61-08-01 DOUGLAS: Amdt. 273 Part 507 Federal Register April 8, 1961. Applies to All DC-6 and DC-7 Series Airplanes Which Do Not Have Antifuel Transfer Check Valves Installed In The Fuel System. \n\n\tCompliance required within the next 330 hours' time in service. \n\n\t(a)\tInstall a placard in the flight compartment in full view of the pilot to read as follows: "Fuel Transfer in Flight Prohibited." \n\n\t(b)\tRevise the operations procedures section covering fuel system management in the FAA approved airplane flight manual to incorporate fuel system operation procedures in accordance with the above placard and delete the item which states that check valves are installed. Flight manual revisions must be approved by FAA. \n\n\tWhen check valves to prevent transfer of fuel are installed, this AD is no longer applicable. \n\n\t(Douglas Service Bulletin No. DC-6-294 dated November 19, 1948, revised February 17, 1950, covers an acceptable installation of check valves.) \n\n\tThis directive effective May 9, 1961.