Which airspace class requires an operational transponder?

Class A, B, and C airspace require aircraft to have an operational transponder.

Class A airspace is typically found at higher altitudes (above 18,000 feet mean sea level) and is under positive control by air traffic control (ATC), necessitating effective communication and identification capabilities from aircraft, which transponders provide. In Class B airspace, which generally surrounds busy airports, the transponder is critical for ATC to manage the high volume of air traffic and to ensure the safety of all aircraft in the vicinity. Similarly, Class C airspace, which usually includes airports with a certain level of traffic but not as congested as Class B, also requires an operational transponder to facilitate communication and location tracking for both ATC and other aircraft.

The other classes of airspace, like Class D, require transponders only under certain conditions, and Class E and G airspace do not typically mandate transponder use unless specific air traffic services are in play or flying under Instrument Flight Rules (IFR). Therefore, the need for a transponder in Classes A, B, and C arises from the operational demands and safety requirements associated with managing air traffic effectively in those areas.