Sky DutyA quick disclaimer before the detail: this guide is an overview to help you get oriented, not a regulatory or legal ruling. Whether a specific operation falls under Part 91 or Part 135 is a determination for the FAA and, for an operator, for your Flight Standards office or principal operations inspector. Use this to ask better questions, not to answer them for a given flight.
The one-sentence version
Part 91 — formally the General Operating and Flight Rules — is the baseline rulebook that applies to nearly all civil flying in the United States. Part 135 — Operating Requirements: Commuter and On-Demand Operations — governs commercial charter and air-taxi flying that carries people or property for compensation or hire, and it adds a substantial layer of requirements on top of the Part 91 baseline. Put simply: almost everyone operates under Part 91, and some operators additionally hold a Part 135 certificate because of the kind of flying they sell. Part 121 — Operating Requirements: Domestic, Flag, and Supplemental Operations — sits at the far end: it governs the scheduled airlines and large commercial carriers, and it carries the most demanding requirements of the three. So the short answer to "the difference between Part 91, 121, and 135" is a ladder of commercial scale and oversight: Part 91 is the private baseline, Part 135 is on-demand charter and air taxi, and Part 121 is the airlines.
What Part 91 covers
Part 91 is the foundation. It contains the rules that apply to how aircraft are operated in general — right-of-way, altitudes, equipment, preflight responsibilities, and the operating limits every pilot learns. Most non-commercial flying lives here: personal and recreational flights, owner-flown aircraft, corporate and business flying where a company operates its own aircraft for its own purposes rather than for hire, and most flight instruction. Fractional ownership programs are addressed under Part 91 Subpart K, sometimes written as 91K.
The defining feature of Part 91 operations is that the operator is not holding out to the public to carry people or property for compensation or hire in the way that triggers commercial certificate requirements. That single distinction — who you are flying for, and whether it amounts to common carriage — is what pushes an operation from Part 91 into Part 135 or Part 121 territory.
What Part 135 covers
Part 135 governs commuter and on-demand operations — the charter and air-taxi world. If a business carries passengers or cargo for compensation or hire on demand, it generally operates under a Part 135 certificate. Holding that certificate is not a formality; it comes with operations specifications (OpSpecs) issued by the FAA that spell out exactly what the operator is authorized to do, which aircraft, which areas, and under which conditions.
The practical result is that a Part 135 operator is running a more structured, more supervised operation than a Part 91 flight department, with the certificate holder retaining operational control of every flight conducted under it.
Part 91 vs Part 121 vs Part 135: the three-way picture
Part 91 and Part 135 are the two rulebooks most flight departments live between, but the picture is clearer with the third part in frame. Part 121 — Operating Requirements: Domestic, Flag, and Supplemental Operations — governs the scheduled airlines and large commercial carriers. You are unlikely to be choosing between Part 91 and Part 121, but seeing all three side by side shows the ladder of commercial scale and oversight that the parts climb: from private flying, to on-demand charter, to the airlines.
| Part 91 | Part 135 | Part 121 | |
|---|---|---|---|
| Who it covers | General operating and flight rules — the baseline for nearly all civil flying, mostly non-commercial | Commuter and on-demand operations — charter and air taxi for compensation or hire | Scheduled and large commercial air carrier operations |
| Certificate required | No air carrier or operator certificate (aircraft and pilots are still certificated) | Air carrier or operator certificate authorizing Part 135, with OpSpecs | Air carrier certificate authorizing Part 121, with OpSpecs |
| Typical operators | Private owners, corporate flight departments, owner-flown aircraft, most flight instruction, and fractional ownership programs (Subpart K) | Charter and air-taxi operators, on-demand and some commuter operators | Scheduled passenger airlines and large cargo carriers |
| Oversight level | Baseline — least prescriptive | More structured — OpSpecs, an approved inspection program, and Subpart F duty and rest limits | Most demanding — the airline standard, where Part 117 flight, duty, and rest rules apply |
Treat this table as orientation, not a ruling. The regulations themselves — 14 CFR Parts 91, 121, and 135, and the certification and operations specifications that go with them — are the authority, and where a specific operation lands is a determination for the FAA and your Flight Standards office. One note on duty and rest, because it is a common mix-up: the structured Part 117 flight, duty, and rest rules apply to Part 121 air carriers, not to Part 135. Part 135 has its own flight-time, duty, and rest limits in Subpart F, and the general Part 91 baseline does not carry structured limits like those — though fractional programs under Part 91 Subpart K do have their own flight, duty, and rest rules.
When an operator needs a Part 135 certificate
The short answer is: when the flying amounts to carrying passengers or cargo for compensation or hire in a way that constitutes common carriage. The longer answer is that this is one of the genuinely nuanced questions in the regulations. It turns on concepts like "holding out" to the public, common versus private carriage, and who has operational control — and small changes in the facts can change the answer.
This is exactly the kind of question where a guide should hand you off rather than pretend to decide. If you are weighing whether an arrangement requires a 135 certificate, that is a conversation for the FAA and your Flight Standards office, ideally before the flying starts. The cost of guessing wrong is high, and the determination is fact-specific.
What changes operationally
Stepping from Part 91 into Part 135 changes the shape of the operation in several areas at once.
Crew and management. Part 135 operators typically must designate required management personnel — commonly a Director of Operations, a Chief Pilot, and a Director of Maintenance — and crews face recurrent testing and checks, such as competency checks and, for instrument operations, instrument proficiency checks and line checks. A Part 91 department has far more latitude in how it staffs and checks its pilots.
Duty and rest. Part 91 does not impose the structured flight-time, duty, and rest limits that commercial operations carry. Part 135 has its own limits in Subpart F. Note that these are not Part 117 rules — Part 117 applies to Part 121 air carriers, not Part 135. This is where a tracking tool earns its keep, but it is worth being precise: keeping a clean record of who was on duty and how much they flew is a different thing from computing whether a crew member is legal. Sky Duty tracks duty periods and flight time; it does not calculate duty-time legality, and this guide does not either.
Maintenance. Part 135 operators generally follow an approved aircraft inspection program rather than relying solely on the standard annual and 100-hour inspection cycle a Part 91 operator might use, and the recordkeeping expectations are more formal. Either way, keeping inspection status visible before a flight goes out is good practice.
Operational control. Under Part 135, the certificate holder retains operational control and is accountable for the conduct of each flight, which drives more deliberate dispatch, release, and record practices than a typical Part 91 operation needs. For the specifics that apply to your certificate, the authority is the FAA and your operations specifications — not a summary like this one.
Where Sky Duty fits — for both
Sky Duty is built for small flight departments, and it supports both Part 91 and Part 135 operation types. Whichever rulebook you fly under, the day-to-day jobs are similar: schedule the fleet and crew, log the flights, keep maintenance and inspection status visible, and track trip costs. Sky Duty puts those in one offline-first iPhone and iPad app, with operation types and crew roles — PIC, SIC, dispatcher, mechanic, and more — that reflect how these departments actually work. You can read the segment overviews for Part 91 operators and Part 135 charter to see how each is set up. Charter teams evaluating Part 135 software specifically can start on that overview.
One boundary is worth restating, because it is the most common question from Part 135 operators. Sky Duty is a scheduling, logbook, maintenance, and expense tool — the operational backbone described on the flight department software page. It tracks duty and flight time, but it does not compute Part 117 or Part 135 duty-time legality, and it is not an EFB. It keeps the record; determining legality against the rules that apply to your certificate stays with the operator.
