A complete guide to transponder squawk codes used in aviation. Learn what 7700, 7600, and 7500 mean, how squawk codes work, and how AeroScope tracks them in real-time.
Squawk codes are four-digit octal codes assigned to aircraft by Air Traffic Control (ATC). Each code is set on the aircraft's transponder and broadcast continuously, allowing ATC radar systems to identify and track individual aircraft. The term "squawk" originates from the World War II-era IFF (Identification Friend or Foe) system codenamed "Parrot" — controllers would tell pilots to "squawk your parrot."
Each digit in a squawk code ranges from 0 to 7 (octal notation), creating 4,096 possible combinations from 0000 to 7777. Certain codes are reserved internationally for emergency situations, while others have specific meanings depending on the region and airspace. ATC assigns discrete squawk codes to aircraft entering controlled airspace, enabling positive identification on radar displays.
Squawk codes are transmitted by an aircraft's transponder, a radio transmitter/receiver that operates on 1030 MHz (interrogation) and 1090 MHz (reply). There are three primary transponder modes used in aviation:
The original transponder mode that transmits only the four-digit squawk code. When interrogated by Secondary Surveillance Radar (SSR), the transponder replies with the assigned squawk code. This allows ATC to identify the aircraft but provides no altitude information.
An extension of Mode A that adds pressure altitude reporting. The transponder transmits both the squawk code and the aircraft's barometric altitude (in 100-foot increments). Most controlled airspace requires at least Mode C capability for separation assurance.
The modern standard used by ADS-B (Automatic Dependent Surveillance-Broadcast). Mode S transponders transmit a unique 24-bit ICAO address, squawk code, altitude, GPS position, velocity, callsign, and additional data. This is the data that AeroScope captures and analyzes in real-time.
Three squawk codes are reserved internationally for emergency situations. These codes trigger immediate attention from ATC and are monitored by military and security services worldwide.
| Code | Meaning | Description | Response |
|---|---|---|---|
| 7700 | General Emergency | Used for any in-flight emergency including engine failure, fire, medical emergency, fuel emergency, structural damage, or any situation requiring immediate assistance. | ATC provides priority handling, clears airspace, alerts emergency services, and coordinates with the nearest suitable airport for landing. |
| 7600 | Radio Failure (COMMS Lost) | Indicates the pilot has lost the ability to communicate with ATC via radio. The transponder is still functioning, so ATC can see the aircraft but cannot talk to it. | ATC provides separation based on last known intentions. The pilot follows lost communication procedures (squawk 7600, fly last assigned altitude and route, then proceed to destination). |
| 7500 | Hijack / Unlawful Interference | The most serious squawk code, indicating the aircraft is being hijacked or subject to unlawful interference. ATC will first attempt to confirm the code is intentional. | Security protocols activated immediately. Military intercept may be scrambled. ATC clears surrounding airspace and coordinates with national security agencies. |
A common mnemonic for remembering emergency codes: 75 — taken alive (hijack), 76 — radio fix (comms lost), 77 — going to heaven (emergency).
| Code | Usage | Description |
|---|---|---|
| 1200 | VFR (US/Canada) | Standard code for Visual Flight Rules traffic in the United States and Canada. Aircraft flying VFR without ATC services use this code. |
| 1000 | IFR (some regions) | Used in some European and international regions for Instrument Flight Rules traffic. In ICAO regions, 1000 indicates an IFR flight receiving ATC services with Mode S. |
| 2000 | Entering controlled airspace | Used by aircraft entering controlled airspace from an area where no squawk code was previously assigned. Often used as a default before ATC assigns a discrete code. |
| 7777 | Military interceptor | Reserved for military interceptor aircraft on active intercept missions. This code should never be assigned to civilian aircraft under any circumstances. |
| 4000 | Military (restricted areas) | Used by military aircraft operating within designated restricted or military operating areas. The specific usage varies by country and military branch. |
| 0000 | Not yet assigned | Indicates that no discrete code has been assigned by ATC. Aircraft may briefly display 0000 during startup or when transitioning between ATC facilities. |
Different countries and regions use specific "conspicuity" squawk codes for various types of uncontrolled traffic. These help ATC identify the type of operation being conducted.
1200 — VFR traffic (the universal US VFR code)
1277 — Search and Rescue (SAR)
1255 — Firefighting aircraft
0100 — FAA flight inspection
4400-4477 — Reserved for NORAD/ADIZ operations
7000 — VFR conspicuity (equivalent of US 1200)
2000 — Entering controlled airspace (no prior code)
0030 — Parachute dropping
7001 — VFR conspicuity (Mode S equipped)
0033 — Aerobatic displays
7000 — VFR conspicuity (standard across Europe)
2000 — Entering controlled airspace
1000 — Mode S equipped IFR (no discrete code needed)
0021-0027 — Reserved for specific national uses
7010 — Used in some regions for VFR in TMAs
AeroScope's real-time flight tracking platform captures and analyzes squawk codes as part of its multi-stage enrichment pipeline. Here is how squawk code monitoring integrates with AeroScope's capabilities:
You can launch AeroScope to see live squawk codes for all aircraft in your area right now.
Squawk 7700 indicates a general emergency. When a pilot sets their transponder to 7700, it alerts air traffic control and triggers priority handling. This code is used for any in-flight emergency including engine failure, medical emergencies, fuel emergencies, and other critical situations.
Squawk 7600 indicates radio failure (COMMS lost). The pilot can no longer communicate with air traffic control via radio but the aircraft transponder is still functioning. ATC will provide separation and the pilot follows lost communication procedures.
Squawk 7500 indicates hijacking or unlawful interference. This is the most serious squawk code and immediately triggers security protocols. ATC will attempt to confirm the code is intentional to rule out accidental selection.
In the United States, the standard VFR (Visual Flight Rules) squawk code is 1200. In Europe and the UK, the VFR conspicuity code is 7000. Aircraft flying VFR without specific ATC-assigned codes use these to indicate they are operating under visual flight rules.
There are 4,096 possible squawk codes, ranging from 0000 to 7777 in octal notation. Each digit can be 0-7, giving 8 x 8 x 8 x 8 = 4,096 combinations. Many codes are reserved for specific purposes while others are assigned dynamically by ATC.
Yes. AeroScope monitors all aircraft squawk codes in real-time. When an aircraft transmits 7700, 7600, or 7500, AeroScope's threat scoring engine automatically flags the aircraft, raises its threat score, and generates an alert for monitoring operators.