In what situation is an operating mode C transponder required?

An operating mode C transponder is required to provide altitude information to air traffic control (ATC) when flying within certain controlled airspace classifications. Specifically, the requirement exists within Class A, Class B, and Class C airspace.

Class A airspace extends from 18,000 feet MSL up to and including FL 600, where altitude reporting is crucial for ensuring safe separation among high altitude traffic. In Class B airspace, which surrounds major airports, a mode C transponder helps ATC manage dense traffic effectively, allowing them to provide appropriate altitude and proximity information to pilots. Class C airspace, also near airports, has similar traffic management needs, where understanding each aircraft's altitude is essential to maintaining safe operations.

The inclusion of altitude reporting particularly enhances ATC's ability to maintain situational awareness and manage the traffic effectively, making it a critical safety measure. Thus, operating mode C transponders are a necessary piece of equipment for aircraft operating within these classes of airspace.