Squawk codes: 7700, 7600, 7500 and more
A squawk is a four-digit code the pilot sets on the transponder. Each digit is 0–7, so there are 4,096 codes. Most are assigned by air traffic control to a specific flight so it can be identified on radar. A few codes are reserved and mean the same thing worldwide.
Emergency codes
7700 Emergency
A general emergency: engine failure, a medical problem on board, fire, pressurisation trouble. Controllers get a prominent alert and the aircraft gets priority. The crew sets it themselves or when asked by the controller.
7600 Radio failure
The crew cannot hear ATC or cannot transmit. The aircraft then follows set procedures (keeps the assigned level and route, approaches per the flight plan) and ATC clears the way.
7500 Unlawful interference (hijack)
The code for a hijacking or another threat to the aircraft. The controller confirms it indirectly and follows security procedures. For the crew’s safety, flights squawking 7500 are not speculated about publicly.
Other common codes
7000 VFR in Europe
The default code for a flight under visual flight rules (VFR) that ATC has not given its own code. Used by most European states.
1200 VFR in the US and Canada
The same as 7000 in Europe: a VFR flight without an assigned code.
2000 No code assigned
The aircraft is entering secondary-radar airspace and has not been given a code yet (typically after an ocean crossing).
On the Spotterra map, aircraft squawking an emergency code are highlighted, and alerts for 7500, 7600 and 7700 nearby always get through, even when your notifications are otherwise muted.
An emergency code alone does not say how serious a situation is — it is often a precautionary landing. Reliable information comes from aviation authorities and airlines.