When flying in and out of non-towered airports, the generally accepted standard practice regarding transponder operation is to keep the transponder on and in altitude reporting mode (Mode C or S) throughout the flight, including when operating in the vicinity of non-towered airports. There is no standard practice or recommendation to turn off the transponder at 10 miles away from a non-towered airport. In fact, doing so would be contrary to best practices for enhancing situational awareness and safety. The Aeronautical Information Manual (AIM) provides comprehensive guidance on transponder operation, emphasizing its role in air traffic control (ATC) services and collision avoidance systems like TCAS (Traffic Alert and Collision Avoidance System).[1]

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The AIM states that pilots should operate their transponder with Mode C (altitude reporting) at all times in all airspace unless otherwise requested by ATC or when operating on an airport surface not in a movement area or in areas of heavy aircraft congestion on the airport surface. This continuous operation allows ATC to track aircraft and provides crucial information to other aircraft equipped with TCAS, even in uncontrolled airspace. While ATC services are not provided at non-towered airports, the transponder still contributes to the overall safety of flight by making the aircraft visible to other transponder-equipped aircraft and to ATC facilities that may be monitoring the airspace. Turning off the transponder at 10 miles out would reduce this visibility and potentially compromise safety, especially in areas with mixed VFR and IFR traffic or where other aircraft might be utilizing traffic advisory systems.[1] [2] [3]

Relevant Regulations and Guidance

• 14 CFR Part 91.215 (Transponder requirements): This regulation outlines the airspace where transponders are required, which includes Class A, B, and C airspace, and within 30 nautical miles of a Class B airport, among others. While non-towered airports are often in Class G or E airspace, the general expectation is for continuous transponder operation unless specifically exempted or instructed otherwise.[4]
• Aeronautical Information Manual (AIM) 4-1-20 (Transponder Operation): The AIM explicitly advises pilots to operate their transponder with Mode C at all times in all airspace unless otherwise requested by ATC. It highlights the benefits of continuous transponder operation for ATC and for collision avoidance systems.[1]
• FAA Safety Briefing (May/June 2017 - "See and Avoid: The Transponder's Role"): This publication reinforces the importance of transponders for both ATC and for other pilots using traffic display systems. It emphasizes that even in uncontrolled airspace, a transponder helps other pilots "see" you electronically, supplementing visual scanning.[2]
• Advisory Circular (AC) 90-66B (Non-Towered Airport Flight Operations): While this AC primarily focuses on communication and traffic pattern procedures at non-towered airports, it does not suggest turning off transponders. The emphasis is on clear communication and visual scanning, with transponder operation being a complementary safety measure.[3]

Conclusion

Therefore, the practice of turning off a transponder at 10 miles away from a non-towered airport is not a generally accepted standard practice and is contrary to FAA guidance and best practices for aviation safety. Continuous transponder operation enhances situational awareness for other aircraft and ATC, even in uncontrolled environments.

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