What Is a Mode S Code and Why Does Every Aircraft Have One?

The Invisible ID Tag

Every aircraft registered in the FAA Civil Aviation Registry is assigned a Mode S code — also called a transponder code or ICAO address. This 24-bit identifier, expressed as a six-character hexadecimal number such as A1B2C3, is permanently assigned to the aircraft and does not change when the aircraft is sold or re-registered. It functions as a permanent digital fingerprint for the airframe.

How Mode S Works

Mode S is a selective interrogation system used in air traffic control radar. When a ground radar station interrogates an aircraft, the aircraft Mode S transponder responds with its unique code along with position and altitude information. Because each aircraft has a unique code, the ground system can identify individual aircraft without ambiguity — a significant improvement over earlier systems where all aircraft responded to every interrogation.

ADS-B and Flight Tracking

Mode S codes are central to ADS-B (Automatic Dependent Surveillance-Broadcast), the modern surveillance system that the FAA mandated for most aircraft operating in controlled airspace. ADS-B-equipped aircraft broadcast their Mode S code, GPS position, altitude, speed, and other data automatically and continuously. This broadcast can be received by both ATC ground stations and by anyone with an ADS-B receiver — which is why flight tracking apps like Flightradar24 can show near-real-time positions of aircraft.

Mode S in the Registry

The FAA Civil Aviation Registry includes the Mode S code for every registered aircraft. This is an important field for cross-referencing registry data with flight tracking data. Given a tail number from the registry, you can find the Mode S code; given a Mode S code from a flight tracking service, you can look up the aircraft registry record.

What the Code Tells You

The structure of U.S. Mode S codes is not random — they are assigned in blocks, and the code range can tell you the country of registration. U.S.-registered aircraft have Mode S codes in the range A00000 through AFFFFF. Foreign aircraft have codes in different ranges according to ICAO standards.

When you look up an aircraft on this site, the Mode S code is displayed in the registry record. Combined with the data sources available to researchers, it enables powerful cross-referencing across aviation datasets.