A transponder replies to radar with a four-digit code, the squawk, so the controller can identify you. Each digit runs from 0 to 7, which gives 4,096 codes.
Codes with fixed meanings
| Code | Meaning |
|---|---|
| 7500 | Unlawful interference (hijack) |
| 7600 | Radio communication failure |
| 7700 | Emergency |
| 2000 | Entering from airspace without radar control, no code assigned (ICAO) |
| 7000 | VFR conspicuity code in much of Europe |
| 1200 | VFR code in the United States and Canada |
Avoid selecting 7500, 7600 or 7700 by accident while changing codes: in a real aircraft, move the knob to standby first. Controllers can use the squawk generator to assign codes that avoid the reserved ones.
Declaring an emergency
There are two levels:
- MAYDAY, MAYDAY, MAYDAY for distress: grave and imminent danger, immediate help needed. Examples: engine fire, serious structural damage, a medical emergency that cannot wait.
- PAN PAN, PAN PAN, PAN PAN for urgency: a serious problem that does not need immediate help. Examples: a single engine failure in a twin with the other running normally, a sick passenger.
After the declaration, give your callsign, the nature of the problem, your intentions, your position and level, and, if time allows, fuel endurance and persons on board. ATC will give you priority. Squawk 7700 if asked or if you have not been given a code.
If the radio fails
- Check the obvious: volume, frequency, headset, the right radio selected.
- Try the previous frequency, then 121.500 MHz, the international emergency frequency.
- Squawk 7600.
- Follow the lost-communication procedure for the airspace. In general, if you are in visual conditions, continue in visual conditions and land at the nearest suitable airport. If not, follow your last clearance, the route and levels you were cleared for, and the published procedures for the arrival.
- Keep transmitting blind ("transmitting blind") in case only your receiver has failed.
For flight simulation and training only. Real-world procedures vary by country and operator; always follow the published charts and your local rules.
Keep reading
- How air traffic control works: The controller positions you meet on a flight, what each one does, and when you change frequency.
- Radio phraseology basics: How to structure a call, what you must read back, and how to say numbers and letters on the radio.