What is the term 'transponder code' used to describe?

• A. A maintenance diagnostic code used on ground.
• B. A selectable code transmitted by the aircraft’s transponder to identify the aircraft to ATC.
• C. A code for cockpit ambiance settings.
• D. A code used for weather updates downlink.

Answer: (B)

The transponder code is the four-digit squawk code that the aircraft’s transponder transmits in response to ATC radar interrogations, used to identify the airplane on radar. ATC assigns this code, and the pilot programs it into the transponder so that, when interrogated by secondary surveillance radar, the aircraft appears with that unique code on ATC screens. This allows controllers to distinguish and track each aircraft in the airspace, separate them as needed, and link the radar return to the specific flight.

It isn’t a maintenance diagnostic code, it isn’t related to cockpit ambiance or cabin settings, and it isn’t a weather downlink identifier. Those systems serve different purposes, whereas the transponder code specifically identifies the aircraft to ATC.