At what altitude is Mode C altitude encoding transponder equipment required?

Mode C altitude encoding transponder equipment is specifically required in controlled airspace above 10,000 feet MSL (Mean Sea Level). This requirement is established to enhance safety and situational awareness by enabling air traffic control to receive and display an aircraft's altitude information.

In controlled airspace, having this equipment helps prevent mid-air collisions, as air traffic controllers can accurately monitor the vertical position of aircraft. The stipulation specifically applies to altitudes at or above 10,000 feet, ensuring compliance with regulations intended to support safe operations in higher airspace where traffic levels may increase.

While other options might incorrectly imply that a transponder is needed at lower altitudes or in specific airspace classifications alone, it is important to remember that the requirement for Mode C transponders is fundamentally tied to classifying altitudes in relation to the MSL threshold of 10,000 feet, establishing this critical safety measure.