Direct Answer
Part 91, Part 121, and Part 135 are the three FAA operating rules. Part 91 is general, non-commercial flying. Part 135 is on-demand and commuter air carriers — smaller commercial operations. Part 121 is scheduled and large air carriers — the airlines and large charter, and the most demanding regime. Which one applies turns on two things: whether you carry others for compensation or hire, and the size and schedule of the operation.
The framework lives in 14 CFR §119.1 (who must hold a certificate) and §119.21 (which operation runs under which part). The seat and payload thresholds that separate Part 135 from Part 121 come from the definitions in 14 CFR §110.2.
This is a legal characterization, not a documentation one. Which rule applies to a specific operation is a determination for an aviation attorney and your FAA Flight Standards office. FileFlo does not make that call — it does the records work each regime requires, keeping the pilot, training, maintenance, manual, and authorization records audit-ready.
One Framework, Two Questions
People often picture these three parts as a ladder defined by aircraft size: small planes are “Part 91,” bigger ones are “Part 135,” and jets are “Part 121.” That is not how the regulation works. The rules are organized by the nature of the operation first, and aircraft size second. A single Pilatus PC-12 or a King Air can be flown under Part 91 for the owner’s own business, and the very same airframe can require a Part 135 certificate the next day if it is used to carry the public for compensation.
The applicability rule lives in 14 CFR §119.1. It reaches persons operating as an air carrier or commercial operator in air commerce, and it expressly also reaches noncommon carriage and private carriage for compensation in aircraft with fewer than 20 passenger seats and a payload capacity under 6,000 pounds. Once §119 applies, §119.21 assigns each kind of operation to a part: domestic, flag, and supplemental operations are conducted under Part 121, while commuter and on-demand operations are conducted under Part 135.
So the analysis is two questions, in order. Question one (the commercial test): are you conducting common carriage for compensation or hire? If not, you are generally in Part 91. If yes, §119 generally requires a certificate, and you are in 121 or 135. Question two (the scale test): defined by the aircraft and the schedule in 14 CFR §110.2, do the seats, payload, and scheduled-versus-on-demand character land you in Part 135 or push you up into Part 121?
This is a legal question — defer it to counsel
The boundaries between these parts have been litigated and interpreted for decades, and the outcomes are fact-specific. Nothing here is legal advice. If there is any chance your operation is for-hire, or sits near a size threshold, get a written determination from a qualified aviation attorney and confirm it with your FAA Flight Standards office before the operation. FileFlo is the records layer for operators who already know which rules apply; it does not classify your operation or render legal opinions.
This guide adds the third regime to the two-way comparison most people start with. If you only need the commercial line — whether a flight is private or for-hire — see our dedicated breakdown of Part 91 vs Part 135: compensation or hire, which goes deep on holding out and the four-element common-carriage test. Here we keep that line and add where Part 121 begins.
Which Rule Applies: A Two-Step Decision
Here is the same logic laid out as a decision you can walk through. Treat it as an orientation map, not a legal conclusion — every branch ends with “confirm with counsel.”
Step 1 — Are you carrying others for compensation or hire?
If you are flying your own or your company’s aircraft for your own purposes and not holding out to carry the public for value, you are generally under Part 91. If value flows to you for carrying passengers or cargo — and “value” is read broadly by the FAA — you have likely crossed into commercial territory and 14 CFR Part 119 generally requires a certificate. This first question separates Part 91 from the commercial regimes (Parts 121 and 135).
Step 2 — Is it common carriage — are you holding out to the public?
Common carriage means holding out a willingness to serve the public (or a segment of it). Common carriage for hire in smaller aircraft is the heart of Part 135. Even private (contract) carriage for compensation can require a certificate under §119.1 when the aircraft is under 20 seats and 6,000 lb payload. Either way, if you are for-hire you are now choosing between Part 135 and Part 121.
Step 3 — How big is the aircraft, and is the service scheduled?
This is the 14 CFR 110.2 scale test. On-demand or commuter operations in airplanes of 30 seats or fewer and 7,500 lb payload or less (and rotorcraft) are Part 135. Scheduled or large service — turbojets, more than 9 passenger seats, or more than 7,500 lb payload — is Part 121. This step is what separates the two commercial regimes.
| Attribute | Part 91 | Part 135 | Part 121 |
|---|---|---|---|
| Type of flying | General / non-commercial | On-demand & commuter air carrier | Scheduled & large air carrier |
| For compensation or hire? | No (not common carriage for hire) | Yes | Yes |
| Certificate under Part 119? | No | Yes (air carrier / operating certificate) | Yes (air carrier certificate) |
| Typical passenger seats | Any (not for-hire) | 30 or fewer (common carriage); commuter 9 or fewer | More than 9 seats, or scheduled turbojet, or > 30 (supplemental) |
| Payload capacity | N/A | 7,500 lb or less | More than 7,500 lb |
| Operation names (§119.21) | — | On-demand, Commuter | Domestic, Flag, Supplemental |
| SMS required? | Not generally (voluntary) | Yes — deadline May 28, 2027 | Yes (long-standing) |
Seat, payload, and operation-type values reflect the definitions in 14 CFR §110.2 and the operation-to-part mapping in §119.21. They are general orientation, not a substitute for a legal determination on your specific operation. The SMS deadline applies to Part 135 and certain air-tour operators.
Part 91 — General Operating Rules
Part 91 is the foundation. It contains the general operating and flight rules that apply to nearly all civil aircraft in U.S. airspace — right-of-way rules, equipment requirements, altimeter and transponder rules, and much more — and it is the regime under which non-commercial flying is conducted. Private owners, corporate flight departments flying their own people on company business, flight training, and personal flying generally live here. Part 91 is not “unregulated”; it is simply the rule set for flying that is not for-hire common carriage.
For larger and turbojet aircraft, Part 91 Subpart F (§91.501 and following) provides specific operating rules and a defined, narrow set of cost-bearing arrangements — company carriage incidental to a business, time-sharing, interchange, and joint-ownership agreements — that can be conducted without a Part 135 certificate. These are technical, condition-laden exceptions, not a general license to carry the public for money, and they are a frequent setting for illegal-charter findings when the substance does not match the paperwork.
What typically lives under Part 91
The Part 91 trap: the flight-department company
A common structure — putting an aircraft in a separate LLC whose only business is flying — can inadvertently create an entity that is providing transportation for compensation, which is exactly what Part 135 regulates. This is the “flight department company” trap. Whether a given ownership structure stays inside Part 91 is a legal question. See the flight department company trap and corporate flight department records.
Even though Part 91 is the lightest regime structurally, it still generates real compliance documents — aircraft maintenance and airworthiness records, pilot currency, lease paperwork under truth-in-leasing, and (increasingly) voluntary SMS records. For the exact line between Part 91 and Part 135, our companion guide on compensation or hire and common carriage is the deep dive.
Part 135 — On-Demand & Commuter Air Carriers
Part 135 is the commercial regime for on-demand and commuter operations — the home of most charter, air-taxi, and smaller scheduled service. It is a substantial step up from Part 91: a Part 135 operator holds an air carrier or operating certificate issued under Part 119, runs operations against FAA-issued operations specifications (OpSpecs), maintains manuals and training programs, and must keep extensive records proving qualified crews and airworthy aircraft.
Under 14 CFR §110.2, the two operation types that run under Part 135 are defined like this:
On-demand operation
Passenger-carrying common carriage (including public charter and negotiated-terms flights) in airplanes or powered-lift — including turbojet-powered ones — with a passenger-seat configuration of 30 seats or fewer (excluding crew seats) and a payload capacity of 7,500 pounds or less. It also covers noncommon/private carriage in aircraft under 20 seats and under 6,000 lb payload, essentially any rotorcraft operation, all-cargo work at 7,500 lb or less, and limited scheduled service that does not rise to commuter frequency.
Commuter operation
Scheduled passenger operations in non-turbojet airplanes (or powered-lift) with 9 passenger seats or fewer and 7,500 lb payload or less — or rotorcraft — run with a frequency of at least five round trips per week on at least one route between two or more points, per published schedules.
The simple way to hold the numbers
For common-carriage passenger flying, Part 135 tops out around 30 seats and 7,500 pounds of payload. Above that — or once you fly scheduled turbojet service, or airplanes with more than 9 seats — you are generally into Part 121. Rotorcraft are the big exception: even large helicopters typically operate on-demand under Part 135. (Commuter, the scheduled flavor of 135, is capped much lower at 9 seats and non-turbojet.)
Part 135 is also where the SMS mandate now bites. The FAA requires a Safety Management System for Part 135 operators (and certain air-tour operators) with a single compliance deadline of May 28, 2027. That is a genuine convergence toward the kind of organized safety oversight Part 121 has long required. SMS is something you build and run with your safety leadership — FileFlo holds the resulting records, it does not run the program.
To go deeper on the Part 135 commercial line and the certification path, see Part 91 vs Part 135: compensation or hire, how to get a Part 135 certificate, and the FAA’s grey-charter crackdown.
Whichever rule you operate under, can you prove it on demand?
FileFlo does not decide whether you are Part 91, 121, or 135, and it does not give legal advice — that is your aviation attorney’s job. What it does is the records work each regime demands: classifying, version-controlling, and tracking expirations on your pilot, training, maintenance, manual, and OpSpecs records so they are audit-ready, not reconstructed in a panic. Starter at $89/mo, Professional at $299/mo. 5-day free trial, no credit card required.
Part 121 — Scheduled & Large Air Carriers
Part 121 is the regime for the airlines and large air carriers — the most extensive and demanding of the three. It governs scheduled passenger service in larger aircraft and non-scheduled (supplemental) operations above the Part 135 thresholds. Part 121 carriers carry the heaviest operational, dispatch, maintenance-program, training, and crew-qualification requirements in the system, reflecting the scale and public exposure of airline-style operations.
Per §119.21, three operation types run under Part 121, each defined in §110.2:
Domestic operation
Scheduled passenger service within the 48 contiguous states and D.C., using turbojet airplanes, airplanes with more than 9 passenger seats, or aircraft with a payload capacity of more than 7,500 pounds.
Flag operation
The same aircraft criteria as domestic, but service that touches Alaska, Hawaii, U.S. territories, or points outside the United States — essentially scheduled operations with an international or extra-territorial leg.
Supplemental operation
Non-scheduled (charter-style) common carriage above the Part 135 ceiling — generally airplanes with more than 30 passenger seats or more than 7,500 lb payload, plus certain turbojet and large propeller airplanes operated outside scheduled service.
Domestic vs. flag vs. supplemental, simply
Within Part 121, the names mostly describe where and how you fly: domestic = scheduled within the contiguous U.S.; flag = scheduled with an Alaska/Hawaii/territorial/international leg; supplemental = non-scheduled (charter) at large-aircraft scale. All three sit under the same Part 121 rulebook, which is why an operation that grows past the 135 thresholds faces a major step-change in requirements, not a minor one.
Most FileFlo aviation readers are Part 91 flight departments or Part 135 operators rather than Part 121 carriers, but the boundary matters: a growing 135 operator adding larger jets or scheduled service needs to know precisely when it would be pushed into Part 121, because the certificate, manuals, and recordkeeping obligations change wholesale.
The Part 135 → Part 121 Boundary
Because so many searches ask specifically “when does Part 135 become Part 121?”, it is worth isolating that line. Both are commercial regimes, so the commercial question is already answered — what separates them is the scale and service test in 14 CFR §110.2. An operation is generally pushed from 135 into 121 when any of these is true:
And what generally keeps a commercial operation in Part 135: airplanes (including turbojets) with a passenger-seat configuration of 30 seats or fewer and a payload capacity of 7,500 pounds or less in on-demand service, scheduled non-turbojet service at 9 seats or fewer (commuter), and essentially any rotorcraft operation. The recurring numbers to remember are 9 seats (the scheduled / domestic line for airplanes), 30 seats (the on-demand common-carriage ceiling), and 7,500 pounds of payload.
Crossing the line is not a paperwork tweak
Moving from Part 135 to Part 121 is a wholesale change — a different certificate, different manuals, different training and dispatch systems, and a much larger compliance footprint. An operator contemplating larger aircraft or scheduled service should plan that transition deliberately, with counsel and the FAA, well before the first qualifying flight. The exact application of the §110.2 thresholds to your fleet and schedule is a legal and certification question, not a self-service one.
A quick note on a related “Part” people confuse with these: Part 145 is not an operating rule at all — it governs certificated repair stations (maintenance organizations), not how you fly. It sits alongside 91/121/135 rather than competing with them. If your maintenance is performed by a repair station, that is a Part 145 relationship feeding the airworthiness records your operating certificate requires.
The Records Each Regime Requires — and Where FileFlo Fits
Here is where the legal question hands off to a documentation one. Deciding which rule applies belongs to your aviation attorney and FAA Flight Standards office. But once you know, each regime carries a continuous obligation to prove compliance with current, organized, retrievable records — and the heavier the regime, the heavier the recordkeeping. A surveillance inspector does not take your word that you are compliant; they ask to see the documents. These are the record families that matter, and how FileFlo keeps them audit-ready.
Pilot qualification & currency records
Part 91 / 135 / 121 pilot recordkeepingWhy it matters
Every regime requires evidence that crews are certificated, checked, and current — lightest under Part 91, far more extensive under 135 and 121. The operation, not just the pilot certificate, must prove qualification for the flights it conducts.
How FileFlo keeps it audit-ready
FileFlo classifies and tracks pilot qualification and currency records, surfacing expirations before they ground a flight.
Maintenance & airworthiness records
Aircraft maintenance recordkeepingWhy it matters
Inspection-program and continuous-airworthiness records must be complete and current for each aircraft — the same airframe maintained to a heavier program once it moves from Part 91 into a certificated operation, often with a Part 145 repair station feeding the records.
How FileFlo keeps it audit-ready
FileFlo indexes maintenance and inspection records against each tail number and program, keeping the airworthiness picture complete and retrievable.
Operating manuals — kept current
14 CFR §135.21 (Part 135)Why it matters
Commercial operators must prepare, keep current, and use a General Operations Manual; Part 121 manual obligations are heavier still. A stale or superseded manual is a finding. (Part 91 flight departments increasingly maintain manuals voluntarily.)
How FileFlo keeps it audit-ready
FileFlo version-controls each manual revision with effective dates and a retained history, so the live version is always identifiable and superseded ones never get mistaken for current.
OpSpecs & authorization documents
14 CFR §119.43 (certificated operators)Why it matters
Part 135 and 121 operators run against operations specifications that must be maintained and mirrored in the operating manual; each authorization implies underlying evidence the conditions are still met. The crossing into a certificated regime is, in large part, the issuance of these OpSpecs.
How FileFlo keeps it audit-ready
FileFlo version-tracks OpSpecs revisions and links them to the manual excerpts and records that must stay in sync.
Related reading: What records a Part 135 operator must keep · Part 135 pilot records required by the FAA · Operations specifications (OpSpecs) explained · General Operations / Maintenance Manual requirements · How to prepare for a Part 135 surveillance audit · Single-pilot operator records
FileFlo is the records layer — not legal counsel and not the certification team
To be unambiguous: FileFlo is a compliance document intelligence platform that classifies, indexes, version-controls, and tracks expirations on your compliance documents. It does not decide whether your operation is Part 91, 121, or 135, render legal opinions, obtain your certificate, file your application, write your manuals, run your SMS, or provide legal, financial, or tax advice. Which rule applies belongs to your aviation attorney and your FAA Flight Standards office; certification belongs to your certification team and required management personnel. Once you know which regime you operate under, keeping the record that proves your compliance complete, current, and audit-ready is the document problem FileFlo solves. (FileFlo does not claim SOC 2 certification.)
Frequently Asked Questions
What is the difference between Part 91, Part 121, and Part 135?
Part 91 is the FAA's general operating rule for non-commercial flying — your own or your company's aircraft flown for your own purposes, with no holding out to carry the public for compensation. Part 135 governs on-demand and commuter air carriers: smaller-scale commercial carriage for compensation or hire. Part 121 governs scheduled and large air carriers — the airlines and big charter — and it is by far the most demanding regime. The dividing lines are set by 14 CFR Part 119 and the definitions in 14 CFR 110.2: whether you are conducting common carriage for compensation pushes you from Part 91 into 121 or 135, and the aircraft size (passenger seats and payload) and whether the service is scheduled decide between 135 and 121. FileFlo is the records layer once you operate under any of these rules; it does not decide which rule applies to you — that is a determination for aviation counsel and your FAA Flight Standards office.
What is the difference between Part 121 and Part 135?
Both Part 121 and Part 135 govern commercial carriage for compensation, but they sit at different scales. Part 135 is on-demand and commuter operations in smaller aircraft — broadly, common-carriage passenger operations in airplanes (including turbojets) with a passenger-seat configuration of 30 seats or fewer and a payload capacity of 7,500 pounds or less, plus essentially any rotorcraft and most all-cargo work at that size. Part 121 is the scheduled and large air-carrier regime: it captures operations in turbojet airplanes, airplanes with more than 9 passenger seats, or aircraft with a payload capacity of more than 7,500 pounds, as well as supplemental operations above 30 seats. In plain terms, when an operation outgrows roughly 30 seats, runs as a true scheduled airline service in larger aircraft, or otherwise crosses the 110.2 thresholds, it moves from Part 135 to the much heavier Part 121. The exact seat and payload numbers come from the definitions in 14 CFR 110.2; whether your specific operation crosses them is a legal call.
How do I know if my operation is Part 91 or Part 121 or Part 135?
Work it in two steps. First, the commercial question: are you conducting common carriage for compensation or hire — holding out to the public a willingness to carry passengers or cargo for value? If not, you are generally Part 91. If yes, 14 CFR Part 119 generally requires a certificate and you are in Part 121 or 135. Second, the scale question, decided by the aircraft and the schedule under 14 CFR 110.2: on-demand and commuter operations in airplanes of 30 seats or fewer and 7,500 pounds payload or less (and rotorcraft) are Part 135; scheduled service and larger aircraft — turbojets, more than 9 seats, or more than 7,500 pounds payload — fall under Part 121. The same airplane can be Part 91 one day and Part 135 the next depending on the operation. This is a fact-specific legal determination for an aviation attorney, not something a blog post — or FileFlo — can decide for you.
When does a Part 135 operation become Part 121?
An operation crosses from Part 135 into Part 121 when it exceeds the 14 CFR 110.2 size or service thresholds. The clearest triggers: operating turbojet airplanes in scheduled or large-carrier service, operating airplanes with more than 9 passenger seats, or operating aircraft with a payload capacity of more than 7,500 pounds — those define domestic and flag operations, which are conducted under Part 121. Supplemental (non-scheduled large) operations above 30 passenger seats or 7,500 pounds payload are also Part 121. By contrast, on-demand common-carriage passenger work in airplanes of 30 seats or fewer with 7,500 pounds payload or less stays under Part 135. The boundary is therefore driven by aircraft size and the scheduled-versus-on-demand character of the service, not by how many flights you fly per se. Confirm any borderline case with aviation counsel before you operate it; the certificate, manuals, and authorizations differ enormously between the two.
How many passenger seats can a Part 135 aircraft have?
For common-carriage passenger operations, Part 135 on-demand service is defined in 14 CFR 110.2 around airplanes and powered-lift — including turbojet-powered ones — with a passenger-seat configuration of 30 seats or fewer (excluding crew seats) and a payload capacity of 7,500 pounds or less. Commuter operations under Part 135 are narrower still: scheduled, non-turbojet airplanes with 9 passenger seats or fewer and 7,500 pounds payload or less, run at least five round trips per week on at least one route. Above those thresholds — more than 30 seats, more than 9 seats in scheduled turbojet service, or more than 7,500 pounds payload — the operation generally moves to Part 121. Rotorcraft of essentially any size can operate on-demand under Part 135. These are the regulatory definitions; whether your aircraft and operation fit them is a determination for your aviation attorney and FAA Flight Standards office, not for FileFlo.
Is a private business jet flown for the company Part 91 or Part 135?
If a company flies its own jet to carry its own officials, employees, and guests on company business, and does not hold out to the public to carry passengers for compensation, that flying is generally conducted under Part 91 (often using Part 91 Subpart F operating rules for large and turbojet aircraft). The moment the same aircraft is used to carry the public for compensation or hire — selling seats or trips, or carrying outside customers for value — it generally needs a Part 135 certificate, because that is common carriage for hire. This is exactly the gray zone where the flight-department company trap and illegal grey charter live: a structure that looks private on paper but in substance carries others for value. Whether a specific arrangement is genuinely Part 91 or has crossed into Part 135 is a legal determination for an aviation attorney; FileFlo keeps the records once you know which rule applies.
Which is safer or more regulated, Part 121 or Part 135?
Part 121 is the most heavily regulated of the three regimes. It imposes the most extensive operational, training, dispatch, maintenance-program, and crew-qualification requirements, reflecting scheduled airline-scale operations. Part 135 is also a demanding commercial-operator regime — far beyond Part 91 — but its requirements are calibrated to on-demand and commuter operations in smaller aircraft. Part 91 is the baseline general operating rule and carries the lightest structural requirements because it is not for-hire common carriage. One area is converging: the FAA now requires a Safety Management System (SMS) for Part 135 operators and certain air-tour operators, with a single compliance deadline of May 28, 2027, narrowing one historical gap between 135 and 121. Comparing 'safety' across regimes is a regulatory and operational question, not a documentation one — FileFlo simply proves the records each regime requires are current and audit-ready.
How does FileFlo help across Part 91, Part 121, and Part 135?
FileFlo does not decide which operating rule applies to you, write your manuals, file your certificate application, run your SMS, or interact with the FAA — those belong to your aviation attorney, your certification team, and your required management personnel. What FileFlo does is the records work each regime demands: it is a compliance document intelligence platform that classifies, indexes, version-controls, and tracks expirations on the documents an operator must keep, whether that is a Part 91 flight department's maintenance and pilot records or a Part 135 operator's training records, manuals, OpSpecs, and authorization documents. Because the regime decides which records you must prove, knowing whether you are Part 91, 121, or 135 comes first — and that is a legal call. Once you know, proving you are compliant with current, organized, audit-ready records is the document problem FileFlo solves. FileFlo does not provide legal, financial, or tax advice and does not claim SOC 2 certification.
Know which rule applies — then prove you follow it
Whether you are a Part 91 flight department or a Part 135 air carrier, the day-to-day job of proving compliance is a records problem — and that is what FileFlo solves. It classifies, version-controls, and tracks expirations on your pilot, training, maintenance, manual, and OpSpecs records so they are audit-ready on demand. AI document classification. 600+ document types. One-click FAA surveillance binder. Starter at $89/mo, Professional at $299/mo. No credit card required for the 5-day free trial. FileFlo does not give legal advice or get you certified — it organizes and proves your compliance documents.
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Reviewed by Chad Griffith, Founder, FileFlo — compliance document intelligence. Last reviewed June 15, 2026. The applicability framework and operation-to-part mapping are verified against the Cornell Legal Information Institute eCFR (14 CFR §119.1 and §119.21); the seat, payload, and operation-type thresholds are verified against the definitions in 14 CFR §110.2 (on-demand, commuter, domestic, flag, and supplemental operations). The FAA Safety Management System (SMS) deadline of May 28, 2027 applies to Part 135 operators and certain air-tour operators. This article is a compliance-document perspective and is not legal, financial, or tax advice; whether any specific operation falls under Part 91, 121, or 135 is a fact-specific determination for a qualified aviation attorney and your FAA Flight Standards office.