FAA Part 135 vs. Part 121: Air Carrier Operations Explained

The FAA regulates commercial air transportation under several different parts of the Federal Aviation Regulations (FARs). The two most common for domestic passenger operations are Part 121 (scheduled airline service) and Part 135 (charter and on-demand operations).

Part 121 — Scheduled Air Carriers

Part 121 applies to large airlines operating scheduled service — carriers like American, Delta, United, Southwest, and FedEx. Part 121 imposes the most stringent safety requirements:

• Pilots must hold an ATP certificate
• First officers must hold at least an ATP (as of 2013)
• Strict crew rest and duty time limits
• Aircraft must meet specific maintenance and equipment standards

Part 135 — On-Demand and Charter

Part 135 covers charter flights, air taxi operations, and scheduled commuter service in aircraft with 9 or fewer passenger seats. Requirements are less stringent than Part 121 but more demanding than general aviation:

• PIC must hold a commercial or ATP certificate
• IFR operations require an instrument rating
• Specific crew rest requirements apply

Browse FAA Part 135 certificate holders on AviationDepth.

Other FARs in Commercial Aviation

• Part 91 — General aviation (non-commercial) flight operations
• Part 121 — Scheduled airline service
• Part 133 — Rotorcraft external-load operations (helicopter long-line)
• Part 137 — Agricultural aircraft operators (crop dusters)