COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Aerospatiale (Socata) TB20 (Trinidad) Series Aeroplanes AD/TB20/18 Fuel System 10/90 Applicability: Model TB20 and TB21 with serial numbers 1 through 730. Requirement: For model TB20: Install a check valve into the fuel system in accordance with Socata Service Bulletin number 47/1 and kit 9128. For model TB20 and TB21: Replace the Dukes pump P/N 463400 with a Weldon pump P/N TB20/6101/5000 (TB20) or P/N TB20/6101/5003 (TB21) respectively in accordance with kits 9132 and 9133. Note: DGAC AD 89-183-(A)R1 refers. Compliance: Prior to 31 October 1991. Background: The inability of the normally fitted electric fuel boost pump to operate in a dry condition if the engine driven fuel pump loses its prime when the engine is stopped, could result in air being drawn into the fuel lines causing power interruption or loss. Installation of a check valve will prevent fuel from siphoning back into the fuel tank when the engine is stopped and the fuel selector is not in the off position; and installation of an electric fuel boost pump which incorporates a solid rotor design, will displace air in the system within its operating parameters.