Transponder and ADS-B Out requirements
Where a Mode C transponder and ADS-B Out are required, including the Mode C veil and the 10,000-ft rule.
Two pieces of surveillance equipment determine where you may fly in busier airspace: the Mode C transponder (which reports your altitude to ATC radar) and ADS-B Out (which broadcasts your GPS position). Since 2020, the rules largely overlap.
You generally need a Mode C transponder and ADS-B Out:
- In and above Class A, B and C airspace.
- Within the Mode C veil — the 30-nm ring around the primary airport of most Class B airspaces — from the surface up to 10,000 ft.
- Above 10,000 ft MSL over the contiguous U.S. (excluding airspace at and below 2,500 ft AGL).
- In Class E at and above 10,000 ft MSL (with the 2,500-ft-AGL exclusion), and over the Gulf of Mexico in certain areas.
There are sensible exceptions: aircraft originally certificated without an electrical system (classic tailwheel types, balloons, gliders) may operate without these in some of this airspace, provided they stay outside the Class A/B/C cores and below the relevant ceilings.
ADS-B Out must meet the performance rules of 14 CFR 91.225 and 91.227, and the pressure altitude it reports must agree with your altimeter (the "data correspondence" rule). If you fly into Class B or C without the required equipment, you need a prior authorization from ATC.
Bottom line: in and around busy airspace, assume you need a working transponder and ADS-B Out, and verify your installation is current.
*Reference and training only. Consult 14 CFR 91.215/91.225 and the FAA AIM.*