What Is a Squawk Code? The Four-Digit Numbers Behind Every Flight

If you've ever looked at a flight tracking app or listened to aviation radio, you may have heard references to "squawking" a code, or seen a four-digit number like 7700 associated with an aircraft. These are transponder codes — known informally as squawk codes — and they're a fundamental part of how aircraft are identified and managed in the sky.

What Is a Squawk Code?

A squawk code is a four-digit octal number (meaning each digit runs from 0 to 7) assigned to an aircraft by air traffic control. It's transmitted by the aircraft's transponder — the same device that broadcasts ADS-B signals — and it allows controllers to identify specific aircraft on radar and in their systems.

There are 4,096 possible squawk codes (since each of the four digits can be 0–7, that's 8⁴ = 4,096 combinations). These are allocated globally in a managed system, with different ranges reserved for specific purposes or geographic regions.

The term "squawk" comes from an early military identification system called IFF (Identification Friend or Foe) developed in World War II. The system was called PARROT, and interrogating it was called "squawking the parrot" — the name stuck long after the technology changed.

How Squawk Codes Are Used

When an aircraft wants to enter controlled airspace or is on an IFR (Instrument Flight Rules) flight plan, ATC assigns it a unique squawk code. The controller might say:

"Speedbird 247, squawk 3421."

The pilot sets 3421 on the transponder Mode A selector. From that point on, whenever ATC radar interrogates the transponder, the aircraft broadcasts code 3421. The radar display associates that code with the aircraft's identity — its flight plan, callsign, type, and destination — so the controller can instantly identify it among dozens of other targets.

The Special Codes Everyone Should Know

While most squawk codes are arbitrary and assigned per-flight, a handful are universally reserved for specific emergency and operational situations.

Squawk 7700 — General Emergency

This is the most important emergency code. An aircraft squawking 7700 is declaring a general emergency. This could mean anything from an engine failure to a medical emergency on board, a structural problem, or any situation where the crew needs immediate priority.

On radar, 7700 typically triggers a visual alert for controllers — the target often flashes or changes colour. ATC will clear airspace around the aircraft, offer direct routing, and coordinate emergency services if needed.

Seeing 7700 on a tracking app doesn't always mean a catastrophic emergency — it might be a precautionary declaration for a medical issue or minor technical problem. But it always means the crew has decided they need priority handling.

Squawk 7600 — Lost Communications (NORDO)

If a pilot loses radio contact with ATC — radio failure, or "NORDO" (No Radio) — they squawk 7600. This tells controllers what has happened without requiring voice communication (which is impossible if the radio is broken).

An aircraft squawking 7600 will follow a specific set of procedures: continue to their destination, descend via the standard approach, and look out for light gun signals from the control tower (a physical traffic light-style signalling system that controllers can use to direct aircraft without radio).

Squawk 7500 — Hijack

The most serious of the emergency codes. 7500 indicates that the aircraft is being hijacked. It can be set discreetly by the crew during a hijacking — the selection requires a specific sequence that would be hard to do accidentally.

Squawking 7500 triggers an immediate, co-ordinated response involving military assets and ground emergency services. It is taken extremely seriously.

Squawk 1200 — VFR in the USA

In the United States, aircraft flying under VFR (Visual Flight Rules) without an ATC clearance routinely squawk 1200. This is the standard "I'm here but not on a flight plan" code. You'll see it frequently on tracking apps when looking at small private aircraft.

Squawk 2000 — Entry Into Controlled Airspace (ICAO)

In ICAO airspace (most of the world outside the US), aircraft that haven't yet been assigned a code squawk 2000. It signals that the aircraft is entering controlled airspace from uncontrolled airspace without a transponder code yet.

Squawk 0000 — Not Usually Used

Code 0000 is reserved and not assigned to flights. Some military aircraft use it in specific contexts.

Squawk 7777 — Military Intercept

Reserved for military aircraft conducting intercept operations. If a fighter is scrambled to intercept an aircraft, 7777 may be used.

Mode A, Mode C, and Mode S

The basic squawk code system is called Mode A — it transmits the four-digit code but nothing else.

Mode C adds altitude information. When ATC interrogates a Mode C transponder, it receives the squawk code plus the aircraft's pressure altitude. This is what allows controllers to see altitude on their radar displays.

Mode S (used in ADS-B) is far more sophisticated. It allows selective interrogation of individual aircraft, transmits a unique 24-bit ICAO aircraft address (not just the four-digit squawk), and forms the basis of the ADS-B system. Every Mode S transponder has a unique hex code assigned to the aircraft's registration — this is how tracking apps identify individual aircraft even when two flights might have the same squawk.

Reading Squawk Codes on Tracking Apps

On ADS-B tracking platforms (including the data behind What Plane?), you'll sometimes see an aircraft's squawk displayed. Normally it's an assigned code that means nothing in particular — just that aircraft's temporary identifier.

But if you ever see:

These stand out immediately and are usually accompanied by news coverage or ATC audio being shared online if the situation is serious.

Can You Monitor Transponder Codes?

ATC radio is freely available to listen to in most countries, and many spotters and enthusiasts use apps and web streams to listen to live ATC. You can hear controllers assigning squawk codes in real time, cross-referencing them with the aircraft you're tracking.

Popular ATC audio streaming services include LiveATC.net and various airport-specific feeds. Pairing live ATC audio with a real-time tracking app creates a surprisingly immersive aviation monitoring experience.

How What Plane? Uses This Data

What Plane? is built on ADS-B data — the Mode S system that underlies all modern transponder technology. When an aircraft broadcasts its ADS-B signal, it includes its ICAO 24-bit address, flight details, GPS position, altitude, and speed. The app uses your location to identify the nearest aircraft and show you all of this in a clean, intuitive interface.

The home screen widget keeps this information a glance away — so even if you just catch a contrail out of the corner of your eye, you'll know instantly what you're looking at.

Download What Plane? free on the App Store.