Airworthiness Directives; Air Tractor, Inc. Models AT-502, AT-502A, AT-502B, and AT-503A Airplanes

This amendment adopts a new airworthiness directive (AD) that applies to certain Air Tractor, Inc. (Air Tractor) Models AT-502, AT- 502A, AT-502B, and AT-503A airplanes. This AD lowers the safe life for the wing lower spar cap established in AD 2001-10-04 R1 and further reduces the safe life for airplanes that incorporate or have incorporated Marburger Enterprises, Inc. winglets. This AD also requires you to eddy-current inspect the wing lower spar cap immediately prior to the replacement/modification to detect and correct any crack in a bolthole before it extends to the modified center section of the wing and report the results of this inspection to the Federal Aviation Administration (FAA). This AD is the result of reports of several cracks originating in the outboard \3/8\-inch hole of the main spar lower cap on Air Tractor Models AT-502, AT-502A, AT-502B, and AT-503A airplanes at times lower than the established safe life. The actions specified by this AD are intended to prevent fatigue cracks from occurring in the wing lower spar cap before the established safe life is reached. Fatigue cracks in the wing lower spar cap, if not detected and corrected, could result in the wing separating from the airplane during flight.