A transponder test is a scheduled check that verifies the air-traffic-control transponder and its altitude-reporting circuitry continue to perform their intended functions. The inspection measures transmitter output power, frequency stability, reply accuracy, and the automatic pressure-altitude output at Mode C interrogations across sufficient test points. It subjects the unit to Mode A/C all-call signals at 235 interrogations per second, 3 dB above minimum trigger level. Because the transponder and the altimeter system operate as an integrated pair, the test is carried out together with the altimeter inspection required under FAR 91.411.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a transponder inspection in aviation?
A transponder inspection is the scheduled evaluation that ensures the transponder is transmitting on the correct frequency, measures transmitter output power, frequency stability, and reply accuracy, and verifies that altitude encoding data agree with the encoder tested under FAR 91.411, all so the unit communicates accurate identification and altitude to ATC radar systems and prevents erroneous data from being transmitted to ATC. The same inspection is the biennial check required by FAR 91.413 and must be performed every 24 calendar months by an FAA-certificated repair station holding a radio rating Class III and follows the checklist in FAR 43 Appendix F. Transponder inspections and tests must be tested and inspected after installation or maintenance where data correspondence error could be introduced. Transponder recertification is the successful completion of this inspection: when the technician records the new inspection date, the unit is recertified for another 24 months and will again share identification, altitude, and speed with ATC until the next cycle.
What are the aircraft transponder testing requirements?
The principal requirement is that the aircraft transponder must be certified every 24 calendar months in accordance with 14 CFR 91.413. This inspection must be entrusted to an FAA certificated repair station that holds both a radio rating Class III and a limited rating appropriate to the test to be performed. The altimeter and encoder must be tested within the same 24-month period as dictated by 91.411 and 91.217. All work is accomplished by an FAA certified AP-IA mechanic, guaranteeing that the transponder continues to meet the performance and environmental standards of TSO-C74b, TSO-C74c, or TSO-C112 and thereby remains usable in controlled airspace.
How to test an aircraft transponder?
To test an aircraft transponder, begin by placing the aircraft inside a shielded hangar and switch the transponder to test mode. Most operators choose to test without disconnecting the antenna. Instead they slip the antenna transmission absorption covers over the blade and connect a ramp test set directly to the transponder's antenna. The external tester - like an ARTS-7000 - simulates ground-radar signals and displays every reply, while the control unit is set to Altitude Reporting ‘ON'. Follow the instructions on the tester's screen, observe the panel and flight instruments, and verify Mode-S Address and Mode-S Diversity tests.
