Airworthiness Directives; Air Tractor, Inc. AT-400, AT-500, AT-600, and AT-800 Series Airplanes

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We are revising an earlier proposed airworthiness directive (AD) that applies to certain Air Tractor, Inc. (Air Tractor) AT-400, AT-500, AT-600, and AT-800 series airplanes. The earlier NPRM proposed to supersede Airworthiness Directive (AD) 2007-13-17, which applies to certain Air Tractor Models AT-602, AT-802, and AT-802A airplanes. AD 2007-13-17 currently requires you to repetitively inspect the engine mount for any cracks, repair or replace any cracked engine mount, and report any cracks found to the FAA. The earlier NPRM proposed to retain the inspection actions of AD 2007-13-17 for Models AT-602, AT-802, and AT-802A airplanes, including the compliance times and effective dates; establish new inspection actions for the AT-400 and AT-500 series airplanes; incorporate a mandatory terminating action for all airplanes; and terminate the reporting requirement of AD 2007-13-17. The earlier NPRM resulted from a Model AT-502B with a crack located where the lower engine mount tube is welded to the engine mount ring, and the manufacturer developing gussets that, when installed according to their service letter, terminate the repetitive inspection requirement. Since issuance of the NPRM,