If you fly in Class A, Class B, or Class C airspace, inside the 30-nautical-mile Mode C Veil around a major airport, or anywhere in the contiguous 48 states at or above 10,000 feet MSL, your aircraft needs an operating Mode S transponder and ADS-B Out equipment broadcasting your position. Those are the core aircraft transponder and ADS-B requirements the FAA has enforced since January 1, 2020. The equipment must be paired with a WAAS-enabled GPS position source, tested every 24 months, and verified after installation through a public performance report. The details below cover where the rules apply, what hardware satisfies them, what paperwork the installer owes the FAA, and what happens if you fly without compliance.
Where the Equipment Is Required
14 CFR 91.215 sets the transponder boundary. You need a functioning Mode S (or Mode A with altitude reporting) transponder in Class A, Class B, and Class C airspace, within the 30-nautical-mile ring around each major airport listed in Appendix D of Part 91 (from the surface up to 10,000 feet MSL), and anywhere in the contiguous 48 states and D.C. at or above 10,000 feet MSL unless you’re within 2,500 feet of the ground.1eCFR. 14 CFR 91.215 – ATC Transponder and Altitude Reporting Equipment and Use Pilots call the 30-mile ring the Mode C Veil.
ADS-B Out sits on top of that. 14 CFR 91.225 requires ADS-B Out anywhere a transponder is required, with one wrinkle at altitude: in Class A airspace (at and above 18,000 feet) the equipment must meet the 1090ES standard.2eCFR. 14 CFR 91.225 – Automatic Dependent Surveillance-Broadcast (ADS-B) Out Equipment and Use Below 18,000 feet you can satisfy the mandate with either a 1090ES transponder or a 978 MHz Universal Access Transceiver.
Performance thresholds for the broadcast itself come from 14 CFR 91.227. Navigation Accuracy Category for Position must be less than 0.05 nautical miles, Navigation Integrity Category must be less than 0.2 nautical miles, and Source Integrity Level must be no greater than 10⁻⁷ per flight hour.3eCFR. 14 CFR 91.227 – Automatic Dependent Surveillance-Broadcast (ADS-B) Out Equipment Performance Requirements In practice this means a WAAS-enabled GPS source. A basic panel GPS from the 1990s won’t qualify.
What Counts as Compliant Equipment
A compliant 1090ES installation has three parts: a Mode S transponder certified to TSO-C112, ADS-B Out capability certified to TSO-C166b (or the newer TSO-C166c), and a WAAS-enabled GPS position source.2eCFR. 14 CFR 91.225 – Automatic Dependent Surveillance-Broadcast (ADS-B) Out Equipment and Use Most modern units combine Mode S and ADS-B Out in a single box. Any panel-mount WAAS GPS approved after 2010 typically meets the accuracy thresholds in 14 CFR 91.227.
Before your shop opens the panel, confirm your aircraft’s ICAO 24-bit address through the FAA Aircraft Registry.4Federal Aviation Administration. Aircraft Inquiry This six-character hexadecimal code gets programmed into the transponder during setup. If it’s wrong, every broadcast will carry the wrong identity and your performance report will fail. The installer also has to confirm the existing antenna can handle 1090 MHz transmission without excessive signal loss and that the electrical system can power the new unit.
1090ES or 978 UAT?
If you fly at or above 18,000 feet, or ever plan to, 1090ES is your only option. UAT is restricted to operations below FL180 within the United States.5Federal Aviation Administration. Frequently Asked Questions It’s also a domestic-only frequency. No other country accepts 978 UAT; Australia, Europe, Canada, Hong Kong, and South Africa mandate 1090ES exclusively. Cross a border with a UAT-only installation and you’re immediately non-compliant.
UAT does carry one genuine advantage for low-altitude domestic flying. The 978 MHz frequency delivers Flight Information Services — Broadcast, which pushes free in-cockpit weather to the aircraft: NEXRAD radar, METARs, TAFs, PIREPs, NOTAMs, icing forecasts, and turbulence data.6Federal Aviation Administration. ADS-B In Pilot Applications A 1090ES-only installation does not receive FIS-B. To get that weather with a 1090ES transponder you need a separate ADS-B In receiver, either panel-mount or a portable box paired with a tablet.
UAT-equipped aircraft still need a Mode C or Mode S transponder for TCAS compatibility. For turboprops, jets, and anyone who might leave U.S. airspace, 1090ES is the practical default. For piston aircraft that will never see FL180 and never cross a border, UAT typically costs less.
Exemptions and ATC Deviations
A narrow set of aircraft don’t need ADS-B Out at all. Aircraft that were never certificated with an engine-driven electrical system, and haven’t had one installed since, are exempt, along with balloons and gliders. These aircraft can operate in the Mode C Veil (outside Class B and C boundaries) and above 10,000 feet MSL without ADS-B equipment.2eCFR. 14 CFR 91.225 – Automatic Dependent Surveillance-Broadcast (ADS-B) Out Equipment and Use In practice this covers vintage taildraggers and some experimental aircraft running magneto-only ignition with no generator or alternator.
If your ADS-B equipment fails in flight, or your aircraft isn’t equipped, you can request a deviation from ATC. For an inoperative system, call the controlling facility at any time, including mid-flight, and request clearance to continue. If the aircraft has no ADS-B Out installed at all, request the deviation at least one hour before the proposed operation.2eCFR. 14 CFR 91.225 – Automatic Dependent Surveillance-Broadcast (ADS-B) Out Equipment and Use Controllers grant these routinely. Using them as a long-term substitute for compliance is a good way to draw attention from your local FSDO.
Installation Paperwork
Installing a Mode S transponder with ADS-B Out counts as a major alteration and requires FAA Form 337. The description of work must reference the applicable 14 CFR sections and the FAA-approved data (typically a Supplemental Type Certificate) used to substantiate airworthiness.7Federal Aviation Administration. Instructions for Completion of FAA Form 337 (AC 43.9-1G) If any part of the installation will be concealed behind skin or structure, the technician has to include a signed pre-closure inspection statement before closing things up.
Form 337 is executed in duplicate. One signed copy goes to you as the owner. The second must be forwarded to the FAA Aircraft Registration Branch within 48 hours of the aircraft being approved for return to service. Record all part numbers, serial numbers, and TSO approvals in the aircraft logbook, and update weight-and-balance records if the installation changed either.
The Post-Installation Performance Report
After installation, a verification flight is required to confirm the system performs within legal tolerances. Fly within FAA ADS-B ground station coverage and perform normal maneuvers, including turns, climbs, and descents, so the system exercises its full range of position and velocity reporting. Then request a Public ADS-B Performance Report from the FAA using your tail number or ICAO hex address and the flight date in Zulu time. Reports are typically available within an hour of landing.8Federal Aviation Administration. Public ADS-B Performance Report Request
The PAPR evaluates four categories: required message elements, integrity and accuracy values, kinematics, and formatting.9Federal Aviation Administration. Public ADS-B Performance Report (PAPR) User’s Guide The pass thresholds are:
- NACp of at least 8
- NIC of at least 7
- SIL equal to 3
- SDA of 2 or 3
Any parameter flagged red means the installation failed and needs troubleshooting before you fly into ADS-B-required airspace. Most failures come back to the GPS side: a position source that isn’t WAAS-capable, an antenna with a poor sky view, or wiring between the GPS and transponder that introduces data errors. Fix the problem and fly the verification again. There’s no limit on retests.
The 24-Month Transponder Inspection
Your transponder isn’t a set-and-forget installation. Under 14 CFR 91.413, every ATC transponder must be tested and inspected within the preceding 24 calendar months before it can be used.10eCFR. 14 CFR 91.413 – ATC Transponder Tests and Inspections The biennial check verifies that the reply frequency stays within tolerance of 1090 MHz, that receiver sensitivity meets minimum triggering levels, that RF output power falls within range, and that the Mode S address responds correctly only to its assigned code.11eCFR. 14 CFR Part 43, Appendix F – ATC Transponder Tests and Inspections
Only specific shops can perform the work: a certificated repair station with the appropriate radio rating, the aircraft manufacturer if it did the original installation, or a facility operating under a continuous airworthiness maintenance program. Most owners pair the transponder check with the pitot-static system test, since both run on the same 24-month cycle.
Any time a transponder is installed or undergoes maintenance that could introduce a data correspondence error, it must also be tested against Part 43, Appendix E, even if the 24-month cycle hasn’t expired. The post-installation bench test and the verification flight both happen before the airplane returns to regular service.
Penalties for Flying Without Compliance
Flying into transponder- or ADS-B-required airspace without the right equipment, and without an ATC deviation, exposes you to certificate action and civil penalties. The FAA can suspend or revoke your pilot certificate depending on the circumstances, and the maximum civil penalty for an individual under the current inflation-adjusted schedule exceeds $100,000 for serious violations.12Federal Register. Revisions to Civil Penalty Amounts, 2025 Most first-time inadvertent airspace busts result in lower penalties or remedial training. Deliberate or repeated violations are treated far more harshly. An unauthorized entry can also trigger an immediate grounding until the aircraft proves compliance.
Privacy Options for Owners
ADS-B broadcasts your position and identity in the clear, and anyone with a cheap receiver can pick them up. Two FAA programs limit how easily that data ties back to you.
Privacy ICAO Address
The PIA program assigns your aircraft a temporary, alternate ICAO address that isn’t linked to your registration in the Civil Aviation Registry. Your transponder broadcasts the substitute code instead of your real hex address, breaking the connection between the signal and your identity. To qualify, the aircraft must be U.S.-registered, equipped with certified 1090ES ADS-B Out avionics compliant with TSO-C166b, and carry a passing PAPR on file from within the past 180 days. You also need a third-party call sign from an FAA-approved provider to use alongside the PIA.13Federal Aviation Administration. Privacy ICAO Address Application PIA works for domestic operations only. International flights require your original ICAO address, and aircraft equipped with 978 UAT are not eligible.
Limiting Aircraft Data Displayed
LADD works on the FAA’s data-sharing side rather than the transponder side. You submit a request online, by email to ladd@faa.gov, or by mail, and the FAA restricts the flight data it shares with third-party tracking services like FlightAware and FlightRadar24.14Federal Aviation Administration. Limiting Aircraft Data Displayed (LADD) You can choose FAA Source blocking, which limits data to FAA internal use only, or Subscriber blocking, which allows data to flow to selected vendors while keeping you off public tracking sites. The FAA processes LADD requests on the first Thursday of each month, and requests received by the 15th of the preceding month generally take effect the following month.
Neither program is airtight. Signals stay receivable by anyone with the right equipment, and crowd-sourced tracking networks can sometimes correlate movements even with a PIA in use. Combined, PIA and LADD make casual tracking substantially harder.