Comprehensive Aviation Regulation, Licensing, and Airspace Management Practice Test

A Mode C transponder provides altitude data to air traffic control by encoding the aircraft’s altitude information and sending it along with the standard identification. This altitude readout helps ATC maintain vertical separation between aircraft, which is crucial for safe spacing in congested airspace.

It’s required in all the airspace classes where ATC must actively monitor and separate aircraft: Class A, Class B, Class C, and Class D. In addition, there’s a designated Mode C veil around many Class B airports, a roughly 30 nautical mile radius, where operating an altitude-encoding transponder is required for flights, even if they’re not otherwise in controlled airspace. That combination ensures ATC can see both where you are and how high you are in the most used portions of controlled airspace.

This device is different from a communications radio or from an emergency beacon; it’s specifically for reporting altitude to the radar system, not for voice communication or emergency signaling.