Is it illegal to fly with a transponder off?

Yes, you can in the US in Class D, E & G airspace according to 14 CFR 91.215. You will need to placard the transponder INOP, and make a note in the aircraft logbook. But, you must stay out of any Mode C Veils, and further than 30 miles from Class B airports.



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If requested by ATC, pilots are allowed to turn off the transponder.

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It's also worth noting that if the transponder is switched off you generally only lose two pieces of information: the flight identification (mode A & mode S data) and altitude (mode C data). Primary radar will still paint a target (albeit unidentified) as long as the aircraft is within radar range.

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If it's failing and/or on fire, you want the power off. Two: operationally, it is sometimes necessary for ATC to request planes on the ground turn off their transponders to declutter the radar.

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NATO aircraft operating around Ukraine turned off their transponders all at once leading up to this morning's Russian mobilization announcement. They did it all at once a couple hours before the announcement.

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A Primary Radar System. Typically located on the control tower or other strategic location on the airport, the Primary Radar antenna is able to detect and display aircraft that are not equipped with or have malfunctioning transponders or ADS-B.

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Radio Procedures for Class B Airspace Unlike Class C airspace, a simple acknowledgement by ATC is not sufficient for you to enter Class B airspace. You must also receive clearance and a unique transponder squawk code to enter.

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