Squawk Codes
Authority: FAA Aeronautical Information Manual (AIM) ยง4-1-20 / ICAO Annex 10
Aviation Mode A/C transponders use 4-digit octal squawk codes. Universal emergency codes are: 7500 (Unlawful Interference / Hijacking), 7600 (Radio Communication Failure), 7700 (General Emergency), and 1200 (VFR in US/Canada).
| Squawk Code | Emergency Condition | ATC Action & Priority | Pilot Memory Phrase |
|---|---|---|---|
| 7500 | Unlawful Interference / Hijack | Highest Priority Silent Alert | 75 - Man with a knife |
| 7600 | Radio Communication Failure (NORDO) | ATC clears airspace ahead | 76 - Radio in need of fix |
| 7700 | General Emergency (Mayday) | Immediate priority handling | 77 - Going to heaven |
| 1200 | VFR Flight (US / Canada) | Standard unassigned VFR | Visual Flight Rules |
| 2000 | IFR Flight (ICAO Unassigned) | Standard international IFR | International IFR |
History & Origin
Transponders trace back to WWII-era Identification Friend or Foe (IFF) systems, which the British code-named "Parrot" -- radar operators instructing aircrews to activate it would say "squawk your parrot," and the verb survived long after the codename itself was retired. Modern Mode A transponders inherited that legacy vocabulary along with the underlying idea: a device that identifies an aircraft to ground radar automatically rather than requiring visual or radio confirmation.
Each squawk code is a four-digit number using only digits 0 through 7, since the underlying signal is transmitted in octal rather than decimal. That gives 4,096 possible codes -- enough for controllers to assign aircraft in a given airspace a unique discrete code for routine tracking, while a small set of codes are permanently reserved by ICAO and national aviation authorities for specific emergency and status conditions rather than ever being assigned to a routine flight.