PA.III.A.K4FAA-S-ACS-6C · Nov 2023Knowledge
Appropriate use of transponder(s).
Appropriate use of transponder(s).
Bottom line
The transponder is the airborne portion of the secondary surveillance radar system. When required to operate their transponder/ADS-B, pilots must always operate that equipment with altitude reporting enabled unless otherwise instructed by ATC or unless the installed equipment has not been tested and calibrated.
Key numbers
| Number | What it means |
|---|---|
| Mode 3/A Code 1200, regardless of altitude, unless otherwise instructed by ATC | VFR code |
| 4,096 possible codes, four digits from 0 to 7 | Codes |
FAA references
- PHAK ch. 14 (p. 14-25) — FAA-H-8083-25C (2023)
- AIM 4-1-20 (Transponder and ADS-B Out Operation) — AIM Basic 2/20/25 w/ Change 1 (8/7/25) & Change 2 (1/22/26)
- 14 CFR 91.413 (ATC transponder tests and inspections.) — eCFR current as of 2026-03-05
- AIM 6-2-2 (Transponder Emergency Operation) — AIM Basic 2/20/25 w/ Change 1 (8/7/25) & Change 2 (1/22/26)
- AIM 6-4-2 (Transponder Operation During Two‐way Communications Failure) — AIM Basic 2/20/25 w/ Change 1 (8/7/25) & Change 2 (1/22/26)
- 14 CFR 91.215 (ATC transponder and altitude reporting equipment and use.) — eCFR current as of 2026-03-05
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