Answered By AOPA
You plan on flying your single engine aircraft to an airport that is located under the 10 NM outer airspace ring of a Class C airport. The aircraft you fly does not have a transponder. Can you make this flight into this airport? (We are assuming that the Class C is not part of a larger airspace complex—e.g., underlying a Class B)
AOPA Staff Answer
You would be able to make this flight with no transponder; as a transponder is not required underneath the outer airspace shelf of Class C airspace. However, it is required inside Class C airspace and above the Class C airspace up to 10,000’ MSL, so keep that in mind. Also remember that if you are transponder equipped FAR 91.215(c) requires that you operate that transponder, including mode C (if installed), anytime you are in controlled airspace.
How about in the case of ADS-B?
It's the same requirement. You would not be required to have ADSB out under the 5-10 mile shelf of Class C airspace. FAR 91.225 is the governing regulation for ADSB. The general rule of thumb is if you needed a mode C transponder there before January 1, 2020, you'll need ADSB out there today.