Transponder Inspections

FAR 91.413 — Transponder Inspections

FAR 91.413 requires ATC transponder testing and inspection every 24 calendar months. Learn the rule, who can perform it, and DPE oral exam questions.

In Plain English

FAR 91.413 sets the inspection requirements that keep your ATC transponder legal to use. The rule has two main parts:

24-month test and inspection: You can't use a transponder covered by §91.215(a), §121.345(c), or §135.143(c) unless it has been tested and inspected within the preceding 24 calendar months and found to comply with Appendix F of Part 43.
Post-installation/maintenance check: After any installation or maintenance where a data correspondence error could be introduced (for example, work that could affect the altitude data sent to ATC), the integrated system must be tested per paragraph (c) of Appendix E to Part 43.

The tests must be performed by a properly rated certificated repair station, the holder of a continuous airworthiness maintenance program under Part 121 or §135.411(a)(2), or the aircraft manufacturer if it installed the transponder.

Why it matters: an out-of-date transponder check makes the aircraft unairworthy for operations that require a transponder — including most controlled airspace and Mode C veil flying.

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Regulation Text

14 CFR § 91.413

§ 91.413 ATC transponder tests and inspections. (a) No persons may use an ATC transponder that is specified in 91.215(a), 121.345(c), or § 135.143(c) of this chapter 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 of this chapter; and (b) Following any installation or maintenance on an ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), appendix E, of part 43 of this chapter. (c) The tests and inspections specified in this section must be conducted by— (1) A certificated repair station properly equipped to perform those functions and holding— (i) A radio rating, Class III; (ii) A limited radio rating appropriate to the make and model transponder to be tested; (iii) A limited rating appropriate to the test to be performed; (2) A holder of a continuous airworthiness maintenance program as provided in part 121 or § 135.411(a)(2) of this chapter; or (3) The manufacturer of the aircraft on which the transponder to be tested is installed, if the transponder was installed by that manufacturer. [Docket 18334, 54 FR 34311, Aug. 18, 1989, as amended by Amdt. 91-267, 66 FR 21066, Apr. 27, 2001; Amdt. 91-269, 66 FR 41116, Aug. 6, 2001]

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You are flying VFR in Class E airspace at 8,500 feet MSL during the day. What are your minimum visibility and cloud clearance requirements?

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Oral Exam Questions a DPE Might Ask

Q1How often must the ATC transponder be tested and inspected?

Per FAR 91.413, the transponder must be tested and inspected within the preceding 24 calendar months and found to comply with Appendix F of Part 43.

Q2Who is authorized to perform the transponder test and inspection required by FAR 91.413?

FAR 91.413(c) allows the tests to be done by a properly equipped certificated repair station with the appropriate radio or limited rating, a holder of a continuous airworthiness maintenance program under Part 121 or §135.411(a)(2), or the aircraft manufacturer if it installed the transponder.

Q3If a mechanic performs maintenance on the transponder, is the 24-month check enough before flight?

No. Under FAR 91.413(b), following any installation or maintenance where a data correspondence error could be introduced, the integrated system must also be tested and found to comply with paragraph (c) of Appendix E to Part 43.

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