COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Airtractor AT-300, 400 and 500 Series Aeroplanes AD/AT/18 Parking Brake Valve 5/97 Applicability: All model AT-250, AT-300, AT-301, AT-302, AT-400, AT-400A, AT-401, AT-402, AT-501 and AT-502 series aircraft not fitted with Scott P/N 4500A-2 parking brake valve. Requirement: Unless previously carried out, replace Gerdes P/N A-850-5 or Cleveland P/N 60-9, as applicable, parking brake valve with Scott P/N 4500A-2 parking brake valve in accordance with Snow Engineering Co. Service Letter #76, dated December 12, 1988. Note: FAA AD 96-24-08 Amdt 39-9833 refers. Compliance: At next scheduled maintenance release issue after 31 May 1997, or by 31 December 1997, whichever occurs first. This Airworthiness Directive becomes effective on 24 April 1997. Background: Advice has been received that during flight the parking brake valve lever can slip from the OFF to the ON position and has caused several aircraft to overturn when landing.