May 18, 2011 -- United Airlines stopped using the flight numbers 93 and 175 after its planes with those flight numbers were used in the terrorist attacks on the United States on Sept. 11, 2001.

9 a.m. ET (approximately): A second plane (United Airlines Flight 175, a Boeing 767 headed from Boston to Los Angeles with 56 passengers, 7 flight attendants and 2 pilots) strikes Two World Trade ...