Airworthiness Directives; Air Tractor, Inc. Models AT-250, AT- 300, AT-301, AT-302, AT-400, AT-400A, AT-401, AT-402, AT-501, and AT- 502 Airplanes

This amendment adopts a new airworthiness directive (AD) that applies to certain Air Tractor, Inc. (Air Tractor) Models AT-250, AT- 300, AT-301, AT-302, AT-400, AT-400A, AT-401, AT-402, AT-501, and AT- 502 airplanes that are equipped with a Gerdes part number (P/N) A-850-5 or Cleveland P/N 60-9 parking brake valve. This action requires replacing the parking brake valve with a Scott P/N 4500A-2 parking brake valve. This AD results from several reports of the parking brake valve inadvertently slipping to the ``PARK'' position during flight, which causes constant pressure on the brakes. When the pilot applies the brake upon landing, this pressure causes the airplane to overturn. The actions specified by this AD are intended to prevent the airplane from overturning because of extreme pressure applied to the brake if the parking brake valve inadvertently slips to the ``PARK'' position during flight.