What is a squawk code?
Every aircraft's transponder broadcasts a 4-digit code (0000 to 7777, each digit 0 to 7) called a squawk. Air traffic control assigns most aircraft a fairly random code just to tell them apart on radar. A handful of codes are reserved worldwide to mean something specific regardless of who assigned them, those are the ones worth knowing. You'll see the squawk on every aircraft's card on the map, and in the Squawk column of the aircraft list.
Emergency codes
| Code | Meaning |
|---|---|
| 7700 | General emergency, set by the crew or ATC for any in-flight emergency |
| 7600 | Radio/communications failure |
| 7500 | Unlawful interference (hijack), never set as a test, always taken seriously |
If you ever spot 7700, 7600, or 7500 on the map, that's a real aircraft genuinely declaring one of these, it doesn't necessarily mean anything is seriously wrong (radio failures and precautionary emergency squawks are far more common than the code names sound), but it's always worth knowing what you're looking at.
Conspicuity codes
| Code | Meaning |
|---|---|
| 7000 | VFR conspicuity, the standard "no specific code assigned" squawk for UK/European visual flight rules traffic (the US equivalent is 1200, which you may see mentioned in American sources) |
| 2000 | IFR conspicuity, the instrument-flight-rules counterpart to 7000, for IFR flights that haven't been given a specific code by ATC |
UK military & special-activity codes
These are the ones most relevant to this site, published in the UK AIP for military low flying, display flying, and range/danger area activity. If you're watching Mach Loop or an airshow, these are the codes you're most likely to actually see.
| Code | Meaning |
|---|---|
| 7001 | Military low flying, fixed-wing military aircraft descending below 2,000ft into the UK Low Flying System (this is the code you're most likely to see at Mach Loop) |
| 7002 | Operating within a notified Danger Area |
| 7003 | Red Arrows transit or display |
| 7004 | Aerobatics, civil or military, solo or formation |
| 7005 | High-energy manoeuvres, military fast jets before sustained high-G flying |
| 7006 | Autonomous operations within a Temporary Reserved Area |
| 7007 | Open Skies Treaty observation flight |
| 7010 | Aerodrome traffic pattern, circuit traffic near an airfield, when instructed by ATC |
| 0033 | Parachute dropping in progress |
Ground & technical codes
You won't see these on real aircraft in the sky, they're set by fixed ground equipment, not aircraft transponders, but they occasionally show up in raw ADS-B data and are worth knowing what they actually mean.
| Code | Meaning |
|---|---|
| 0000 | SSR (radar) data unreliable |
| 0001 | Ground-based Height Monitoring Unit |
| 0002 | Ground transponder testing |
| 7776 / 7777 | UK SSR site/ground monitors, fixed test transponders used to calibrate radar sites. (Worth flagging: in the US, 7777 means something completely different, military interceptor aircraft on an active air-defence mission. That's not what it means here.) |
Everything else
The vast majority of squawk codes you'll see (the 4-digit number on most aircraft) are just an ordinary ATC-assigned identifier with no special meaning of its own, useful for telling one aircraft apart from another on radar, not a code to decode. There's no dedicated national code for gliders or for NATS' Traffic/Deconfliction Service either, gliders normally just use 7000, and those services assign a discrete code per aircraft rather than using one fixed number.