In which type of airspace is an operating mode C transponder required to be on at all times?

An operating Mode C transponder is required to be on at all times in Class B airspace. Class B airspace is the busiest and most controlled type of airspace, extending from the surface up to a designated altitude around major airports. The requirement for a Mode C transponder in this airspace is crucial for maintaining safe separation and situational awareness among the various aircraft operating in the area, including both commercial and general aviation flights. This technology enables air traffic controllers to monitor the altitude of an aircraft, which enhances safety by allowing the controllers to effectively manage the high volume of traffic and prevent potential collisions.

In Class B specifically, the transponder provides critical data that supports the operational structure of busy terminal airspace, ensuring efficient traffic management. Other airspace classes, such as Class C and Class D, have their own transponder requirements, but they do not mandate Mode C at all times in the same way that Class B does.