Under which class of airspace must aircraft operate with a Mode C transponder, per FAR, Part 91.215?

Aircraft are required to operate with a Mode C transponder in Class A airspace, which is defined as airspace from 18,000 feet MSL up to and including flight level 600. The requirement for a Mode C transponder is crucial as it enables air traffic control to effectively monitor and manage aircraft operations in this busy and high-altitude environment.

The necessity for a Mode C transponder in Class A airspace stems from the altitude at which this airspace is found, where a large volume of air traffic is present, including jet aircraft operating at high speeds. This transponder provides essential altitude information to air traffic controllers, facilitating safe separation between aircraft and reducing the risk of mid-air collisions.

While other classes of airspace, such as Class B and Class C, also have transponder requirements, they are not as universal as in Class A. Class B airspace requires a Mode C transponder for the operation of all aircraft, while Class C generally requires it for aircraft operating within and above the airspace. However, the overarching requirement for a Mode C transponder is mandated in Class A airspace, making it essential for safe operations at higher altitudes.