Air Force One is the official air traffic control call sign of a United States Air Force aircraft carrying the President of the United States. In common parlance the term refers to those Air Force aircraft specifically designed, built, and used for the purpose of transporting the president. The Presidential aircraft is a prominent symbol of the American presidency and its power. The idea of designating specific military aircraft to transport the President arose in 1943, when officials of the United States Army Air Forces – the predecessor to the U.S. Air Force – became concerned with relying on commercial airlines to transport the President. A C-87 Liberator Express was reconfigured for use as a presidential transport; however, it was rejected by the Secret Service amid concerns over the aircraft's safety record. A C-54 Skymaster was then converted for presidential use; this aircraft, dubbed the Sacred Cow, transported President Franklin D. Roosevelt to the Yalta Conference in February 1945, and was subsequently used for another two years by President Harry S. Truman.