Definition
A federal regulation that prohibits the operation of an aircraft equipped with an ATC transponder unless that transponder has been tested and inspected within the preceding 24 calendar months. The inspection must be performed by an appropriately certified person, and the results must be recorded in the aircraft's maintenance records.
Plain English
A rule saying that if your aircraft has a transponder, that transponder must be checked and signed off at least once every two years before you can legally fly with it.
Context Anchor
Seen in aircraft logbook and preflight paperwork checks, especially when confirming required inspections before a flight.
Derivation
14 CFR means Title 14 of the Code of Federal Regulations, which is the section of U.S. federal law covering aeronautics and space. Part 91 covers general operating and flight rules. Section 91.413 is the specific paragraph within Part 91 dealing with transponder tests and inspections. The numbering is just a filing system: title, part, section.
Why Pilots Care
A current inspection keeps the transponder legal for flight in controlled airspace and ensures reliable radar returns and altitude reporting.
Intuition Check
Do not read this as just a random legal number. It points to a specific rule about whether the aircraft's transponder inspection is current and legal.
Example Sentence 1
During the preflight assessment, the pilot checked the maintenance logs to confirm the transponder inspection required by 14 CFR part 91, section 91.413 was still current.
Example Sentence 2
An FAA inspector may request proof that the transponder complies with 14 CFR part 91, section 91.413 during a ramp check.