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Squawk codes: 7700, 7600, 7500 and the rest

The four digits behind every target on an air traffic controller's radar.

"Squawk" is aviation slang for the code an aircraft's transponder sends when a secondary surveillance radar asks it who it is. When a controller says "squawk four five two one", the pilot dials 4521 into the transponder, and from then on the radar can tell that aircraft apart from every other one on the scope. Most codes are ordinary and temporary. A handful are special — and three of them make every controller in range sit up.

Why there are 4,096 codes

A squawk code has four digits, and each digit runs only from 0 to 7 — there is no 8 or 9 on a transponder. That is a legacy of the original design, in which each digit was three binary pulses: eight possibilities per digit, 8 × 8 × 8 × 8 = 4,096 codes in all. That sounds like a lot until you remember how many aircraft are airborne over a continent at once, which is why codes are carefully managed and reused.

How controllers use them

When a flight departs or enters controlled airspace, ATC assigns it a code that is unique in that area. The radar system then links that code to the flight plan, and the label on the scope — the data block — shows the callsign instead of a bare number. Codes are allocated to control units in blocks so that two neighbouring centres never hand out the same one, and a flight crossing several regions may be asked to change code as it goes.

Modern Mode S transponders carry a unique 24-bit aircraft address and can send the callsign directly, so in much of Europe aircraft on Mode S radar coverage are told to squawk a shared code (1000) and are identified by their Mode S data instead — easing the pressure on the old 4,096-code pool. You can read more about Mode S, Mode C and ADS-B in how ATC radar works.

The special codes

Conspicuity codes vary from country to country, so pilots check the national rules for the airspace they fly in. The three emergency codes, though, are the same everywhere.

7700: emergency

Setting 7700 triggers an alarm on every radar display that sees the aircraft. Controllers will usually already know — a crew with a serious problem also declares "MAYDAY" or "PAN PAN" on the radio — but 7700 guarantees the aircraft stands out, even on a frequency the crew is not talking to. Traffic around it is moved out of the way, the crew is offered priority and whatever routing they need, and the emergency services are alerted at the destination. Once the crew is in contact, the controller may ask them to keep 7700 or move to a normal code; either way the flight keeps its priority.

7600: radio failure

An aircraft that has lost its radios cannot hear instructions or ask for anything. Squawking 7600 tells every controller what has happened, and the crew then follows the published lost-communications rules, so the controller can predict exactly what it will do and clear the airspace ahead of it. Under ICAO rules, an aircraft in instrument conditions in radar airspace keeps its last assigned speed and level for seven minutes, then continues on its flight plan route, descends to arrive as close as possible to its expected time, and flies the approach. (Some states, including the United States, publish their own version.) Controllers also try other ways to reach the crew — another frequency, a relay through another aircraft, or asking them to acknowledge with an ident.

7500: unlawful interference

7500 is the silent alarm for a hijacking — a way to tell ATC without saying it aloud in the cockpit. Because the consequences of a false 7500 are serious, crews are careful never to pass through the emergency codes by accident when changing code, and controllers handle any 7500 according to strict security procedures.

"Squawk ident" and friends

A controller who wants to be sure which target is which says "squawk ident". The pilot presses the IDENT button and the target flashes or highlights on the scope for about eighteen seconds — instant, unmistakable identification. Other instructions you will hear: "squawk standby" (stop replying, for example on the ground), "squawk Charlie" or "squawk altitude" (turn on altitude reporting), and "recycle squawk" (reset the transponder when its reply looks wrong).

In the simulator

Every aircraft in the ATC Radar Simulator carries a squawk code on its data block and flight strip. Emergencies appear exactly as they would for a real controller: a 7700 flight declares a MAYDAY on the radio and needs priority, and a 7600 flight cannot hear you at all — it keeps its last clearance for a short time, then flies its own approach, and your job is to keep everyone else clear of it. (The simulator compresses the timings so the scenario plays out in a session.) Building a flight plan? The surveillance letters in item 10 describe the transponder — see the ICAO Flight Plan Builder.