If you're interested in aviation, or like listening to inflight ATC communications, you've likely heard of 'squawk codes.' These are four-digit codes given by ATC to all aircraft in the airspace for communication purposes. However, squawk codes can also be used for important communications between aircraft and the ground.
KEY TO COMMUNICATION
The primary goal of a squawk code is to provide effective communication between the ATC and the aircraft. Prior to departure, aircraft will be given squawk codes, which will be used by ATC to direct the aircraft during its flight. This code will show up on ATC screens and helps provide basic information such as speed and altitude.
Squawk codes are four digits, with each being a number between zero and seven. This gives thousands of possible combinations for air traffic control to give to aircraft. That being said, there are certain combinations that can't be used. This is because there are a few reserved squawk codes, which we'll come to a bit later.
Pilots have to enter squawk codes into their transponder to communicate with flight controllers. Only if the correct squawk code is entered into the transponder will it appear on ATC screens with the correct information. The transponder is constantly communicating with the ground and providing "pings" with information.
At times, aircraft might be asked to change their squawk codes once airborne. This might be to better communicate with other towers. Alternatively, this can also be required if entering restricted airspace. This means a flight might have multiple squawk codes depending on where it is flying and on what path.
ENSURING SAFETY AND SEPARATION
Aviation is an industry driven primarily by safety-based practices, and squawk codes are no exception. These four-digit codes are very important to ensure aircraft separation, especially during the takeoff and landing phases of a given flight.
Air traffic control uses squawk codes to monitor aircraft positions on information screens, ensuring that they do not break minimum separation while flying. Having the transponder off or inputting incorrect squawk codes has the potential to result in a safety threat and security situation, as has happened in the past.
Squawk codes are particularly helpful in crowded airspace. This is because, under such conditions, air traffic controllers have the busy task of managing multiple aircraft simultaneously. Since most arrival and departure path are already set, the corresponding controllers must ensure that there are no aircraft in the vicinity.
Perhaps the most publicly known squawk codes are those used for emergency conditions. These three ICAO-assigned squawk codes can be used by any aircraft with a transponder to explain their emergency to flight controllers.
The first emergency code is Squawk 7500. This code is used to indicate that the aircraft has been hijacked and requires emergency support from security services and air traffic control. The code has become popular due to its use in the world of cinema, with the movies 7500 and Flight 7500 alluding to the code in their titles.
The second emergency code is Squawk 7600. This code is used to communicate to air traffic control that the aircraft in question has lost communications with the tower. In the case that contact cannot be established, planes will be directed using aviation light signals. These light signals can be used to provide aircraft with clearance to land or to indicate unsafe conditions in case radio contact fails.
The third emergency code is Squawk 7700. This code is used to communicate all emergencies onboard a flight, and is perhaps the best-known example. Depending on the nature and severity, crews may conduct checks before formally declaring an emergency. When squawk 7700 is declared, all nearby ATCs are also informed of the emergency and pilots can fly the plane as they see fit regardless of the rules.