Definition
An airspace area within a 30 nautical mile radius of the primary airport of a Class B airspace area, measured from the surface up to 10,000 feet MSL, in which aircraft must be equipped with an operable Mode C transponder (altitude reporting) to operate.
Plain English
A circle of air drawn 30 nautical miles around the main airport of a busy Class B airspace. If you fly inside that circle below 10,000 feet, your transponder must be working and reporting your altitude.
Context Anchor
Seen in Mode C altitude-reporting and transponder equipment requirements, especially near major airports with heavy traffic.
Why Pilots Care
Failing to have Mode C active inside this area violates regulations and can result in airspace incursions or enforcement action.
Intuition Check
Do not read “30-mile circle” as any ordinary 30-mile area drawn by the pilot. In this FAA context, it means a specific 30-nautical-mile area around certain listed primary airports, tied to altitude-reporting equipment requirements.
Example Sentence 1
Before departing, the pilot confirmed the transponder was operating in Mode C because the route passed inside the 30-mile circle around the Class B airport.
Example Sentence 2
Charts show the 30-mile circle so pilots know exactly where the altitude-reporting rule begins.