A squawk code is the four-digit code an aircraft’s transponder sends so air traffic control can tell flights apart on a radar display. Each digit runs from 0 to 7, which gives 4,096 possible codes. Three of them are reserved for emergencies: 7500 for unlawful interference, 7600 for radio communication failure and 7700 for a general emergency.
§01What is a squawk code?
The squawk is the Mode A identity code. ATC assigns a discrete code to a flight, and the pilot dials it into the transponder. The FAA’s Aeronautical Information Manual refers to these as the “4096 discrete codes” and expects them to be read out digit by digit (2102 is “two one zero two”).1Source 1 · Federal Aviation AdministrationAeronautical Information Manual, Chapter 4, Section 1 (4-1-20 Transponder and ADS-B Out Operation)faa.gov On Mode S the same code travels in a 13-bit field that encodes four octal digits.3Source 3 · Junzi Sun, mode-s.orgThe 1090 Megahertz Riddle: Mode S surveillance repliesmode-s.org
Possible codes=8 × 8 × 8 × 8 = 4,096
- 8
- Values per digit (0–7, octal)
§02Which squawk codes are special?
| Code | Meaning | Reference |
|---|---|---|
| 7500 | Unlawful interference | SERA.13005; FAA AIM 4-1-20 (hijack code) |
| 7600 | Radio communication failure | SERA.13005 |
| 7700 | Emergency | SERA.13005; FAA AIM 4-1-20 (“squawk mayday”) |
| 1200 | VFR, United States (unless ATC instructs otherwise) | FAA AIM 4-1-20 |
| 7000 | Not receiving air traffic services, EU (unless the authority prescribes otherwise) | SERA.13005 |
SERA also notes that a crew facing unlawful interference may select 7700 instead of 7500 if circumstances warrant.2Source 2 · UK Civil Aviation Authority Regulatory LibrarySERA.13005 SSR transponder Mode A code setting (Regulation (EU) No 923/2012)regulatorylibrary.caa.co.uk The absence of 7500 therefore proves nothing.
§03Example: separating a real emergency from noise
The FAA asks pilots changing from 2700 to 7200 to go through 2200, not 7700, because even a momentary pass through 7500, 7600 or 7700 (and the 7600–7677 and 7700–7777 ranges) triggers false alarms at automated ground facilities.1Source 1 · Federal Aviation AdministrationAeronautical Information Manual, Chapter 4, Section 1 (4-1-20 Transponder and ADS-B Out Operation)faa.gov A sound alerting rule therefore requires persistence. It should fire only when the code holds across several consecutive messages, ideally from more than one receiver, then show the aircraft’s altitude and speed trend next to the alert.
§04Common misconceptions
- “A squawk identifies the aircraft.” It identifies the flight to ATC for now. The stable key is the 24-bit address.
- “7700 means a crash.” It covers anything from a medical issue on board to a technical fault, and most such flights land normally.
- “One 7700 frame is enough to alert.” Code transitions and decoding errors produce single-frame blips. Require persistence.
§05How Kimo uses squawk codes
Kimo’s Airspace watch template runs an emergency lane: 7500, 7600 and 7700 page your team after a short persistence check, with the code history, altitude profile and receiver coverage attached. You tune the rules in Alerts. The emergency squawk alerting post and the airspace alerting rules guide explain the thresholds. Aircraft in Kimo demos are simulated.
§06Related terms
- Mode S: carries the squawk alongside a unique 24-bit address.
- ADS-B: broadcasts include the squawk and an emergency indicator.
- Data fusion: confirming an alert across receivers and sources.
Frequently asked questions
Why is it called “squawk”?
Can a squawk code contain an 8 or a 9?
Should a dashboard alert on every emergency squawk?
Sources
3 references- Aeronautical Information Manual, Chapter 4, Section 1 (4-1-20 Transponder and ADS-B Out Operation) (opens in a new tab)Federal Aviation Administrationfaa.gov
4096 discrete codes; code 1200 for VFR; 7500 as the hijack code; avoiding 7500/7600/7700 during code changes.
- SERA.13005 SSR transponder Mode A code setting (Regulation (EU) No 923/2012) (opens in a new tab)UK Civil Aviation Authority Regulatory Libraryregulatorylibrary.caa.co.uk
Codes 7700, 7600, 7500 and 7000; 7700 may be used instead of 7500 if circumstances warrant.
- The 1090 Megahertz Riddle: Mode S surveillance replies (opens in a new tab)Junzi Sun, mode-s.orgmode-s.org
13-bit identity field encoding four octal digits.
External sources were accessed at the time of writing. Kimo product details, customers and figures in examples are illustrative unless a source is cited.
