, Inc. winglets. These winglets are installed in accordance with Supplemental Type Certificate (STC) SA00490LA. The actions specified by this proposed AD are intended to prevent fatigue cracks from occurring in the wing lower spar cap before the originally-established safe life is reached. Fatigue cracks in the wing lower spar cap