Under 14 CFR Part 91.413, no person may use an ATC transponder unless, within the preceding 24 calendar months, it has been tested, inspected, and found to comply with Part 43, Appendix F. This rule applies to:
- AIFR operations only.
- BClass B and C airspace operations only.
- CAll transponders operated in civil aircraft.
- DCommercial operations under Part 135 only.