Transponder (aeronautics) in the context of Airborne Collision Avoidance System

In telecommunications, a transponder is a device that, upon receiving a signal, emits a different signal in response. The term is a blend of transmitter and responder.

In air navigation or radio frequency identification, a flight transponder is an automated transceiver in an aircraft that emits a coded identifying signal in response to an interrogating received signal.

United Airlines Flight 175 was an American domestic passenger flight from Logan International Airport in Boston to Los Angeles International Airport in California that was hijacked by five al-Qaeda terrorists on the morning of September 11, 2001, as part of the September 11 attacks. The aircraft involved, a Boeing 767-200 carrying 51 passengers and 9 crew members (excluding the 5 hijackers), was deliberately crashed into the South Tower of the World Trade Center in New York City, killing everyone aboard and causing the deaths of more than 600 people in the South Tower's upper levels in addition to an unknown number of civilians and emergency personnel on floors beneath the impact zone. Flight 175 is the second-deadliest plane crash in aviation history, surpassed only by American Airlines Flight 11.

Flight 175 departed from Logan Airport at 08:14. Twenty-eight minutes into the flight, the hijackers injured several crew members, forced their way into the cockpit, and murdered both pilots while moving anyone who remained to the rear of the aircraft. Lead hijacker Marwan al-Shehhi, who had trained as a pilot for the purposes of the attacks, was able to usurp the flight controls once the hijackers killed the captain and first officer. Unlike the team on American Airlines Flight 11, the terrorists aboard Flight 175 did not switch off the plane's transponder when they took over the cockpit. Thus, the aircraft was visible on New York Center's radar, which depicted the deviation from its assigned flight path before controllers took notice four minutes later at 08:51 EDT. Upon realizing, the ATC workers immediately made several unsuccessful attempts to contact the cockpit of the hijacked airliner, which twice nearly collided with other planes as it recklessly flew toward New York City. In the interim, three people were able to get through to their family members and colleagues on the ground, passing on information to do with the hijackers as well as casualties suffered by the flight crew.