When is a Mode C transponder required?

A Mode C transponder is specifically required to report altitude information and is critical for maintaining safety in the National Airspace System. The requirement for a Mode C transponder is indeed applicable when operating above 10,000 feet MSL (Mean Sea Level) but not below 2,500 feet AGL (Above Ground Level). This altitude requirement helps air traffic control maintain separation between aircraft flying at higher altitudes, where increased traffic is common, and provides vital information for efficient airspace management.

While there are other cases where a transponder may be necessary, such as in Class D airspace or Class C, the unique altitude aspect of the Mode C requirement makes this answer particularly significant. Class C airspace generally mandates transponder use, but not specifically the Mode C element if operating at lower altitudes. Similarly, Class D airspace does not uniformly require a Mode C transponder unless other conditions apply.