In which airspace is a transponder with Mode C required?

A transponder with Mode C is required in Class B airspace and within 10,000 feet mean sea level (MSL) in Class B airspace primarily for the purpose of enhancing situational awareness and safety for all aircraft operating in those busy environments. Class B airspace is designed to manage the significant volume of air traffic around major airports, where there are high-density traffic patterns. By requiring Mode C transponders, air traffic control can receive altitude information from aircraft, allowing them to maintain safe separation and effectively manage aircraft movements.

Moreover, the 10,000 feet MSL requirement extends the transponder obligation into the airspace above Class B, which often includes busy transition areas and ensures that all aircraft are visible to air traffic controllers and to other aircraft, further enhancing overall safety in the airspace system.

In contrast, while Class C airspace requires transponders as well, it is specifically for the airspace surrounding airports with an operational control tower and certain instrument flight rules (IFR) traffic. Class D airspace only requires transponder use if the aircraft is operating with certain radios and indicated requirements, which is less stringent than Mode C for Class B airspace. The requirement for a transponder at all altitudes is