Definition
The Federal Aviation Regulation that requires the ATC transponder and its altitude reporting equipment to be tested and inspected by an appropriately rated person within the preceding 24 calendar months before the transponder may be used in controlled airspace. No person may use a transponder unless it has passed this inspection.
Plain English
This is the rule that says your aircraft's transponder must be checked and signed off every two years, or you can't legally use it in airspace where ATC is watching.
Context Anchor
Seen in aircraft inspection records, logbooks, preflight planning, and discussions of transponder inspection requirements.
Derivation
14 CFR means Title 14 of the Code of Federal Regulations, the part of U.S. federal rules that covers aviation. Part 91 contains general operating and flight rules, and section 91.413 is the specific rule number for transponder tests and inspections.
Why Pilots Care
An uninspected transponder makes the aircraft illegal for operations in controlled airspace and can result in enforcement action or denied ATC services.
Intuition Check
Do not read “part” and “section” as casual labels or page numbers. Here they point to an exact federal aviation rule that creates a legal requirement.
Example Sentence 1
Before the cross-country, the pilot checked the maintenance logs and confirmed the 91.413 transponder inspection was current.
Example Sentence 2
During the annual inspection, the shop performed the required transponder check to satisfy 14 CFR part 91 section 91.413.