The first launch of the Space Shuttle occurred on 12 April 1981, exactly 20 years after the first crewed space flight, when the orbiter "Columbia", with two crew members, astronauts John W. Young, commander, and Robert L. Crippen, pilot, lifted off from Pad A, Launch Complex 39, at the Kennedy Space Center. The launch took place at A launch attempt two days earlier was scrubbed because "Columbia"'s four primary general purpose IBM System/4 Pi computers (GPCs) failed to provide correct timing to the backup flight system (BFS) when the GPCs were scheduled to transition from vehicle checkout to flight configuration mode.