U.S. airmen observe aircraft assigned to the 60th Air Mobility Wing line up prior to takeoff at Travis Air Force Base, Calif., Sept. 11, 2013, as part of the Freedom Launch honoring the victims of the 9/11 terrorist attacks. The first plane departed at 8:46 a.m., the same time terrorists crashed American Airlines Flight 11 into the North Tower of the World Trade Center, with the remaining 21 aircraft launching consecutively during a 36-minute time frame. Terrorists hijacked four passenger aircraft Sept. 11, 2001. Two of the aircraft were deliberately crashed into the World Trade Center in New York; one was crashed into the Pentagon; the fourth crashed near Shanksville, Pa. Nearly 3,000 people died in the attacks.