What Is the Difference Between Part 91 and Part 135?

How it works

Part 91 and Part 135 are two parts of the United States Federal Aviation Regulations, formally Title 14 of the Code of Federal Regulations (14 CFR). They set out different rules depending on whether a flight is private or commercial, and the distinction is the foundation of how charter is legally allowed to operate in the United States.

14 CFR Part 91 sets the general operating rules for civil aviation and covers, among much else, flights an aircraft owner or a company flies on its own account, with no payment changing hands for carrying passengers or cargo. A business flying its own jet to move its own staff, or an individual flying friends without charging them, normally operates under Part 91. Oversight is lighter: there is no requirement for an Air Operator Certificate, and no requirement that the FAA approve the operator's maintenance and training programme to the same standard as a charter carrier.

14 CFR Part 135 covers commuter and on-demand operations, the rules an operator must follow to carry passengers or cargo for compensation. To fly under Part 135 in the United States, an operator must hold an FAA Air Operator Certificate, which sets out approved aircraft, approved pilots, an approved maintenance programme, and an approved crew training and drug-and-alcohol testing programme. The FAA inspects and audits Part 135 operators on an ongoing basis. Almost every charter flight a private aviation client books in the United States, whether through a broker, an operator directly, or a jet card programme, is flown under Part 135.

The line between the two is what a flight is for, not what aircraft is used or who owns it. The same jet can be flown under Part 91 one day, by its owner, and chartered under Part 135 the next, by a certificated operator, provided the aircraft and crew meet Part 135's separate approval. A Part 91 operator carrying paying passengers without holding a Part 135 certificate is operating illegally in the United States; this is one meaning of the term grey charter.

Part 91 and Part 135 are United States classifications with no direct equivalent numbering elsewhere. The UK, under the Air Navigation Order 2016, and the EU, under Regulation (EU) No 965/2012 on Air Operations, draw a broadly similar line between private and commercial flying and both require an Air Operator Certificate for commercial operations, but the detailed rules differ from the US ones and are not interchangeable with them. This entry describes the United States position as it stood in September 2026; the regulations are subject to change.

Cost impact

Part 135 status adds cost in the United States, and that cost is built into what a charter client pays. A Part 135 operator carries the overhead of its FAA Air Operator Certificate: approved training and recurrent checking for pilots, a maintenance programme often more demanding than Part 91's, drug and alcohol testing, and ongoing audit. These costs sit inside the hourly charter rate quoted by any operator flying under Part 135.

There is also a direct tax consequence in the United States. Payments for commercial air transportation are subject to the federal excise tax on air transportation, administered by the US Internal Revenue Service, and this applies to Part 135 charter but generally not to a genuine Part 91 private operation where no fare is charged. The rate has stood at approximately 7.5% of the amount paid for the flight for some years, though the precise current rate and any segment fee should be checked directly, since tax rules change and this figure is not certain enough to state here as settled fact.

A charter quote for a United States Part 135 flight should show this tax as a separate line, on top of the flight time charge, any fuel surcharge and any handling fees. A Part 91 arrangement, such as cost-sharing among an aircraft's owners, is not a commercial fare and does not carry this tax in the same way, though the rules on what an owner may lawfully charge another party under Part 91 are narrow, and a broker or operator offering a cheaper 'Part 91 charter' than Part 135 competitors is a common warning sign of grey charter.

Example

Suppose, for illustration, a charter operator in the United States quotes an illustrative $10,000 in flight time for a return trip on a midsize jet, flown under Part 135. The federal excise tax, taken here as an illustrative approximate 7.5%, would add around $750 to the invoice, before any fuel surcharge or handling fees, taking the flight-time portion of the bill to roughly $10,750.

Compare that with the same aircraft flown under Part 91 by its own owner, carrying colleagues at no charge: no fare is paid, so there is no excise tax line at all, and no charter contract exists, because no commercial transportation has taken place. The saving is not really a discount; it reflects that a Part 91 flight is not a chartered service and is not open to a member of the public buying a seat or a trip.

Where a client is quoted a materially cheaper flight from a party claiming to operate under Part 91 while effectively selling seats or trips to strangers, the illustrative $750 tax saving and the lower apparent price can be a sign that the flight is not being lawfully offered as United States Part 135 commercial charter.