Additional maintenance requirements

This regulation sets maintenance requirements for Part 135 operators of smaller aircraft (nine passenger seats or fewer). All operators must follow either the manufacturer's recommended maintenance program or an FAA-approved alternative for engines, propellers, rotors, and emergency equipment.

Special rules apply to single-engine aircraft used for passenger flights under IFR. These aircraft must have an engine trend monitoring program that includes oil analysis, performed either at the manufacturer's recommended intervals or every 100 hours (whichever is more frequent). The operator must also maintain written maintenance instructions for specific required equipment and keep detailed records of all trend monitoring results in the engine maintenance logs.

In practice, this means Part 135 operators can't simply perform basic inspections—they must follow structured maintenance programs. For single-engine IFR passenger operations, the additional monitoring requirements provide an extra safety layer by tracking engine health trends that might reveal developing problems before they cause failures.

*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.*