What is a squawk code? ​

A squawk code is a four-digit number assigned to an aircraft by ATC. The aircraft's transponder broadcasts this code, and it shows up on the controller's radar screen next to the aircraft's position. It's how controllers identify who's who.

Each digit ranges from 0 to 7 (octal, not decimal), giving 4,096 possible codes from 0000 to 7777.

How transponders work ​

Aircraft carry a transponder that responds to radar interrogation signals from ground stations. When interrogated, the transponder sends back the assigned squawk code plus altitude information.

There are several transponder modes:

• Mode A sends only the squawk code. Old technology, still used.
• Mode C sends the squawk code plus pressure altitude. This is what lets controllers see your altitude on the radar.
• Mode S sends a unique 24-bit address, the squawk code, altitude, and can respond to individual interrogations. Modern aircraft all have this. It's also what makes TCAS work.
• ADS-B is the newest system. The aircraft broadcasts its GPS position, altitude, speed, and identification continuously without being interrogated. No more waiting for the radar sweep.

Common squawk codes ​

Most codes are assigned individually by ATC. But a few have special meanings everywhere:

• 1200 - VFR flight, no ATC services. If you're flying visually without talking to anyone, you squawk 1200.
• 1000 - Mode S conspicuity code, used in some countries
• 2000 - Default code entering a new flight information region (used internationally)

Controllers assign discrete codes to IFR flights so each aircraft has a unique identifier on the scope. When you hear "squawk 4523," the pilot dials that into the transponder and the controller's radar tag updates.

Emergency codes ​

Three squawk codes are reserved worldwide for emergencies. These trigger immediate alerts on the controller's radar display.

7700 - General emergency. Mayday. The aircraft is in serious trouble. Could be engine failure, fire, medical emergency, structural damage. When a controller sees 7700, that aircraft gets priority over everything.

7600 - Radio failure. The pilot can't communicate by voice. They can still fly the airplane, and ATC can still see them on radar. Controllers will try to provide separation and clear a path. The pilot follows a specific set of procedures for landing without radio contact.

7500 - Hijack. The aircraft has been unlawfully seized. This one is handled very carefully. Controllers will not acknowledge it verbally because the hijacker might be listening. Security protocols activate immediately.

There's a mnemonic for remembering them: 75 - taken alive (hijack), 76 - technical tricks (radio failure), 77 - going to heaven (emergency).

"Ident" ​

Sometimes a controller says "squawk ident." The pilot presses a button on the transponder that makes their radar return flash or bloom on the controller's screen for a few seconds. It's a quick way to visually confirm which target is which, especially in dense traffic.

In radarcontrol.io ​

Aircraft in radarcontrol.io have transponder functionality. Emergency squawk codes trigger the emergency system with different scenario types including decompression, engine failure, fire, and fuel emergencies. Each emergency type requires different handling.

Handling emergencies well earns +200 points. Ignoring them or letting conflicts develop with emergency aircraft hits your score hard.

Try managing emergencies while keeping traffic flowing:

• Atlanta Center (ZTL) - High traffic volume means emergencies add real pressure. Play now.
• New York TRACON (N90) - Tight airspace, less room to maneuver when an emergency pops up.

Related: What is radar identification? | What is TCAS? | What is a conflict alert? | How does ATC separation work?

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