Airworthiness Directives; Air Tractor, Inc. Models AT-300, AT-400, AT-400A, AT-401, AT-401B, AT-402, AT-402A, AT-402B, AT-501, AT-502, and AT-502B Airplanes

This amendment adopts a new airworthiness directive (AD) that applies to certain Air Tractor, Inc. (Air Tractor) Models AT-300, AT- 400, AT-400A, AT-401, AT-401B, AT-402, AT-402A, AT-402B, AT-501, AT- 502, and AT-502B airplanes. This AD requires you to repetitively inspect the vertical fin front spar fitting for cracks and replace any cracked fitting found. This AD also requires you to install a steel doubler as a terminating action for the repetitive inspections. This AD is the result of a report of failure of a \1/4\-inch thick vertical fin front spar fitting. The actions specified by this AD are intended to prevent failure of the vertical fin front spar fitting, which could result in failure of the rear spar fitting. Such failures could lead to loss of directional control of the airplane.