A transponder is a communication device on a plane that communicates with Air Traffic Control (ATC).
There are 3 emergency transponder codes.
Each code is used for a different emergency.
1. 7500 – Hijacking
The 7500 transponder code sends a silent alert to the ATC that the plane has been hijacked without alerting the hijacker.
The usual response to the 7500 transponder code is to scramble fighter jets to escort the plane to a base.
Pilots also normally have to confirm a hijacking with ATC and must comply with ATC instructions.
2. 7600 – Lost Radio/ Communications
Pilots set the 7600 code on their transponders if their onboard radio stops working.
The 7600 code alerts the ATC that a plane cannot communicate with them.
The ATC responds by informing other nearby planes of the situation and instructs them to avoid the plane suffering communication failure to avoid potential collisions.
The pilot of the plane suffering communication failure will remain on high alert, especially if they’re heading to a busy airport, to avoid potential collisions.
The pilot will very cautiously land the plane since they can’t communicate with other planes or the ATC.
3. 7700 – General Emergency
The 7700 code is used for emergencies other than hijackings or communication failures.
Common emergencies for this code include:
Engine failure
Fuel emergency
Landing gear problems
Flight control problems
Pressurization issues
Fires
Issuing this transponder code will alert nearby air traffic controllers, who’ll provide assistance to the pilot based on the emergency.
Real-Life Examples of Emergency Transponder Code Usage
On September 11, 2001, the Korean Air Flight 085 pilot accidentally typed the letters “HJK” to ATC after being informed of the 9/11 terrorist attacks.
The ATC instructed the pilot to change their transponder code to 7500 to confirm a hijacking, which the pilot mistakenly complied with.
The flight was escorted by two F-15 fighter jets and diverted to land in Whitehorse, Yukon, Canada, from its original destination of Alaska, USA.
Once the plane landed, the authorities discovered it was a false alarm.
In 2021, an Air France flight from Paris to Nantes squawked the 7700 code due to a cabin pressurization issue and was instructed to return to Paris only 50 minutes after take-off by the ATC.
How to Remember Emergency Transponder Codes Easily
Transponder codes can be easily remembered using the following rhymes.
7500 (Hijacking): Let the other guy drive.
7600 (Broken radio): Plane needs a fix.
7700 (All other emergencies): Going to heaven.
There Are Over 50 Transponder Codes
The United States has over 50 transponder codes, while the International Civil Aviation Organization (ICAO) has 6, including the 7500, 7500, 7700 codes.
Every country has its own allocated transponder codes, with few being shared across borders.
Pilots are allocated codes by air traffic control for each flight.
Examples of Non-Emergency Transponder Codes
Some examples of non-emergency transponder codes include:
1200: This code is for a plane flying under Visual Flight Rules (VFR).
1255: 1255 is for when the plane is not in contact with ATC while to or from firefighting areas.
2000: 2000 is used when a plane enters a secondary surveillance radar (SSR) area from a non-SSR area.
Difference Between a Transponder Code & Squawk Code
The terms transponder code and squawk code are generally used interchangeably.
The transponder is the device a code is punched in, and the word ‘squawk’ refers to the communication from the aircraft’s transponder.
Not All Planes have Transponders
The vast majority of planes have transponders.
Some small planes aren’t required to have transponders to operate in certain airspace or at specific altitudes.
For example, the FAA requires aircraft to have a Mode-S transponder-based ADS-B transmitter to fly at 18,000 feet.
How Aircraft Transponders Work
The pilot enters the 4-digit code assigned to them by the ATC into the transponder.
The ATC radar screen identifies the aircraft via the 4-digit code, which is transmitted through the secondary surveillance radar.
The air traffic controller then views the blip on their primary radar screen and provides directions to the pilot.
The radar only informs the air traffic controller of the plane’s distance and bearing.
More modern transponders let the transmitting aircraft identify other nearby aircraft and alert them of the emergency, so they maintain distance.
In conclusion:
Pilots use emergency transponder codes to alert air traffic control of emergencies.
The 3 main transponder codes are 7500 for hijacking, 7600 for lost radio communication, and 7700 for all other emergencies.
Pilots set the emergency codes on their transponders, transmitting the signal to nearby air traffic control towers.
The ATC then provides the pilot with guidance depending on their emergency.
Helen Krasner holds a PPL(A), with 15 years experience flying fixed-wing aircraft; a PPL(H), with 13 years experience flying helicopters; and a CPL(H), Helicopter Instructor Rating, with 12 years working as a helicopter instructor.
Helen is an accomplished aviation writer with 12 years of experience, having authored several books and published numerous articles while also serving as the Editor of the BWPA (British Women Pilots Association) newsletter, with her excellent work having been recognized with her nomination of the “Aviation Journalist of the Year” award.
Helen has won the “Dawn to Dusk” International Flying Competition, along with the best all-female competitors, three times with her copilot.
