What is the altitude requirement for transponder operation below Class A airspace?

The correct altitude requirement for transponder operation below Class A airspace is that it must be at or above 10,000 feet MSL (Mean Sea Level). This regulation is in place to enhance situational awareness and improve the safety of all aircraft operating in the vicinity, particularly when flying in airspace where radar surveillance is limited. Below this altitude, transponder use is typically not mandated unless operating within certain specified areas, or in controlled airspaces where transponders are required for identification and tracking.

The designation of 10,000 feet MSL is significant for maintaining aircraft separation and preventing potential mid-air collisions. Operating transponders at this altitude allows air traffic control to effectively monitor and manage air traffic, even outside the more restrictive Class A airspace. This aligns with the Federal Aviation Administration's (FAA) guidance on airspace operation standards and the promotion of safety for all flights.

In this context, the other options do not meet the regulatory requirements for transponder operation below Class A airspace, as they either set altitude limits that are too low or provide incorrect thresholds for transponder use.