At what altitude is a commercial pilot required to maintain a transponder?

A commercial pilot is required to maintain a transponder when flying at or above 10,000 feet MSL (Mean Sea Level). The transponder allows aircraft to be identified by air traffic control radar, contributing to safer and more efficient airspace management.

In general aviation, the requirement for a transponder increases with altitude due to the potential for more complex air traffic at higher altitudes. At 10,000 feet MSL and above, the risk of encountering other aircraft increases, and having the transponder active helps facilitate safe separation and tracking.

This requirement aligns with the regulations aimed at enhancing situational awareness for pilots and controllers alike. While there are also stipulations for operating in certain airspace classes that may necessitate a transponder at lower altitudes, the specific altitude requirement for maintaining a transponder under most conditions is indeed 10,000 feet MSL.