Final answer:
The squawk code that indicates a hijack emergency is 7500. It is a four-digit transponder code used by pilots to alert air traffic control of a hijacking discreetly. Other related emergency codes include 7700 for general emergencies and 7600 for communication failure.
Step-by-step explanation:
The mode 3 code that indicates a hijack emergency is known as squawk code 7500. In aviation, when a pilot inputs this discrete transponder code into the aircraft's transponder, it alerts air traffic control (ATC) to a possible hijacking situation without alerting passengers or potential hijackers onboard. Squawk codes are four-digit numbers dialed into the transponder and they are used internationally to communicate various situations. The code 7500 specifically is reserved for communicating the state of emergency related to a hijacking scenario. Pilots are trained to use this mode of communication as a covert means to indicate emergencies without escalating the situation with a potential threat on board.
Other emergency squawk codes include 7700 for general emergencies and 7600 for radio communication failure. Pilots and air traffic controllers are both trained to recognize these codes and respond accordingly. The use of the hijack code is a serious matter, and once the code is transmitted, it triggers protocols that involve coordination among various agencies, including law enforcement and emergency response teams.