A squawk code is a four-digit number a pilot sets on the aircraft's transponder so controllers can identify it on radar. Three codes signal trouble anywhere in the world: 7500 means a hijacking, 7600 means radio failure and 7700 means a general emergency. In the U.S., 1200 is the code for small planes flying by visual flight rules (VFR) that aren't talking to ATC.
The emergency codes
Emergency
A general emergency, like an engine problem, a medical emergency on board or a pressurization issue. It flags the aircraft on every radar display in range so controllers can give it priority.
Lost radio communication
The radios have failed. The pilot follows preset lost-communication rules, and controllers clear the way ahead. Near an airport, the tower can still talk to the pilot with colored light signals.
Hijacking
Unlawful interference with the flight. It lets a crew alert ATC silently, and controllers have special procedures to confirm it and notify authorities.
Other common codes
VFR (U.S. and Canada)
Small planes flying by visual flight rules that haven't been given a code by ATC.
Gliders (U.S.)
VFR gliders that are not in contact with ATC.
Firefighting (U.S.)
Aircraft working a firefighting area that are not in contact with ATC.
Search and rescue (U.S.)
VFR aircraft flying authorized search and rescue missions.
VFR (Europe and much of the world)
The ICAO standard VFR code when no other code has been assigned.
Military intercept and radar testing (U.S.)
Used by military interceptors on active air defense missions, and by ground equipment that tests radar sites.
How transponder codes work
Each digit runs from 0 to 7, so there are 4,096 possible codes. Controllers give each instrument flight its own code, which links the radar target to its flight plan. That's how the controller's screen shows each aircraft's callsign, altitude and speed next to its target.
Why you see 7700 flights in the news
Flight tracking sites highlight aircraft squawking 7700, so emergencies get noticed quickly online. Many of them end in a normal landing: crews often declare an emergency as a precaution, because it gets them priority handling, a direct route and fire and rescue crews standing by.
Behind every 7700 is a controller clearing the sky and lining up help on the ground. That's worth a thank-you.
Common questions
What does squawk 7700 mean?
A general emergency. It alerts every controller with the aircraft on radar so the flight gets priority handling.
What does squawk 7600 mean?
The aircraft has lost radio communication. The pilot follows preset lost-communication procedures and controllers keep the way clear.
What does squawk 7500 mean?
Unlawful interference, such as a hijacking. It lets the crew alert ATC without saying anything on the radio.
What is the VFR squawk code?
1200 in the United States and Canada, and 7000 in Europe and many other countries.
Why is it called squawk?
The term is usually traced to World War II, when the identification system that came before the transponder was code-named "Parrot."