U. S. PLANE SHOT DOWNor1WASHINGTON, March 31.—(AP) —British fighter planes from an aircraft 'carrier shot down a U, S. Army transport airplane and its crew of six by error in the Atlantic this week, the war departments and the British admiralty announced [jointly today.The fighters .were protecting an Allied convoy, said the announcement, and apparently mistook the four-engine C-54 transport; plane for a German Fbcke-Wulf 200 bomber when identification was not established as the transport plane approached the convoy.There were no passengers on the plane, which was loaded with cargo. It* was flown by a civilian contract crew, all of whom were lost. They, were:Crew .Captain Charles S. Garber, Arlington, Va.; First Officer George Edward Shelton. Alexandria. Va.: