The term functions as a noun phrase, referring to the four specific commercial airliners that were hijacked by 19 al-Qaeda terrorists on the morning of September 11, 2001. These aircraft were commandeered shortly after takeoff from airports in the northeastern United States and were subsequently used as weapons in a coordinated series of terrorist attacks.
The four aircraft involved were: American Airlines Flight 11, a Boeing 767, which struck the North Tower of the World Trade Center; United Airlines Flight 175, also a Boeing 767, which struck the South Tower of the World Trade Center; American Airlines Flight 77, a Boeing 757, which was crashed into the Pentagon; and United Airlines Flight 93, a Boeing 757, which crashed into a field in Shanksville, Pennsylvania, after passengers and crew attempted to regain control from the hijackers.
The use of these aircraft as guided missiles marked a significant tactical shift in terrorism, transforming civilian transportation infrastructure into instruments of mass destruction. This event fundamentally altered global aviation security protocols, leading to the creation of the Transportation Security Administration (TSA), reinforced cockpit doors, and expanded federal air marshal programs, and it directly precipitated the U.S.-led War on Terror.
