Which category of airspace requires a transponder?

Class B airspace generally requires all aircraft, regardless of whether they are operating under visual flight rules (VFR) or instrument flight rules (IFR), to be equipped with an operable transponder. This requirement is in place to enhance both flight safety and traffic management in busy airspace areas. Class B airspace typically surrounds the busiest airports and has a higher level of air traffic density, necessitating strict adherence to regulations to avoid collisions and ensure orderly flow of aircraft.

Transponders facilitate communication and identification of aircraft by air traffic control (ATC) and help with situational awareness for other pilots. They transmit information such as the aircraft's altitude and identification code, which aids in tracking and ensuring separation from other aircraft in this highly congested airspace.

In contrast, Class G airspace does not require transponder equipment, while Class E airspace has variable requirements that can depend on the specific circumstances and altitude. Class D airspace generally requires transponders only if the air traffic control facility asks for it, rather than as a blanket requirement.