What a squawk actually is
A squawk code — properly a Mode A code — is a four-digit number the crew dials into the transponder. When ground radar interrogates the aircraft, the transponder replies with that number, letting the controller tie a blip on the screen to a specific aircraft. It is also carried in ADS-B broadcasts, which is how flight trackers display it.
Each digit runs from 0 to 7 only — the code is octal, a legacy of the system's 1950s origins. That gives 8 × 8 × 8 × 8 = 4,096 possible codes, which is why you will sometimes hear a transponder described as "4096-code capable".
Four thousand sounds like plenty until you consider how much traffic Europe handles at once. Codes are therefore a managed resource: assigned on departure, sometimes changed in flight, and recycled constantly. A code is a temporary label, not an identity.
The wartime IFF equipment that transponders descended from was codenamed "Parrot". Controllers asked crews to "squawk" a code, and to "strangle the parrot" when switching it off. The parrot is long gone; the verb stuck.
The three emergency codes
Three codes are reserved worldwide. They are deliberately spaced apart on the dial so that turning towards one does not accidentally pass through another.
| Code | Meaning | What it looks like in practice |
|---|---|---|
7500 | Unlawful interference (hijack) | Extremely rare. On trackers, an appearance is far more often a mis-keyed code than a real event. |
7600 | Radio failure | The aircraft can fly but cannot talk. It will follow published lost-communication procedures. |
7700 | General emergency | The broadest of the three, and the one you will actually see. Covers anything from a serious failure to a passenger taken ill. |
Setting any of these lights up the controller's display immediately and, in most systems, alerts adjacent sectors too. The crew do not need to explain first — the code does the shouting so they can concentrate on flying.
A 7700 on a tracker is usually undramatic. Most result in a routine diversion to the
nearest suitable airport, frequently for a medical issue on board rather than a technical one. Squawking
7700 gives the crew priority handling; it is a tool for reducing risk, and using it early is good
practice, not evidence that something is going badly wrong.
The codes you will actually see
The overwhelming majority of squawks are administrative. In UK airspace a handful of general-purpose codes come up constantly:
| Code | Used for |
|---|---|
7000 | The general conspicuity code, used across the UK and much of Europe when flying VFR without a discrete code assigned. By far the most common code on any UK tracker. |
2000 | Set when entering UK airspace from an area where no code was assigned, and in some non-radar situations. |
7004 | Aerobatics and display flying. |
7010 | Aerodrome traffic pattern — circuits at an airfield. |
0033 | Parachute dropping in progress. |
7777 | Reserved. Ground transponder monitoring and certain military operations — never set by ordinary traffic. |
If a light aircraft on the map is showing 7000, that is the aviation equivalent of a blank
expression. It means nobody has given it anything more specific to display.
The UK also operates listening squawks: codes that tell a particular radar unit "I am monitoring your frequency but not receiving a service". They let a controller call you directly if a conflict develops. They are unit-specific and are published in the UK AIP.
Discrete codes and how they get assigned
An aircraft under an air traffic service is normally given a discrete code — one unique to that flight within the area. In Europe, blocks of codes are distributed between countries and units through a centralised system so that two aircraft in adjacent sectors do not end up wearing the same label.
This is also why a code can change mid-flight. Crossing from one country's system to another may bring a fresh assignment, and you will see the squawk field on a tracker update accordingly while everything else about the flight stays the same.
Why squawks are slowly mattering less
Mode S and ADS-B both carry the aircraft's permanent 24-bit hex address, which is globally unique and never recycled. That makes it a far better identifier than a four-digit code from a pool of 4,096. Modern systems increasingly correlate traffic by hex address and flight ID, using the Mode A code mainly for legacy compatibility and for the emergency signalling that remains genuinely useful.
The squawk is not going away soon — too much equipment depends on it — but on a tracker it is now the hex code, not the squawk, that reliably answers "which aircraft is this?"
Code allocations change and vary by country. This page is written for enthusiasts reading a flight tracker. Anyone flying should use the UK AIP and current official publications as the authoritative source.