Airworthiness Directives; Air Tractor, Inc. Models AT-400; AT-401, AT-401B, AT-402, AT-402A, and AT-402B Airplanes

The FAA is adopting a new airworthiness directive (AD) for certain Air Tractor, Inc. (Air Tractor) Models AT-400, AT-401, AT-401B, AT-402, AT-402A, and AT-402B airplanes. This AD requires you to lower the safe life for the wing lower spar cap for certain Models AT-402A and AT-402B airplanes and those that incorporate or have incorporated Marburger Enterprises, Inc. (Marburger) winglets. For Models AT-400, AT-401, AT-401B, AT-402, and certain AT-402A, airplanes, this AD requires you to repetitively inspect the wing lower spar cap in order to reach the safe life. We also developed an alternative method of compliance (AMOC) to the requirements of this Ad for certain Models AT- 402A and AT-402B airplanes. The AMOC includes repetitive eddy current inspections, modification of the center splice connection, and lower spar cap replacement. This AD is the result of reports of cracks in the \3/8\-inch bolt hole of the wing lower spar cap before reaching the approved safe life. We are issuing this AD 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, if not detected and corrected, could result in wing separation and loss of control of the airplane.