Which airspace requires a Mode C transponder?

Which airspace requires a Mode C transponder?

• A. Class B
• B. Class C
• C. Class A
• D. Class D

Answer: (C)

Mode C gives ATC the aircraft’s altitude by encoding it in the transponder signal, which is essential for maintaining vertical separation in controlled airspace. In Class A airspace, all flight operations are under positive ATC control, so every aircraft must be equipped with a functioning altitude-reporting transponder. That universal requirement makes Class A the best answer because it applies to the entire airspace block, ensuring ATC has accurate altitude information at all times. While transponders (often with altitude reporting) are also used in other classes like B and C, the question highlights the airspace where the altitude‑encoding requirement is universally enforced, which is Class A.

Mode C gives ATC the aircraft’s altitude by encoding it in the transponder signal, which is essential for maintaining vertical separation in controlled airspace. In Class A airspace, all flight operations are under positive ATC control, so every aircraft must be equipped with a functioning altitude-reporting transponder. That universal requirement makes Class A the best answer because it applies to the entire airspace block, ensuring ATC has accurate altitude information at all times. While transponders (often with altitude reporting) are also used in other classes like B and C, the question highlights the airspace where the altitude‑encoding requirement is universally enforced, which is Class A.