Transponder test due date, 24 calendar months.

What the rule says

14 CFR 91.413(a): no person may use an ATC transponder that is specified in 91.215(a), 121.345(c) or 135.143(c) "unless, within the preceding 24 calendar months, the ATC transponder has been tested and inspected and found to comply with appendix F of part 43." A test in August 2025 is good through August 31, 2027.

No IFR condition in the text

Unlike the altimeter and static test in 91.411, this rule is tied to the transponder itself. The rule text contains no IFR condition: it covers a transponder that 91.215(a), 121.345(c) or 135.143(c) specifies. The text of 91.215 was not read for this page, so check it for when a transponder is required.

Maintenance trigger

91.413(b): following any installation or maintenance on an ATC transponder where data correspondence error could be introduced, the integrated system must be tested, inspected and found to comply with paragraph (c), appendix E, of part 43.

Who can do the test

91.413(c): a certificated repair station properly equipped and holding a radio rating Class III, a limited radio rating appropriate to the make and model transponder, or a limited rating appropriate to the test; the holder of a continuous airworthiness maintenance program under part 121 or 135.411(a)(2); or the manufacturer of the aircraft on which the transponder is installed, if the manufacturer installed it.

Worked example

Test and inspection on August 5, 2025. The 24th calendar month after August 2025 is August 2027, so the last day is August 31, 2027. On September 15, 2027 the transponder may not be used under this rule until it is tested again.

For squawk code meanings see the squawk codes reference; for the altimeter test see the altimeter and static system page.

FAQ

How often does a transponder need to be tested?

Within the preceding 24 calendar months for a transponder specified in 91.215(a), 121.345(c) or 135.143(c).

Is the transponder test only for IFR flights?

No. The rule is tied to the transponder the regulations require, not to the type of flight.

Does the test expire on the same day two years later?

No. It is good through the end of the 24th calendar month after the month of the test.

Who can test a transponder?

A properly rated repair station, a part 121 or 135 continuous airworthiness maintenance program holder, or the aircraft manufacturer if it installed the transponder.

What if the transponder was repaired?

After installation or maintenance that could introduce data correspondence error, the integrated system must be tested under 91.413(b).

Sources

Retrieved 2026-11-02. Regulation text read through the eCFR API (current as of 2026-10-29); AIM text read from the FAA publication URL below. Rules change; check the current text.

• 14 CFR 91.413, ATC transponder tests and inspections: the 24 calendar month rule, maintenance trigger and who may test.