Part 91 vs Part 135: Which Rules Apply to Your Flight

14 CFR Part 91 — "General Operating and Flight Rules" — is the baseline rulebook every civil flight in US airspace follows. 14 CFR Part 135 — "Operating Requirements: Commuter and On-Demand Operations" — is a second, stricter rulebook layered on top for flights that carry passengers or cargo for compensation or hire. Part 135 is additive, not alternative: a charter flight complies with Part 91 as well, and then meets a longer list of requirements on top of it.

The rules attach to the flight, not to the airplane. The same tail number, the same crew and the same eight seats can fly Part 91 on Tuesday, when the owner uses their own aircraft, and Part 135 on Wednesday, when that aircraft is made available to a charter certificate. "Is this a Part 135 jet?" is therefore the wrong question. "Is this flight being conducted under Part 135, and by whom?" is the right one, and it has a specific answer the operator can give you in writing.

The rule of thumb for anyone buying a flight: if you are paying someone else to fly you somewhere in the United States, the flight has to be conducted under Part 135 — or under Part 121 if it is a scheduled airline. If it is being flown under Part 91 and you are paying for the transportation, it is illegal charter, whatever it is being called on the invoice.

The FAA's test for common carriage, set out in Advisory Circular 120-12A, has four elements: a holding out of a willingness to transport persons or property, from place to place, for compensation. Meet all four and you are a common carrier and you need an air carrier certificate. Miss one and you may not be — which is why the two elements people get wrong are worth understanding precisely.

"Holding out" is much broader than advertising. It is any signal to the public that you are willing to fly people for money: a website, a listed phone number, a standing relationship with a broker, a marketplace listing, or simply a reputation established by word of mouth. There is no minimum audience size, and no requirement that the signal be paid for. Physical advertising is sufficient evidence of holding out but has never been necessary to it.

"Compensation" is broader than a fee, too. The FAA has read it to include reimbursement of costs, free flight hours, and business goodwill — benefit flowing to the person providing the flight, whether or not any money changes hands. That breadth is why so many good-faith "we're just splitting the costs" arrangements fail the test. Even genuine private carriage — a handful of hand-picked customers under contract, with no public holding out at all — generally still requires a certificate once it is carriage of persons for compensation or hire.

Duty and rest is the largest practical difference, and it runs entirely one way. Part 91 sets no federal flight-time or duty limits on the pilots of a private flight; the backstop is the general prohibition on careless or reckless operation and nothing more. Part 135 hard-caps it: on unscheduled on-demand operations a single pilot may not be assigned more than 8 hours of flight time in any 24 consecutive hours (10 hours with two pilots), and the crew must have had a qualifying rest period — at least 10 consecutive hours — in the preceding 24. A Part 135 crew that has run out of duty legally cannot launch. A Part 91 crew in the same seats, on the same day, can.

Training and checking differ in kind, not just degree. Part 135 crews fly under an FAA-approved training program, take a competency check every 12 months, and — for an instrument-rated pilot in command — an instrument proficiency check every 6 months. A Part 91 pilot needs a flight review every 24 calendar months plus instrument currency: six approaches, holding, and course intercepting and tracking within the preceding 6 months. Both are legal standards. They are not the same standard, and the gap widens in exactly the conditions where it matters.

Two other Part 135 requirements have no Part 91 equivalent at all. Passenger-carrying IFR flights need a second in command unless the aircraft is specifically approved for single-pilot operation with a functioning autopilot; under Part 91 a single pilot may fly a single-pilot-certified jet in any weather with no autopilot requirement whatsoever. And Part 135 crews, along with safety-sensitive maintenance staff, sit inside a mandatory FAA drug-and-alcohol testing program under 14 CFR Part 120 — pre-employment, random and post-accident. Part 91 crews do not.

Maintenance oversight steps up. A Part 91 aircraft needs an annual inspection and whatever the manufacturer's program requires. An aircraft on a Part 135 certificate is maintained under the operator's own FAA-accepted inspection and maintenance program, with additional inspection intervals, additional record-keeping, and — on larger aircraft — a continuous airworthiness maintenance program the FAA approves and audits. An aircraft carrying persons for hire also picks up the 100-hour inspection requirement that a private aircraft never sees.

The weather rules are the difference most buyers have never heard of, and they are the one worth remembering. Under Part 91 a pilot may legally begin an instrument approach no matter what visibility is being reported on the ground — the "look-see" approach — and may take off in visibility so low the runway is not usable for a return landing. Part 135 forbids both. A Part 135 flight may not start an approach when the reported weather is below the authorized minimums for that approach, may not depart IFR unless the destination forecast is at or above landing minimums at the estimated time of arrival, and needs a takeoff alternate when the departure airport is below landing minimums.

Part 135 also imposes destination-alternate requirements Part 91 does not, restricts single-engine IFR passenger carriage, and — on the larger aircraft — applies landing runway-length margins that a private flight is free to ignore. The practical consequence for a passenger is occasionally annoying and always in your favor: a Part 135 flight will sometimes decline the short strip nearest your destination, or hold for weather, in a situation where an identical Part 91 flight would simply go.

Tax. A Part 135 flight is commercial air transportation, so the fare carries the 7.5% federal excise tax on domestic air transportation plus a per-passenger domestic segment fee. A Part 91 flight is not subject to that tax; the aircraft pays a higher non-commercial fuel tax instead. This is a genuine line on your invoice, and it is one of the honest reasons a Part 91 quote can look cheaper than a Part 135 one for the same trip.

Insurance is where an illegal Part 91 charter stops being a paperwork problem. An aircraft insured for private use carries a policy written for private use. Flying paying passengers outside the purpose the policy states gives the insurer grounds to deny the claim — and that exposure does not land on the broker who arranged the flight. It lands on the passengers and their families.

Oversight and recourse. A Part 135 certificate holder has a named FAA principal operations inspector, an approved set of operations specifications defining exactly what it may and may not do, and a certificate the FAA can suspend or revoke. There is a regulator with an open file on the operator. On a Part 91 flight there is no operating certificate to act against. The DOT's charter-broker disclosure rule at 14 CFR Part 295 — which requires a broker to state in writing that it is an agent rather than the operator, and to name the direct air carrier flying you — is built around Part 135 carriers, so a gray-charter arrangement gives you nothing to hold anyone to.

None of the above makes Part 91 suspect. It is the correct and legal rule for the large majority of private flying. You own an aircraft, or your company does, and you fly it for your own transportation: that is Part 91, and there is nothing irregular about it. Corporate flight departments carrying their own employees on company business fly under Part 91 too, provided the transportation is incidental to the company's business and the company is not in the business of transportation.

Part 91 Subpart F also permits a defined set of cost-reimbursement arrangements on large and turbojet-powered multi-engine aircraft — time-sharing, interchange and joint-ownership agreements under 14 CFR 91.501 — where the charge is capped at an enumerated list of expenses: fuel, oil and lubricants; crew travel, food and lodging; hangar and tie-down costs away from base; insurance obtained for the specific flight; landing and airport fees; customs and permit fees; in-flight catering; passenger ground transportation; and flight-planning and weather services, plus an additional amount equal to the fuel, oil and lubricant cost. These are narrow written agreements with capped costs. They are not a general license to charge someone for a ride.

Separately, 14 CFR 61.113(c) lets a private pilot share the pro rata share of operating expenses — fuel, oil, airport expenditures and rental fees — with passengers, provided the pilot pays at least an equal share and the flight has a genuine common purpose. The FAA has been explicit that advertising such flights to the general public turns expense-sharing into holding out, and therefore into charter requiring a certificate.

Fractional programs are their own case. NetJets, Flexjet, PlaneSense and most of the large fractional names fly under Part 91 Subpart K — "Part 91K" — a fractional-specific regime finalized in 2003 that keeps Part 91's tax and operational treatment while adding Part-135-style requirements on crew duty, training and maintenance. Part 91K is not Part 135, and it is not something you can book a single trip on. The definition of the term, and what it means for someone buying a fractional share, is set out in the Part 91K glossary entry linked below rather than repeated here.

Ask one question and get the answer in writing: which certificate is this flight being operated under, and who holds it? A real answer names a company and a Part 135 certificate designator — four letters. "We work with a network of certificated operators" is not an answer, because your flight is on exactly one certificate and the person selling it to you knows which.

Check that whoever you are paying discloses their role. Under 14 CFR Part 295 an air charter broker must tell you in writing that it is a broker rather than a direct air carrier, and must identify the carrier operating your flight. A broker that will not put that in writing has answered the question.

Ask who holds operational control. Under Part 135 the certificate holder — not the broker, not the aircraft owner, and not you — must hold the authority to start, conduct or terminate the flight. It is the test the FAA applies most often when it investigates suspected illegal charter, and it is the test a gray arrangement fails. If the answer involves the aircraft owner, the broker, or anything described as shared, stop there.

Finally, sanity-check the price against the certificate rather than against the market. A quote well under the going rate for the route, aircraft and date can be a legitimate repositioning leg with the operator's costs already committed, or it can be an owner with no cost basis to recover and no certificate to lose. The two look identical on a screen and could not be more different in the air. Verify the certificate first, then decide which one you are looking at.