About an hour after takeoff from Dulles International Airport yesterday morning, Flight 77, a Boeing 757 headed for Los Angeles with 64 people aboard, became a massive missile aimed at the White House. The target would change suddenly, but the symbolism was equally devastating. The diving plane carved out a massive chunk of the Pentagon. The unidentified pilot executed a pivot so tight that it reminded observers of a fighter jet maneuver. The plane circled 270 degrees to the right to approach the Pentagon from the west, whereupon Flight 77 fell below radar level, vanishing from controllers' screens. Aviation sources said the plane was flown with extraordinary skill, making it highly likely that a trained pilot was at the helm, possibly one of the hijackers. Someone even knew how to turn off the transponder, a move that is considerably less than obvious.