A transponder is not required unless an aircraft is operating: In Class A, Class B, or Class C airspace. Above 10,000 feet Mean Sea Level (MSL), excluding airspace below 2,500 feet Above Ground Level (AGL).
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However, if you wish to operate in class A, B, or C airspace, or at an altitude of over 10,000' MSL, or within a 30 nautical mile radius of the primary airport in class B airspace, you will need a transponder and altitude encoder (commonly referred to as mode C).
However, if you wish to operate in class A, B, or C airspace, or at an altitude of over 10,000' MSL, or within a 30 nautical mile radius of the primary airport in class B airspace, you will need a transponder and altitude encoder (commonly referred to as "mode C").
You must receive official clearance. Also, pilots must hold a minimum of a private pilot certification to enter Class B airspace. Sport pilots, recreational pilots, and student pilots are not authorized to operate in Class B airspace.
Class B airspace surrounds the busiest airports from the surface to 10,000 feet MSL. The dimensions of Class B airspace vary depending on the needs of the airport. Class C airspace extends from the surface to 4,000 feet MSL.
You must be equipped with ADS-B Out to fly in most controlled airspace. Any airspace that requires the use of a Transponder, described in 14 CFR 91.215, also requires aircraft to be equipped with a Version 2 ADS-B Out system.
Class D airspace: No transponder is required unless otherwise specified by ATC (Pilots only require two-way radio communication in this class of airspace).