An operable 4096-code transponder with encoding altimeter is required in which airspace?

• A. Class A
• B. Class A, Class B (and within 30 miles of the Class B primary airport), and Class C airspace
• C. Class B
• D. All controlled airspace

Answer: (B)

The key idea is that an automatic altitude-reporting transponder (4096-code with encoding) is required where ATC surveillance and vertical separation rely on altitude data. In the U.S., that means you must have this equipment in Class A airspace, in Class B airspace, and in Class C airspace, and also anywhere inside the Class B outer area (roughly within 30 miles of the Class B primary airport). These are the areas where ATC needs altitude information to identify and separate aircraft effectively. Outside those areas, such as most Class D and Class E, the altitude-encoding transponder is not required. So the airspaces that require it are Class A, Class B (including the 30-mile Class B veil), and Class C.