General requirements

This regulation establishes baseline requirements for operating aircraft under Part 135 (commuter and on-demand operations).

First, your aircraft and equipment must comply with all applicable FAA regulations—you can't operate if something doesn't meet regulatory standards.

Second, all required instruments and equipment must be both FAA-approved and working properly (with limited exceptions found in §135.179). You can't legally fly with inoperative required equipment unless specifically permitted.

Third, the regulation sets technical standards for transponder equipment. While the specific dates have passed, the rule essentially requires that any transponder installed after January 1, 1992, must meet Mode S standards (TSO-C112). The regulation clarifies that temporary transponder swaps during maintenance, reinstallations after maintenance, or moving equipment between aircraft in your fleet don't count as new "installations" requiring Mode S upgrades.

This ensures Part 135 operators maintain properly functioning, approved equipment that meets modern performance standards.

*This is a plain-English summary for study only. The official 14 CFR text on this page is controlling — always read the current regulation and consult a CFI.*