What Are Squawk Codes?

What are aircraft squawk codes? You can see them in SkyAware, VRS, and other ADS-B flight tracking software. Why do squawk codes exist? Are they just random numbers or can they tell us useful information? Are there commonly used squawk codes? Can they be received reliably? Read on to learn more.The "squawk" code is a 4-digit octal number that is assigned by air traffic control, typically before a flight departs or when an aircraft enters controlled airspace. The pilots will enter the assigned squawk code into the aircraft's transponder. Squawk codes are part of the older Mode A transponder system used with traditional radar surveillance and are still used today for compatibility with legacy radar surveillance systems. The Mode A squawk code was introduced so that traditional radar surveillance positions could be matched up to a specific flight for air traffic control purposes. When an aircraft is interrogated by a ground-based radar site, the aircraft responds with its programmed squawk code. The radar system correlates the squawk code response with the radar-based position and provides the combined information to the air traffic controller.

Squawk codes are not globally unique and their assignment is coordinated by air traffic control (ATC) to avoid aircraft in the same area having the same code assigned. Most of the time the squawk code is not very interesting to casual observers as it typically appears to be a random number. However, there are special squawk codes assigned by ATC in specific regions that may tell you something about the flight.

For example, "1200" in the USA means that the flight is flying under Visual Flight Rules (VFR) and is not typically in direct contact with ATC. "1200" is a shared code so you may see many aircraft transmitting it at the same time in a given area. In other parts of the world such as Europe, "7000" is used for VFR flights. The most widely recognized squawk codes are the 3 codes which indicate emergency situations. These emergency squawk codes are established as a worldwide standard by ICAO: 

• 7500 - Aircraft hijacking
• 7600 - Radio failure/lost communications
• 7700 - General emergency - This indicates any other kind of emergency, for example, an onboard medical emergency or a mechanical problem. It is important to realize that most aircraft "emergencies" are just situations requiring urgency or priority. It is very rare for an aircraft to be in a true situation of distress.

Also remember that the 3 emergency squawk codes may not necessarily be used even in a corresponding emergency. An aircraft may remain on its previously assigned squawk code during an emergency.

Other special codes can be used to designate special types of flights and these assignments typically vary based on which entity controls the airspace in question. There may also be squawk codes assigned locally to your region. For example, in Houston, Texas, the local air traffic control facility preassigns squawk codes to certain locally-based aircraft that frequently operate in controlled airspace, such as television news helicopters. These aircraft will simply be told to "squawk preassigned" rather than being given a new unique code for each flight. By observing your local aircraft traffic data, you may be able to discern squawk code patterns that are unique to your area!

It is important to realize that decoding of squawk codes can be unreliable in consumer-grade receivers because of the way the code is transmitted. For example, this means that you may briefly see an incorrect squawk code displayed by mistake. Usually this is temporary and the software will update with the correct code the next time the data is seen transmitted from the aircraft. However, there is no way to be sure which is the correct code at any given time. Unfortunately there is not much we can do to prevent this issue from occurring, so always take the squawk code with a grain of salt!

For those interested in joining the ADS-B community, FlightAware offers step-by-step instructions to build a PiAware Receiver for under $100.

ADS-B hosts located in areas needing additional coverage might be eligible for a FlightFeeder, a free, prebuilt ADS-B receiver.

FlightAware invites you to join the ADS-B network. You'll become part of a community of aviation hobbyists across the globe and start seeing the skies in a totally different way.