No person may use an ATC transponder unless it has been tested and inspected within at least how many calendar months?

• A. 24 calendar months
• B. 12 calendar months
• C. 36 calendar months
• D. 60 calendar months

Answer: (A)

A transponder must be tested and inspected every 24 calendar months. This biennial interval ensures the unit encodes altitude correctly and responds reliably to radar interrogations, so ATC can accurately see your aircraft’s position and altitude. The test is performed by an FAA-approved technician, and the aircraft records must show the date of the inspection. If the last test is older than 24 months, the transponder isn’t considered current for operation in airspace where a functioning transponder is required.