Without ADS-B Out, you can fly in any airspace except the ADS-B rule airspace defined by FAR 91.225 (see above). Note that ADS-B is not required in Class D airspace, or under a Class B or Class C airspace shelf, unless it lies within a Mode C veil.
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Before entering Class C airspace, the pilot MUST establish communication with the radar service. Radio contact with radar and/or tower must be maintained when in this airspace. You may request Flight Following Radar Service outside the 10 mile shelf. It may be granted on a workload permitting basis.
The ceiling of a Class C airspace should be 4,000 feet above the primary airport's field elevation. The surface area extends from the surface to the upper limit of the airspace. The floor of the airspace between the 5 and the 10 NM must extend from no lower than 1,200 feet AGL to the upper limit of the airspace.
Class D airspace: No transponder is required unless otherwise specified by ATC (Pilots only require two-way radio communication in this class of airspace).
The purpose of Class C airspace is to enhance aviation safety in the terminal area and to decrease the risk of midair collisions by providing traffic separation service.
The regulation 14 CFR 91.225(e) allows aircraft not certificated with an electrical system, including balloons and gliders, not equipped with ADS-B Out to operate within 30 nautical miles of a Class B primary airport—basically, within its Mode C veil—while remaining outside of any Class B or Class C airspace.
Although the cost of installing equipment varies from aircraft to aircraft, the Civil Aviation Safety Authority (CASA) estimates the median price of installing a good quality certified ADS-B (out) unit at about $8,000, or $4,000 after the rebate.
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 may fly without ADSB under the Class B or Class C shelf provided you remain outside of any Mode C veil. §91.225 tells you where you cannot fly. It says you cannot fly within the lateral boundaries of the Class B or C airspace or above the airspace. It does not say anything about under the shelf.
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).
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.
A Mode C transponder gives air traffic control (ATC) your altitude and an identification code. Specifically, a Mode C transponder is required if you wish to operate in Class A, B, or C airspace, at an altitude of over 10,000' MSL, or within a 30-nautical mile radius of the primary airport in Class B airspace.
Class E airspace is more related to weather requirement than it is to communication and in most cases does not require ATC communications. Class G (it's called “uncontrolled” airspace) never requires VFR pilots to be in communication with ATC. I know you are wondering: There is no class F airspace in the United States.