During every flight, controllers on the ground need to know which aircraft they are talking to. A transponder code, four digits assigned by air traffic control, supplies that need: the pilot sets this number in the aircraft's transponder, and the device instantly transmits the identifier together with the airplane's position and altitude. Because a transponder code makes 4,096 possible codes, standard non-emergency numbers are reserved for routine situations defined by the International Civil Aviation Organization, letting ATC categorize aircraft without using radio time. On Mode S transponders the same code travels alongside information like Automatic Dependent Surveillance Broadcasting, Traffic Collision Avoidance, and data that supports detailed surveillance.
Some of the 4,096 codes are kept strictly for crisis moments and are shown on controller screens in red; emergency codes like 7500, 7600, and 7700 denote hijacking, radio failure, or a general emergency. If controllers want an aircraft to stop broadcasting while still keeping its assigned code, they relay ‘SQUAWK STANDBY’, and later ‘SQUAWK NORMAL’ to resume transmission. Together, these squawk codes give pilots and controllers a silent, reliable way to share identity and intent from take-off to touchdown.
| Code | Description |
|---|---|
| 0000 | Military intercept code. Internal ARTCC subsets assigned by Enroute Safety and Operations Support. |
| 0100–0400 | Allocated to Service Area Operations for assignment for use by Terminal/CERAP/industry/unique purpose/experimental activities. |
| 0100–0700 | Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2. For use in oceanic airspace unless another code is assigned by ATC. |
| 0500, 0600, 0700 | External ARTCC subsets. Blocks of discrete codes except that xx00 is used as a non-discrete code after all discrete codes are assigned. |
| 1000 | Used exclusively by ADS-B aircraft to inhibit mode 3A transmission. Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2. For use in oceanic airspace unless another code is assigned by ATC. External ARTCC subset. |
| 1100 | Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2. For use in oceanic airspace unless another code is assigned by ATC. External ARTCC subset. |
| 1200 | Visual flight rules (VFR) flight standard squawk code used in North American airspace when no other has been assigned or is applicable. |
| 1201 | Assigned via FAR 93.95 for use by VFR aircraft in the immediate vicinity of LAX. |
| 1202 | Visual flight rules (VFR) glider operations for gliders not in contact with ATC; effective February 2012. |
| 1203–1272 | Discrete 1200 series codes unless otherwise allocated, designated for DVFR aircraft and only assigned by a flight service station. |
| 1255 | Aircraft not in contact with an ATC facility while enroute to/from or within the designated fire fighting area(s). |
| 1273–1275 | Calibration Performance Monitoring Equipment (CPME) Parrot transponders. |
| 1276 | Air Defense Identification Zone (ADIZ) penetration when unable to establish communication with ATC or aeronautical facility. |
| 1277 | VFR aircraft which fly authorized SAR missions for the USAF or USCG while enroute to/from or within the designated search area. |
| 1300 | Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2. For use in oceanic airspace, unless another code is assigned by ATC. External ARTCC subset. |
| 1400 | External ARTCC subset. |
| 1500 | Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2. For use in oceanic airspace unless another code is assigned by ATC. External ARTCC subset. |
| 1600, 1700 | External ARTCC subsets. |
| 2000 | Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2. For use in oceanic airspace unless another code is assigned by ATC. External ARTCC subset. |
| 2100 | Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2. For use in oceanic airspace unless another code is assigned by ATC. External ARTCC subset. |
| 2200, 2300, 2400 | Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2. For use in oceanic airspace unless another code is assigned by ATC. External ARTCC subsets. |
| 2500, 2600, 2700 | External ARTCC subsets. |
| 3000 | External ARTCC subset. |
| 3100, 3200, 3300, 3400, 3500, 3600, 3700 | External ARTCC subsets. |
| 4000 | Aircraft on a VFR Military Training Route or requiring frequent or rapid changes in altitude. Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2. For use in oceanic airspace unless another code is assigned by ATC. External ARTCC subset. |
| 4100 | External ARTCC subset. |
| 4200, 4300 | Internal ARTCC subsets assigned by Enroute Safety and Operations Support. |
| 4400–4477 | Reserved for use by SR-71, YF-12, U-2, B-57, pressure suit flights, and aircraft operations above FL600. |
| 4401–4433 | Reserved in accordance with FAA Order JO 7110.67 (Federal Law Enforcement). |
| 4434–4437 | Weather reconnaissance, as appropriate. |
| 4440,4441 | Operations above FL600 for Lockheed/NASA from Moffett Field. |
| 4442–4446 | Operations above FL600 for Lockheed from Air Force Plant 42. |
| 4447–4452 | Operations above FL600 for SR-71/U-2 operations from Edwards AFB. |
| 4453 | High balloon operations: National Scientific Balloon Facility in Palestine, Texas; and other providers, some in international operations. |
| 4454–4465 | Air Force operations above FL600 as designated in FAA Order 7610.4. |
| 4466–4477 | Reserved in accordance with FAA Order JO 7110.67 (Federal Law Enforcement). |
| 4500, 4600, 4700 | Internal ARTCC subsets assigned by Enroute Safety and Operations Support. |
| 5000 | Reserved for use by NORAD. |
| 5061, 5062, 5100, 5200 | Reserved for special use by Potomac TRACON. |
| 5100, 5200, 5300, 5500 | Internal ARTCC subsets assigned by Enroute Safety and Operations Support. May be used by DOD aircraft beyond radar coverage but inside US controlled airspace with coordination as appropriate with the applicable Area Operations Directorate. |
| 5400 | Reserved for use by NORAD. |
| 5600, 5700 | External ARTCC subsets. |
| 6000 | External ARTCC subset. |
| 6100 | Reserved for use by NORAD. |
| 6200, 6300 | External ARTCC subsets. |
| 6400 | Reserved for use by NORAD. |
| 6500, 6600, 6700 | External ARTCC subsets. |
| 7000 | External ARTCC subset. |
| 7100, 7200, 7300 | External ARTCC subsets. |
| 7400 | Unmanned aerial vehicle lost link. |
| 7500 | Aircraft hijacking. |
| 7501–7577 | Reserved for use by Continental NORAD Region (CONR). |
| 7600 | Radio failure (lost communications). |
| 7601–7607 | Reserved for special use by the FAA. |
| 7610–7676 | External ARTCC subsets (block of discrete codes). |
| 7700 | Emergency. |
| 7701–7707 | Reserved for special use by the FAA. |
| 7710–7776 | External ARTCC subsets (block of discrete codes). |
| 7777 | Non-discrete code used by fixed test transponders (RABMs) to check correctness of radar stations (BITE). DOD interceptor aircraft on active air-defense missions and operating without ATC clearance in accordance with FAA Order 7610.4. |
What are aviation transponder codes?
Aviation transponder codes, typically called squawk codes, are 4-digit codes given to pilots by ATC for communication purposes while in flight, and the standard VFR transponder code means you are flying under VFR without flight following or ATC-assigned squawk codes.
Transponder codes are four-digit octal numbers transmitted by the transponder in response to a secondary surveillance radar interrogation signal. Each digit ranges from 0 to 7, so the lowest possible squawk is 0000 and the highest is 7777; this makes 4,096 possible codes. A discrete transponder code is assigned by air traffic control and acts as a unique identifier so that controllers can keep track of individual aircraft, separate traffic, and manage the flow of aircraft through crowded airspace. The most common code you will use is 1200, the standard VFR transponder code, but ATC assigns you a different four-digit code for identification or you need to input a specific one based on the situation.
The term ‘squawk’ comes from the early radar beacon system in which the transponder, a little box mounted in the panel of your airplane, automatically replies to secondary surveillance radar with the code selected. Mode A transponders send only the squawk code and identification, Mode C adds pressure altitude transmitted in 100-foot increments, and Mode S transmits the squawk code, identification, pressure altitude, and more detailed data like ADS-B and TCAS. Among the 4,096 codes, the International Civil Aviation Organization has standardized six emergency codes for global use: 7500 is reserved for emergencies only, specifically unlawful interference or hijack, while 7700 is the universal emergency code and 7600 indicates radio communication failure. Whether you are flying VFR under code 1200 or operating under an ATC-assigned discrete code, the four-digit octal sequence you squawk is the key that allows controllers to identify your aircraft instantly on their radar screens.
What is an aircraft emergency transponder code?
An aircraft emergency transponder code is a four-digit number the pilot dials into the transponder to alert air traffic control that the aircraft is in distress and needs priority handling. The three reserved codes are 7500, 7600, and 7700. Emergency codes grab immediate attention.
Squawk 7500 indicates unlawful interference; the pilot sets 7500 for hijacking, and the code will trigger the special emergency indicator in all radar ATC facilities. ATC will never assign 7500 without prior notification from the pilot.
Code 7600 is for lost communication. Squawk 7600 indicates a radio communication failure. When 7600 appears on ATC radar, controllers will try to contact the aircraft on multiple frequencies.
7700 is the emergency squawk code. Squawk 7700 is a ‘catch all' code used for any emergency scenario, like engine failure, medical emergency, onboard fire, or any situation that makes continued flight questionable. Setting 7700 on the transponder shows that the aircraft is in distress, alerts ATC, and triggers the special emergency indicator in all radar ATC facilities.
What is the frequency band for an aircraft transponder?
The standard secondary-surveillance transponder receives interrogation from the secondary surveillance radar on 1030 MHz and replies on 1090 MHz. Number 1090 MHz is the transponder's frequency. Mode S transponders operate on the same 1030 MHz and 1090 MHz frequencies as ATCRBS units do. Mode S Extended Squitter transponder operates on 1090 MHz. The receipt of a valid interrogation causes the aircraft transponder to reply with one of two types of PAM data at 1090 MHz : the mode A squawk identity code and the mode C pressure altitude reply.
A separate band is set aside for long-range surveillance: the 1300-1350 MHz band is for surveillance radar stations and associated airborne transponders. The interrogation and reply frequencies always differ by 63 MHz, so the band is divided into 126 channels for interrogation and 126 channels for reply. Transponder output power is typically about 200 watts.
I found that this instrument broadcasts on a frequency of 1090 MHz. The instructor stated that our transponder worked within a particular radio range. This heavy frequency is a portion of the L-band electromagnetic spectrum.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
