If a crew resets their transponder to the emergency code of 7700 (squawking 7700), all air traffic control facilities in the area are immediately alerted that the aircraft has an emergency situation. It's up to the crew to let ATC know what the exact situation is.

What is a squawk Alert 7700?

7700 is a squawk code that is reserved for emergency situations and immediately alerts Air Traffic Control (ATC) and other units that the aircraft squawking 7700 is in distress. It may be assigned by Air Traffic Control or the pilots may decide to enter it into their transponder themselves.

Why would an aircraft squawk 7700?

Squawk 7700, an aviation autobiography by Peter M. Buffington, tells of his personal adventure into the world of aviation to achieve a professional airline pilot career. This code is used to indicate that the aircraft has been hijacked and requires emergency support from security services and air traffic control.