Christopher A. Hart, acting chairman of National Transportation Safety Board(NTSB) speaks at a press conference in Washington D.C., the United States, on June 24, 2014. US federal accident investigators on Tuesday concluded that "mismanagement" by the pilots of Asiana Flight 214, including confusion over whether one of the airliner's key controls was maintaining airspeed, caused the plane to crash while landing in San Francisco last year. [Photo/Xinhua]