Which airspace requires an altitude-encoding transponder within 30 NM of the airport?

The requirement for an altitude-encoding transponder within 30 nautical miles of an airport primarily applies to Class B airspace. Class B airspace surrounds major airports where there is significant air traffic, and the transponder requirement enhances the ability of air traffic control to monitor and manage that traffic safely. An altitude-encoding transponder provides ATC with the aircraft's altitude information, which is essential in ensuring separation between aircraft and preventing potential collisions.

In addition to being located within Class B airspace, this requirement specifically targets the area around the major airport, particularly within a 30-mile radius, where many aircraft could be transitioning, departing, or arriving. This safety measure is critical for maintaining order in highly congested airspace near busy airports. Other classes of airspace, like Class C and D, have their own set of requirements, but the specific stipulation regarding an altitude-encoding transponder is predominantly tied to Class B airspace to bolster safety in high-traffic scenarios.