Class C airspace
Class C airspace, its two-way-radio and transponder requirements, and its typical 5-/10-nm shape.
Class C airspace surrounds airports that are busy enough to have a control tower and radar approach control, but not as congested as Class B. The typical shape is a 5-nm-radius surface core up to about 4,000 ft above the airport, with a 10-nm-radius shelf from roughly 1,200 ft to 4,000 ft AGL.
To enter, you must establish two-way radio communication with the controlling facility before crossing the boundary — and importantly, if the controller uses your call sign, communication is established and you may enter. (If they say "aircraft calling, remain clear," it isn't.) You also need a Mode C transponder and ADS-B Out.
Unlike Class B, no explicit "cleared to enter" is required — radio contact is the trigger. Class C provides radar service: sequencing to the primary airport and traffic advisories.
VFR weather minimums are 3 statute miles visibility with the standard 500 ft below, 1,000 ft above, 2,000 ft horizontal cloud clearances.
On sectionals, Class C is drawn with solid magenta lines and altitude figures (top and floor of each ring shown as a fraction). A good habit is to have the approach frequency loaded and the ATIS copied before you reach the outer shelf.
*Reference and training only. Consult current charts and the FAA AIM.*