Relevant Discussion: 91.215, 91.217, 91.135, 91.411, 91.413, 99.13, 121.345, 135.143, AIM 3-2-3, 3-2-4, 4-1-20, 5-6-4, 6-2-2, 6-4-2, FAA-H-8083-16, 8083-25
Everything You Need to Know about Mode C Transponders
Key Takeaways:
- Transponders (Mode C and ADS-B Out) are generally required in Class A, B, and C airspace, within 30 nautical miles of specific Class B/military airports (up to 10,000 ft MSL), above the ceiling of Class B/C airspace (up to 10,000 ft MSL), and at or above 10,000 ft MSL across the contiguous U.S. (excluding below 2,500 ft AGL), with specific exceptions for certain aircraft like balloons, gliders, or those without engine-driven electrical systems.
- Transponders transmit pressure altitude (independent of the pilot's altimeter setting) to ATC, must agree with the barometric altimeter within ±125 feet, and should be operated at all times in controlled airspace and regulated areas, including prior to surface movement. Common squawk codes include 1200 (VFR), 7700 (Emergency), 7600 (Lost Comms), and 7500 (Hijack), while the "Ident" button is used when requested by ATC to highlight the aircraft's radar target or acknowledge instructions during radio failure.
- Altitude encoders for IFR flight in controlled airspace require testing and re-certification every two calendar years. Procedures exist for operating with an inoperative transponder (flight to ultimate destination/repairs) or without one (requiring at least one-hour prior authorization from ATC for non-equipped aircraft). Mode S transponders provide enhanced capabilities, including air-to-air datalink for TCAS II resolution maneuvers.
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