What is the primary purpose of a Mode C transponder?

• A. It transmits altitude information to ATC to assist in maintaining vertical separation.
• B. It transmits weather data to ATC.
• C. It provides heading information to ATC.
• D. It transmits altitude information to ATC but only when assigned by ATC.

Answer: (A)

Mode C is the altitude reporting feature of the transponder. When ATC radar asks, the Mode C transponder replies with an encoded altitude value—typically pressure altitude—so controllers can maintain vertical separation even when aircraft are close together on the radar screen. It’s not about weather data or headings; those come from other systems. The altitude data is provided automatically in response to interrogation, not only when ATC explicitly assigns an altitude. So the primary purpose is to give ATC accurate altitude information to help manage vertical separation.