What is a squawk
The squawk (in Italian codice transponder or secondary code) is a four-digit numerical code (from 0000 to 7777) that the pilot sets on the aircraft's transponder. The transponder transmits it in response to radar interrogations (SSR mode, Secondary Surveillance Radar).
The ground-air radar sends an "interrogation" signal; the aircraft's transponder responds with the set squawk code. On the controller's display, next to the aircraft's dot appears that code — uniquely identifying that aircraft in the system.
Three relevant modes: - Mode A: transmits only the squawk code - Mode C: transmits code + altitude (based on QNE 1013) - Mode S: transmits code + altitude + aircraft identification + other data
Mode C is now mandatory in almost all European airspaces. Mode S is mandatory in controlled airspace in Europe.
The four emergency codes
They are universal, recognized by every ATC radar in the world:
7700 — General emergency
To set in case of MAYDAY or serious emergency. On radars an audible and visual alarm appears (red flashing). See MAYDAY entry.
7600 — Communication loss
To set in case of RCF (Radio Communication Failure). Alerts ATC that you can't talk but continue operating normally. See Radio Failure entry.
7500 — Hijack
To set in case of hijacking or hostile presence on board. Immediately activates national security protocols (armed forces, antiterrorism). Extremely rare in general VFR but exists as standard.
7400 — UAV/drone link lost
More recent code for drones that lose command link. Not relevant for piloted aircraft.
Never casually pass through emergency codes while modifying the transponder. If you have to change from 4271 to 7000 and the mechanical selector passes through "7700" even for a second, you've alerted an emergency. Always set the new code before confirming it (digit by digit), or use the transponder's "preset" mode.
Standard operational codes
In addition to emergency codes, there are operational codes assigned for daily use:
7000 — General VFR (Europe)
The default for European VFR: if you're flying VFR and no one has assigned you a specific code, set 7000. It's recognized as "VFR without specific plan" by all European radars.
1200 — General VFR (USA)
The default for American VFR, equivalent to European 7000. Not used in Europe.
2000 — IFR without specific plan
Default code for IFR without assignment. Rarely set — usually the IFR pilot always has a code assigned by ATC.
7777 — Military/civilian test
Reserved for tests and special missions. Not to be used in normal operations.
Codes assigned by ATC
When you enter controlled airspace (CTR, TMA), or when you contact a FIS service that uses radar, the controller will assign you a specific code to identify you uniquely:
HB-PMRSquawk 4271, ident, HB-PMR.
The ident is a command that flashes your signal on the controller's radar — useful to confirm that you are indeed the one they're talking to.
Assigned codes are temporary: at the end of transit, the controller will tell you "squawk 7000" to return to VFR default.
When to set what
Typical sequence of a controlled VFR flight:
- On ground before take-off: 7000 (VFR default)
- Departing VFR without APP contact: 7000
- In contact with APPROACH: assigned code (e.g. 4271)
- Transferred to INFORMATION: often put back on 7000
- In emergency: 7700 (MAYDAY)
- In radio loss: 7600 (RCF)
In a TMZ (Transponder Mandatory Zone) the transponder must be on Mode C/S with active code (usually 7000 if autonomous, or assigned if in radio contact). You can't enter TMZ with transponder off. See RMZ / TMZ entry.
Swiss specifics
In Switzerland Skyguide assigns specific squawk codes for:
- Zurich TMA transit: codes in 4xxx series - Geneva TMA transit: codes in 5xxx series - Regional FIS service: specific codes if under radar surveillance
When flying VFR locally outside TMA, maintain 7000. If the controller doesn't assign you another code, 7000 is the default. If instead you're in TMZ, set 7000 (or assigned) and turn on transponder Mode C/S — it's mandatory.
Summary — to remember
- Squawk = 4-digit code transmitted by transponder.
- Four universal emergency codes: 7700, 7600, 7500, 7400.
- 7000 = European VFR default (1200 in USA).
- Codes assigned by ATC to identify you uniquely.
- Never casually pass through 7500/7600/7700 while changing code.
- In TMZ Mode C/S mandatory.
Sources
- ICAO Annex 10 — Aeronautical Telecommunications, Volume IV (Surveillance Radar and Collision Avoidance Systems)
- ICAO Doc 4444 — PANS-ATM, Chapter 8
- AIP Switzerland — ENR 1.6 (ATS Surveillance Services)
- Aero Locarno · Subject 090 — VFR Communications
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