A transponder is what turns your aircraft from an anonymous blip into a labelled, altitude-tagged target on a controller's screen, and the squawk code is how you and air traffic control agree which target is you.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
What a transponder actually does
A transponder is the airborne half of Secondary Surveillance Radar (SSR). A ground station sends an interrogation, and the transponder replies with a coded pulse train. Unlike primary radar, which just bounces energy off the airframe, SSR gets a cooperative reply that carries information, which is why a controller can see your identity and altitude rather than only your position.
The four-digit code you set is the Mode A identity. Each digit runs from 0 to 7 (the code is octal), so there are 4096 possible codes, which is why Mode A is sometimes called the 4096-code mode. Set 4321 and your reply carries 4321.
Mode A, Mode C and Mode S
The modes build on each other:
- Mode A returns only the identity code you have set.
- Mode C adds your pressure altitude, always referenced to the standard 1013.25 hPa setting, so the controller sees a flight level regardless of the local QNH.
- Mode S adds a permanent 24-bit aircraft address unique to the airframe, allows a radar to interrogate one specific aircraft (selective interrogation, the "S"), and carries a data link. Mode S is the foundation for ADS-B, where the aircraft broadcasts its own GPS position.
Most modern installations combine these and are described as Mode A/C/S. The procedures for how codes are assigned and used are set out in ICAO Doc 4444 (PANS-ATM) and, for the United States, the FAA AIM.
The three codes every pilot must know
Three codes are reserved internationally in ICAO Annex 2 and recognised by air traffic services everywhere:
- 7700: a general emergency. Setting it alerts every SSR station in range.
- 7600: radio communication failure. It tells ATC you can probably still hear or be seen, but cannot talk.
- 7500: unlawful interference, such as a hijack.
These never change between countries, and most training uses a memory aid for them. Setting one of these draws immediate attention, so they are used deliberately, not tested in the air.
Conspicuity and "no code assigned"
When ATC has not given you a discrete code, you use a conspicuity code, and these are regional, not universal:
- In much of Europe, 7000 is the general conspicuity code.
- In the United States, 1200 is the VFR code.
- 2000 is commonly used when entering an area from one without SSR code assignment.
Because these vary by country and are revised from time to time, always set the code current for where you are flying, as published in the local AIP, rather than relying on memory of another region's value.
Common pitfalls
- Codes are octal. There is no 8 or 9 in a squawk code; every digit is 0 to 7.
- Mode C altitude is pressure altitude. It is always on the standard setting, so it will not match your QNH altitude readout, and that is normal.
- Conspicuity codes are local. A code that is correct at home may mean something else abroad, so check the AIP.
- Squawk only when told, except in an emergency. Changing to a discrete code you were not assigned can confuse the picture.
In Pilot EFB
Pilot EFB keeps a quick squawk-code reference, including the reserved emergency codes, so the value you need is a tap away rather than a memory test on a busy frequency. It is a personal reference held on your device; Pilot EFB is offline-first and is not a certified Electronic Flight Bag, so follow ATC instructions and current official procedures as the authority on what to set and when.