Part 135 of the US Federal Aviation Regulations governs on-demand commercial operations: charter, air taxi, and most non-scheduled passenger flying for hire. A company holding a Part 135 certificate has proven to the FAA that it can carry paying passengers to a commercial standard — and it is re-inspected continuously, not certified once and forgotten. If you are paying for a flight in the US on an aircraft with nine or fewer passenger seats (or up to 30 under commuter rules), it must be operated under Part 135; scheduled airlines fly under Part 121.
The certificate touches everything that matters operationally. Pilots meet higher minimums than private flying requires — a Part 135 pilot-in-command needs a commercial certificate, an instrument rating, and at least 1,200 hours for IFR passenger work, plus recurrent training and checkrides every 6–12 months. Crew duty is capped: a 14-hour duty day and 10 hours of flight time for a two-pilot crew, with mandatory rest between duty periods. Aircraft follow a continuous inspection program rather than the lighter annual-inspection regime of private aviation. And the certificate holder must exercise "operational control" — a named, accountable company decides whether the flight goes, not the aircraft's owner or the passenger.
For the customer, Part 135 is the line between charter and gray charter, and it carries your insurance. Commercial policies with passenger liability appropriate to paying customers attach to certificated operations; put money on a Part 91 flight and that coverage is in question the moment something goes wrong. Verification is free and public: the FAA's database lists every certificate holder and the aircraft on its operations specifications, and every aircraft in the Yond catalog lists its operator with direct contacts, so you can confirm the certificate and the tail number in one exchange before committing.
Some useful edge cases. Part 135 is a floor, not a ceiling — operators differ enormously above the minimum, which is what ARGUS and Wyvern audits measure. The same physical jet often flies under both rule sets: Part 91 when the owner is aboard, Part 135 when chartered, with stricter runway and weather margins applying to the charter legs — occasionally why a charter can't use a short strip the owner uses privately. And the broker who sold you the trip is not the operator; the Part 135 certificate holder named in your contract is the party responsible for your flight. Outside the US, the equivalent credential is an Air Operator Certificate.
