Definition
A regulatory inspection and test of an aircraft's ATC transponder and altitude reporting system, required under 14 CFR 91.413 within the preceding 24 calendar months for any aircraft operated with a transponder. The inspection verifies that the transponder transmits correct identification and altitude data and meets the performance standards specified in 14 CFR Part 43, Appendix F.
Plain English
A check, required every two years, that confirms the aircraft's transponder is working correctly and reporting the right altitude to air traffic control.
Context Anchor
Seen in aircraft maintenance records, airworthiness checks, and preflight planning for airspace where a transponder is required.
Derivation
Transponder comes from "transmitter" + "responder" -- a device that responds when interrogated by a radar signal. The inspection ensures that response is accurate and within tolerance.
Why Pilots Care
Keeps the aircraft legal for operations in airspace where a transponder is required and prevents communication or identification problems with ATC.
Intuition Check
Do not read inspection here as a quick visual look at the panel. A transponder inspection is a required equipment test that must be recorded as current.
Example Sentence 1
Before the cross-country, the pilot checked the logbook and confirmed the transponder inspection was still within the 24-month window.
Example Sentence 2
Before departing for Class C airspace the pilot verified the transponder inspection date was within the last 24 months.